October 13, 2017

Modelling panspermia in the TRAPPIST-1 system

James A. Blake1,2*, David J. Armstrong1,2, Dimitri Veras1,2

Abstract The recent ground-breaking discovery of seven temperate planets within the TRAPPIST-1 system has been hailed as a milestone in the development of exoplanetary science. Centred on an ultra-cool dwarf star, the planets all orbit within a sixth of the distance from Mercury to the Sun. This remarkably compact nature makes the system an ideal testbed for the modelling of rapid lithopanspermia, the idea that micro-organisms can be distributed throughout the Universe via fragments of rock ejected during a meteoric impact event. We perform N-body simulations to investigate the timescale and success-rate of lithopanspermia within TRAPPIST-1. In each simulation, test particles are ejected from one of the three planets thought to lie within the so-called ‘habitable zone’ of the star into a range of allowed orbits, constrained by the ejection velocity and coplanarity of the case in question. The irradiance received by the test particles is tracked throughout the simulation, allowing the overall radiant exposure to be calculated for each one at the close of its journey. A simultaneous in-depth review of space microbiological literature has enabled inferences to be made regarding the potential survivability of lithopanspermia in compact exoplanetary systems.

1Department of Physics, University of Warwick, Coventry, CV4 7AL 2Centre for Exoplanets and Habitability, University of Warwick, Coventry, CV4 7AL *Corresponding author: [email protected]

Contents Universe, and can propagate from one location to another. This interpretation owes itself predominantly to the works of William 1 Introduction1 Thompson (Lord Kelvin) and Hermann von Helmholtz in the 1.1 Mechanisms for panspermia...... 2 latter half of the 19th Century. Indeed, Thompson provided an Radiopanspermia • Lithopanspermia • Directed panspermia • excellent summation of the theory in 1871 [1]: Pseudopanspermia • Other mechanisms 1.2 Micro-organisms in low Earth orbit...... 3 “We must regard it as probable in the highest de- Early missions • Spacelab • LDEF • EURECA • MIR-Perseus • gree that there are countless seed-bearing mete- Biopan • EXPOSE • Cosmic rays oric stones moving about through space. If at the 1.3 Micro-organisms in simulation experiments....9 present instance no life existed upon this Earth, Planetary ejection • Atmospheric entry one such stone falling upon it might, by what we 1.4 The TRAPPIST-1 system...... 13 blindly call natural causes, lead to its becoming Discovery • Habitability • Motivation covered with vegetation.” 2 Theory 16 With these premises in place, panspermia has since been cham- 2.1 Equations of orbits...... 16 pioned by a number of prominent scientists, with notable con- General orbits • Circular orbits tributions from Svante Arrhenius [2], Francis Crick [3], Fred 2.2 Impacting the target...... 18 Hoyle [4] and Chandra Wickramasinghe [4, 5, 6]. Naturally, 3 Simulation set-up 19 this level of support for the theory has ensured that numerous 3.1 Simulation script...... 19 branches of thought have developed over the years, to be dis- cussed further in Sec. 1.1. Panspermia would, however, remain 3.2 Additional scripts...... 20 conjecture until the 1980s, a decade which welcomed the ability 4 Results and Discussion 21 to meticulously test the theory by exposing micro-organisms to 4.1 Timescale and fate...... 21 low Earth orbit environments. Such experiments, both pioneer- 4.2 Radiant exposure and survivability...... 22 ing and present-day, will receive attention in Sec. 1.2. 4.3 Discussion...... 26 A particular surge of interest came in the 1990s as a result of the Martian meteorite, ALH84001, which was found to pos- 5 Further work 27 sess structures that could indicate the presence of terrestrial Acknowledgments 28 nanobacteria. Careful testing of ALH84001, shown in Fig.1, References 28 has alluded to the presence of amino acids and polycyclic aro- matic hydrocarbons [7]. Whilst these can be an indication of life, most experts agreed that the compounds most likely formed 1. Introduction abiotically from organic molecules, or via contamination during The concept of panspermia (‘seeds everywhere’) has existed extraction. The past year has sparked a similar rise of interest since the ancient writings of the Greek philosopher Anaxagoras in panspermia, due to the discovery of seven Earth-sized plan- (500 BC–428 BC), although his notion differs from that of the ets in TRAPPIST-1, a system whose compact nature provides current theory. In modern nomenclature, panspermia refers to an unprecedented testbed for the process. This constitutes the the hypothesis stating that seeds of life exist across the entire motivation for our study. Modelling panspermia in the TRAPPIST-1 system — 2/32

of lithopanspermia as a mechanism remains speculative, cer- tain aspects have recently become testable experimentally [15]. An outline of these experiments is provided in Sec. 1.3. The presence of Martian meteorites on Earth provides evidence of the natural transfer of rock between planets within the Solar System. This was anticipated in the mid–late 1800s by the likes of Hermann von Helmholtz and Lord Kelvin, both of whom favoured the lithopanspermia hypothesis [1, 16]. For lithopanspermia to take place, one would expect the process to consist of three separate stages:

1. Planetary ejection – it was Cockell who first realised that in order for life to be transported from one planet to another, it must first be able to survive ejection from the original planet [17]. Such an ejection would result from the impacts of km–sized asteroids and comets, subjecting the rock to extreme forces, accelerations and tempera- Figure 1. The ALH84001 Martian meteorite is ∼9 cm across ture increases. A combination of petrographic studies and has been found to contain organic carbon compounds. and numerical simulations of Martian rocks, ejected at a velocity high enough to allow escape from Mars, have unearthed shock pressure estimates of 5–55 GPa during 1.1 Mechanisms for panspermia launch, alongside post-shock temperatures in the range Whilst we have so far referred solely to the general theory of 100–600 ◦C[18, 19]. Sec. 1.3 will provide further de- panspermia, numerous branches have developed over the last tails of these experiments, alongside others investigating couple of centuries, each proposing a separate mechanism for the effect of hypervelocity impact on the wellbeing of the transfer of life throughout the Universe. Although there micro-organisms. Whilst these impacts are undoubtedly exists no conflict between the various mechanisms, and they energetic in nature, a small proportion of the resulting could all be at work in relative harmony, it is important to ejecta will not exceed the lower limit of 100 ◦C. These distinguish between them. lower temperatures result from an ejection originating in the ‘spall’ zone, referring to the targetted surface of the 1.1.1 Radiopanspermia impacted planet. Here, the shock wave from the impact Proposed originally by Arrhenius in his 1903 work, The Distri- is effectively cancelled by the reflected shock wave from bution of Life in Space, radiopanspermia is the hypothesis that the surface [20]. It has been estimated that more than a micro-organisms can propagate through space, driven by stellar billion fragments of rock have been ejected from Mars at radiation pressure alone [8]. His assertions were driven by the such low temperatures throughout the history of the Solar knowledge that interplanetary space within our Solar System is System, with ∼5 % having the potential to land on Earth. littered with micron-sized dust particles. Such particles, below What’s more, the Earth’s crust has been found to be inhab- a critical size of 1.5 microns, would be driven away by the ited by a number of micro-organisms [21, 22]. To many, radiation pressure of the sun, potentially transporting microbial these findings have strongly supported the lithopansper- stowaways to other planetary systems. mia hypothesis. An intriguing theory, though one which quickly loses ef- 2. Transit in space – if the microbial life is able to survive fectiveness as the size of the particle increases. In this sense, the ejection process, it must then withstand the inter- radiopanspermia as a mechanism for transporting life holds planetary transfer that follows. The scale of this journey solely for very small particles, at most hosting single bacterial will depend on the system in question, and whether the spores [9]. Furthermore, Shklovskij and Sagan asserted that it lithopanspermia is occuring between planets of the same is extremely unlikely that micro-organisms would be able to system, or an entirely separate system. One specific case withstand the ‘lethal’ concoction of the solar UV and cosmic that has been studied in great detail is exchange of ejecta radiation, whilst more recent studies have highlighted the dele- between Mars and the Earth [9, 14, 23]. Here, estimates terious nature of the space vacuum [10, 11, 12]. Experiments of journey times lie within the range of 1–20 million looking into the effect of radiation on DNA stability have pro- years, although simulations appear to suggest that a small vided a final nail in the coffin for radiopanspermia, concluding proportion of meteorites could reach Earth within the first that boulder-sized rocks (∼1 m in diameter) are required to few years of travel [23]. This is the primary reason why provide effective shielding of bacterial spores from galactic the seven recently discovered planets of the TRAPPIST-1 cosmic radiation [13, 14]. These findings instead support a system (see Sec. 1.4) have generated such excitement mechanism involving much larger transportation vessels, such within the fields of panspermia and astrobiology. The as asteroids and comets, known as lithopanspermia. planets of this system all lie within a radius of ∼5 % of the Earth-Sun distance. As such, lithopanspermia within 1.1.2 Lithopanspermia the system is likely to occur on a much shorter timescale Lithopanspermia refers to the hypothesis that micro-organisms when compared with the Earth-Mars case above. Simula- shielded by rocks can travel from planet to planet through ei- tions performed by Krijt et al. (2017) found that ejecta ther interplanetary or interstellar space. Whilst the viability released from planet f could reach the other ‘habitable’ Modelling panspermia in the TRAPPIST-1 system — 3/32

planet g within ∼80 years [24]. The present investigation nucleic acids stand alongside lipids, proteins and carbohydrates aims to extend these results, as detailed in Sec.4.A to constitute the four main macromolecules necessary for life. number of experimental facilities have placed a focus More recently, in 2012, scientists at Copenhagen University dis- on assessing the response of microbial life to the testing covered signatures of glycolaldehyde, a sugar which holds im- environments of outer space, making use of low Earth portance in the production of RNA, around a solar-type young orbit satellites [9, 15]. These will be outlined further in star that resides in the binary system IRAS16293-2422 [33]. Sec. 1.2. The following year, researchers found traces of cyanometha- 3. Atmospheric entry – the final stage of lithopanspermia nimine in a giant ISM gas cloud ∼ 25000 ly from Earth. This to test involves hypervelocity entry from space through molecule is known to produce adenine, a nucleobase that con- the atmosphere of the target planet. As will be discussed tributes to the ladder-like structure of DNA. The same project in Sec. 1.3, numerous experiments have subjected rock also discovered the presence of ethanamine, thought to have a samples containing micro-organisms to temperatures and part to play in the formation of alanine, one of twenty amino pressures typically experienced by meteorites undergoing acids found in the human genome [34]. Finally, in 2015, NASA atmospheric entry [15, 25]. announced that the compounds uracil, cytosine and thymine, all key components of either DNA or RNA, had been successfully 1.1.3 Directed panspermia produced within the laboratory, subject to conditions remanis- In 1973, Crick and Orgel argued that life may have been pur- cent of outer space [35]. posely spread throughout the Universe by an advanced extrater- restrial civilisation [3]. Referred to as ‘directed panspermia’, 1.1.5 Other mechanisms this theory asserts that life may have deliberately been sent to A variety of other hypotheses exist regarding possible mech- seed the Earth. Naturally, this also concerns the seeding of anisms for panspermia. Belbruno and coworkers have shown other habitable worlds from the Earth, a feet that is fast be- that low-energy transfer of rocks among protoplanets in orbit coming plausible due to developments in engineering (solar around young stars should be fairly common, supporting the sails), astronomy (exoplanets, astrometry) and biology (micro- theory of lithopanspermia [36]. Space probes are pushing new bial genetic engineering). Indeed, studies by astroecologists frontiers, reaching further than ever before. This naturally raises have deduced that a number of key nutrients could be obtained the question of whether such probes could cross-contaminate from materials typically found within asteroids [26]. throughout the Solar System and beyond. Whilst numerous In order to prove the hypothesis, it has been suggested protection policies have been implemented, recent research has that a distinctive ‘signature’ may be present in the genetic found that certain microbes may still be immune to clean-room code of early microbial life on Earth, reminiscent of the parent procedures [37]. A rather alternative interpretation was pro- civilisation [27]. Although evidence for such a signature is yet posed by Dehel, who suggests that magnetic fields ejected from to be found, the theory continues to raise interesting questions the Earth’s magnetosphere could lift bacterial spores from the as modern advancements add to its plausibility. atmosphere, and propel them towards other stellar systems [38]. Despite the wide range of theories that exist, the present work 1.1.4 Pseudopanspermia will focus predominantly on the lithopanspermia hypothesis, Another form of panspermia, most notably championed by with particular emphasis on interplanetary transfer within the Chandra Wickramasinghe, is pseudopanspermia. According same stellar system. to this hypothesis, the solar nebula was able to draw in or- ganic molecules that were already present in space during its 1.2 Micro-organisms in low Earth orbit formation. As the planets condensed from the nebula, these The continued development of space flight has enabled nu- molecules would have been incorporated and subsequently dis- merous micro-organisms to be exposed directly to the harsh tributed among the planetary surfaces. It is thought that these conditions of outer space. Prior to testing, such conditions organic compounds then evolved to form life, via a process were thought to be extremely hostile to life. Most notably, the known as abiogenesis [28]. Wickramasinghe originally sug- vacuum of up to 10−14 Pa is an impenetrable barrier for the bio- gested formaldehyde, CH2O, as the main organic component logical processes of growth, metabolism and reproduction [39]. of interstellar dust, although many other ideas were raised [29]. What’s more, micro-organisms in space will undergo exposure Organic molecules in the interstellar medium are most com- to a complex concoction of radiation. Whilst the solar UV monly formed when an inorganic molecule becomes ionised, poses a particularly severe threat, bombardment by high energy usually due to interactions with cosmic rays [30]. Electrostatic particles originating either from the star (e.g. stellar wind) or attraction then ensures that a reactant is drawn towards the from galactic/extragalactic space (cosmic rays) will also induce charged ion. Interstellar dust plays a key role by shielding the adverse effects. A summary of the space environment mea- newly formed organic molecules from UV light, which can sured for various low Earth orbit missions to be discussed in further ionise them [31]. this review is provided in Table 1. To date, a number of intriguing discoveries have been made Despite the hostile environment, certain life forms possess in support of pseudopanspermia. In 2008, radiometric dating of the ability to survive long periods of time in a ‘dormant’ state, organic species residing within the Murchison meteorite indi- such as bacterial and fungal spores. A spore is a resilient cas- cated that the rock was non-terrestrial in origin, implying poten- ing which contains identical genetic information to the micro- tial accretion from the interstellar medium (ISM) [32]. One of organisms that are able to form it. The cores of bacterial spores the most notable molecules investigated was uracil, one of the exhibit extremely low enzyme activity, thought to be due to four nucleobases that make up the structure of ribonucleic acid their notable lack of water content. This contributes to their (RNA), the counterpart of deoxyribonucleic acid (DNA). The resilience, in conjunction with the fact that the spore DNA is Modelling panspermia in the TRAPPIST-1 system — 4/32

Table 1. Space environment conditions measured for various low Earth orbit missions. Extended from Horneck et al. (2010) [9].

Space parameter Spacelab LDEF EURECA MIR-Perseus Biopan EXPOSE

Vacuum pressure (Pa) ∼ 10−4 ∼ 10−6 ∼ 10−5 ∼ 10−4 ∼ 10−6 ∼ 10−4

Irradiance (Wm−2) 1365 ∼1370 1367 1370 ∼1370 ∼ 1370 Undeter- UV fluence (Jm−2) ≤ 103 ∼ 109 ≤ 3 × 103 ∼ 107 ∼ 109 mined (>) 110, 170, (>) 110, 170, (>) 110, 170, Spectral range (nm) (>) 50, 170 (>) 110 (>) 110 290, 300 280, 295 200, 290, 400 Cosmic ionising 0.001 4.8 0.2–0.4 0.037–0.049 0.004–0.074 0.1–0.2 radiation dose (Gy)

Temperature (K) 243–290 264–302 295–318 259–316 235–288 253–332

Gravity (g) ∼ 10−3 ∼ 10−6 ∼ 10−6 ∼ 10−3 ∼ 10−6

Exposure time (days) 10 2107 336 98 10–15 559 mixed with acid-soluble proteins, which aid in chemical and en- subtilis (strain 168) tested at the stated wavelengths. However, zymatic reactivity of the DNA [40]. The structure of the spore survival rates were found to be much lower for samples exposed can subsequently protect the genetic material during periods to the full spectrum of solar radiation [47]. What’s more, the B. throughout which extreme conditions would ordinarily destroy subtilis spores were found to be unhindered by exposure to the the micro-organism. space vacuum alone, suggesting impressive survivability with This property makes spores, alongside other extremophiles, appropriate shielding from the solar UV. Similar results were ideal for testing the survivability of space, a key factor to con- found for bacteriophage T7 samples. Other micro-organisms sider when assessing the viability of lithopanspermia. With a tested by the MEED include the two yeasts, Rhodotorula rubra broad definition, the term ‘extremophile’ is used to describe and Saccharomyces cerevisiae, and the two fungi, Trichophyton micro-organisms that are able to survive in the harshest condi- terrestre and Chaetomium globosum. No significant differences tions on Earth. Extremophiles have been found with tolerances were found between non-irradiated inflight and corresponding against extreme temperatures, lack of sunlight and severe dessi- ground-based control samples in terms of survival rate, whilst cation [41, 42, 43, 44]. A variety of such micro-organisms have S. cerevisiae showed a degree of sensitivity with regards to UV now been tested in low Earth orbit facilities. This section aims irradiation [47]. to provide an overview of the experiments undertaken to date; a briefer summary is provided in Tables 2–4. 1.2.2 Spacelab In the 1983 experiment, Spacelab 1, three separate strains of 1.2.1 Early missions B. subtilis were exposed to the full space environment. This With the dawn of spaceflight came the desire to test micro- included the space vacuum, in conjunction with the full solar organisms within the space environment. In 1966, as part of UV spectrum (>170 nm), or with selected ranges peaking at the Gemini IX and XII missions, cultures of bacteriophage either 220 nm, 240 nm, 260 nm or 280 nm [48]. When exposed T1 and Penicillium roqueforti spores underwent exposure to to vacuum alone, the inflight survival rate of the HA 101 strain space for 16.8 h and 6.5 h, respectively [45]. Survival fractions was found to be 62 % after 10 days exposure. With combined −5 for both experiments were found to be ∼ 10 , demonstrating exposure to the solar UV, survival fractions reduced to ∼ 10−2 the destructive nature of conditions in outer space [45, 46]. after the same period. In this respect, the results are consistent Shielding the spores with a layer of aluminium caused the with those found in the early spaceflight experiments; exposure survival rate to increase by three orders of magnitude, implying to solar UV appears to have the most deleterious effects. A that radiation played a key role in the inactivation of the micro- similar experiment took place during the German Spacelab D2 organisms. mission in 1993 [9, 49]. Following these early experiments, the more developed Mi- crobial Ecology Evaluation Device (MEED) exposure facility 1.2.3 LDEF was implemented during the Apollo 16 mission [47]. Compris- Still holding the record for the longest exposure of micro- ing of 798 sample holders, the MEED made use of optical filter organisms to the space environment, the Long Duration Ex- windows and optional ventilation holes for exposure to both posure Facility (LDEF) NASA mission took place aboard the solar radiation and space vacuum during flight. Samples were Exostack facility between 1984 and 1990 [50]. Samples were exposed first to vacuum for ∼1.3 h and to solar UV radiation either exposed to the full environment of space, or shielded for 10 min. Peak wavelengths for the solar UV (controlled by by quartz filters or aluminium foil. During the flight, the dose the filters) stood at 254 nm, 280 nm and 300 nm. Survival rates of total solar UV radiation stood at an estimated 109 Jm−2 for were varied based on different combinations of space-like expo- the 6 yrs of exposure. This was combined with a 4.8 Gy dose sures, but were found to be relatively high for spores of Bacillus of galactic cosmic ray (GCR) radiation. A survival fraction Modelling panspermia in the TRAPPIST-1 system — 5/32 of 1–2 % was achieved for B. subtilis spores (in a monolayer) ∼1–2×107 Jm−2 of which was UV (>170 nm). Cosmic radi- following the 6 yrs of exposure to the space vacuum alone. The ation doses among the samples varied between 4 mGy and survival rate was much improved for samples immersed in pro- 74 mGy, depending on their positioning within the vessel. tective substances, such as sugars or buffer salts. For instance, Spores of B. subtilis are known to have a D10 value (dose ∼70 % of spore multilayers mixed with 5 % glucose remained leading to 10 % survival) of 1500 Gy, so this was relatively viable following 6 yrs aboard the LDEF [9]. minor. Full details of the conditions for each of the first three Biopan experiments are provided by Horneck et al. (2001) [54]. 1.2.4 EURECA A survival fraction of 10−6 was found for spores exposed to The Exobiology Radiation Assembly (ERA) facility aboard the the full space environment, whilst much higher fractions (0.5– European Retrievable Carrier (EURECA) subjected spores of B. 0.97) were observed for samples that were shielded from the so- subtilis strains and an E. coli plasmid pUC19, alongside cells lar UV. Clay shielding was found to be ineffective when placed of Deinococcus radiodurans and conidia of Aspergillus species, as a ‘shadowing’ filter. Much more protection was received to various aspects of the space environment [51, 52]. These if the spores were mixed with the clay, and similarly positive included the space vacuum, as well as selected wavebands results were obtained when mixing the spores with meteorite and intensities of solar UV radiation. For samples exposed powder. Both of these methods proved more efficient at protect- to the full extent of the solar UV, the micro-organisms were ing the spores than mixing with glucose, which previously had encased in artificial meteorites, comprising of soils, rocks and shown great promise during the LDEF mission [50, 54]. meteorite powder [9]. In these cases, sample holders were either The LICHENS experiment took place in the Biopan 5 facil- completely exposed, or protected by long-pass cut-off filters ity aboard the Foton-M2 satellite, aiming to expose lichens to at wavelengths >110 nm, >170 nm, >280 nm or >295 nm. the full array of space conditions for the first time [55]. Lichens Exposure to the space vacuum lasted 9 mths, whilst solar UV are composite organisms comprising of a stable symbiotic in- exposure occurred for 6 mths. During this time, GCR doses teraction between a fungus and/or cyanobacteria. Found in reached levels of up to 0.4 Gy [9]. some of the most extreme environments on Earth, lichens have Subsequent analysis of the samples revealed that survival shown an ability to survive complete loss of water in periods of rates of spores in multilayers and/or in the presence of protec- severe dessication [56]. They can grow inside rock, between tive substances like glucose are much improved. Conversely, the grains, and are often found in mountainous regions, where spores embedded within artificial meteorites were largely inac- exposure to UV radiation is higher than average [57]. As such, tivated [52]. The solar UV was found to induce DNA strand lichens are a natural choice to test alongside bacterial spores in breaks, significantly reducing survival. A correlation was found space. between the decrease in viability and increase in DNA dam- Shoots of the endolithic lichens Rhizocarpon geographicum age, implying that the solar UV once again had the most and Xanthoria elegans were exposed to the space vacuum, deleterious effect on the micro-organisms [51]. Whilst sur- alongside selected wavebands of the solar UV. Temperatures vival fractions were small after exposure to the solar UV, how- varied between -21.7 ◦C and +21.8 ◦C throughout the 14.6 d ever, they remained significant even for the highest fluences of 8 −2 exposure, during which time the lichens were subjected to ∼ 3 × 10 Jm . In the same experiment, a variety of amino ∼ 2.2 × 107 Jm−2 of solar UV (>170 nm). What’s more, the acids and urea were relatively unaffected by the dehydrating samples also endured a total ionising radiation dose of ∼3 mGy. nature of the space vacuum, provided they were shielded from Post-landing analysis, consisting of Scanning and Transmission the solar UV. Electron Microscopy (S/TEM), revealed that the majority of the 1.2.5 MIR-Perseus algal and fungal cells survived the exposure (83 % in Xanthoria A relatively long-term exposure (3 mths) took place during the and 71 % in Rhizocarpon)[55]. Viable cells were assessed Exobiologie experiment, part of the French Perseus mission by the extent to which their membranes were intact. A full aboard the Russian MIR space station [9, 53]. During the recovery of photosynthetic activity was observed following the mission, spores of the HA 101 and TKJ 6312 strains of B. mission, even for those samples exposed to over 99 % of the subtilis were exposed to the full extent of space (UV >110 nm). solar light. This is thought to be due to a thick, dense upper The temperature of the samples varied between -14 ◦C and cortex possessed by the species in question. Various pigments +43 ◦C. Throughout the 98 days of exposure, the spores received within the lichens may have played a screening role with regards a cosmic ray radiation dose of between ∼37 mGy and ∼49 mGy, to the solar UV, whilst other studies have suggested a thermal whilst the UV fluence remains undetermined [9]. dissipation mechanism to explain the impressive tolerances observed [58]. 1.2.6 Biopan Finally, the Biopan 6 facility aboard the Foton-M3 satel- Numerous exposure experiments have taken place using the lite played host to the Lithopanspermia experiments [59, 60]. ESA’s Biopan facilities aboard the Russian Foton satellites, the Samples of the lichens Rhizocarpon geographicum, Xantho- first of which occurring in 1994 [54]. For the first three Biopan ria elegans and Aspicilia fruticulosa were subjected to similar experiments, the primary focus was to investigate shielding as space conditions to those of the other Biopan experiments. Sam- a means of protecting bacterial spores against the damaging ples exposed to the solar spectrum (>110 nm, >290 nm and properties of solar radiation. With this in mind, spores of the >400 nm) exhibited reduced photosynthetic activity post-flight, HA 101, HA F and TKJ 6312 strains of B. subtilis were sub- when compared with pre-flight controls. Despite this, viability jected to the space environment either unprotected, or shielded fractions were all above 50 %, supporting the positive results by clays, soils, rocks or meteorite powders [54]. obtained for lichens in previous Biopan missions. The samples were exposed for ∼10–15 days, during which Biopan 6 also provided the first testing ground for tardi- time they were subjected to 1–2×108 Jm−2 of solar radiation, grades in space. Tardigrades are among the most resilient Modelling panspermia in the TRAPPIST-1 system — 6/32

Figure 2. (a) The Exostack mission aboard NASA’s Long Duration Exposure Facility, which subjected micro-organisms to up to 6 yrs of space exposure. (b) The Foton–M3 spacecraft, used to host the Biopan 6 and STONE 6 facilities. (c) Sample containers aboard the EXPOSE facility during the EXPOSE-R series of experiments. (d) Sample containers used to expose micro-organisms to the space environment aboard the Biopan 4 facility. (e) A Foton return capsule, containing both the Biopan and STONE experiments. (f) The Exposed facility aboard the Japanese Kibo Module of the ISS, currently undertaking the most recent of astrobiological exposure experiments. Modelling panspermia in the TRAPPIST-1 system — 7/32

Figure 3. (a) Spores of Bacillus subtilis, currently the longest surviving bacterial spore within a space-like environment (see LDEF). (b) The bright orange pigmentation of the lichen Xanthoria elegans (see Biopan 5,6). (c) Artist’s impression of a tardigrade, one of the most resilient species known to exist on Earth (see Biopan 6). species on Earth, surviving extreme temperatures and pres- synthetic activity compared to pre-flight levels. Also shielded sures for significant periods of time [60, 61, 62, 63]. What’s from the solar UV, the black fungus Cryomyces antarcticus was more, they can survive radiation doses exceeding 5000 Gy, found to possess 12.5 % viable cells following exposure [73]. much higher than the other micro-organisms mentioned thus Finally, the SEEDS experiment subjected 2100 seed samples to far [64]. A recent study by Sloan et al. (2017) considered the the full extent of the space environment. An impressive 23 % of likelihood of tardigrade wipeouts from supernovae, gamma- the seeds were able to germinate following the 1.5 yr mission, ray bursts, large asteroid impacts and close encounters with with the highest viability (44 %) found in tobacco [74]. For all other stars [65]. It was found that such an outcome would be the EXPOSE-E experiments reviewed above, the samples were extremely unlikely, and that tardigrades will most probably also exposed to galactic cosmic ray radiation, amounting to survive through any mass extinction event. ∼ 91.1 × 10−6 Gy day−1. Due to the orientation of the module In a similar way to bacterial spores, tardigrades are able to aboard the ISS throughout the mission, EXPOSE-E managed form a quiescent state in times of extreme stress, referred to as to avoid additional radiation from both the radiation belts and cryptobiosis, during which time processes such as metabolism the solar wind [75]. are reversibly shut down. Cryptobiosis will initiate if the tardi- The second facility, EXPOSE-R, was mounted to the Rus- grade encounters an environment that is extremely dessicated, sian module Zvezda aboard the ISS in 2009. For this set of cold, hot, low/high in pressure, toxic or low/high in pH [66, 67]. missions, ∼1200 samples were exposed to a variety of space One particular form of cryptobiosis, known as anhydrobiosis, conditions for 682 days, including solar UV radiation, space has served as the primary focus of research into tardigrade be- vacuum, cosmic rays and temperature variations. As was the haviour. In this process, the tardigrade will contract and lose case for EXPOSE-E, a number of separate experiments took a large quantity (almost all) of its water content, thus allow- place during the facility’s time in operation. The AMINO ex- ing cell stabilisers (most notably trehalose) to be formed and periment exposed organic molecules to space both in their natu- metabolism to be reduced or, in the most extreme cases, haulted ral state and embedded in meteorite powder [76]. Compounds altogether [68, 69, 70]. tested include the amino acids glycine, alanine and aspartic acid, 1.2.7 EXPOSE alongside the dipeptide dileucine. Chosen for their astrobiolog- ical importance, amino acids and dipeptides play a key role in The most recent results obtained from exposure facilities in low the formation of macromolecules considered essential for life. Earth orbit are those of the European Space Agency’s EXPOSE During the AMINO experiment, the samples were subjected to facilties mounted to various modules of the International Space a total UV (100–400 nm) dose of ∼ 1.04×109 Jm−2. The high- Station (ISS). Thus far, three facilities have been launched. est survival rate in unshielded (natural) form was achieved by EXPOSE-E launched in 2008, providing three exposure trays; glycine, with 72 % of the sample remaining viable after expo- the second tray allowed samples to be exposed to the space sure. However, when shielded against the solar UV by meteorite vacuum, solar UV (>110 nm) and cosmic radiation [71]. In the powder, 2-amino isobutyric acid produced the best survival rate ADAPT experiment aboard EXPOSE-E, it was again shown of 78 %. All compounds tested were fairly resilient against that solar UV is the most damaging factor when exposing B. the space environment, except for dileucine, which showed no subtilis endospores to space [72]. If shielded, however, up to signs of preservation if unshielded from UV. Shielding using 8 % of the spores survived the ∼1.5 yrs of exposure. A similar meteorite powder increased survival rates in all cases. This was experiment, PROTECT, found that B. pumilus spores showed a also true in a study of RNA as part of the AMINO experiment, ∼50 % survival rate if shielded from the solar UV. which found a higher level of degradation in samples when The LIFE experiment aboard EXPOSE-E provided a first fully exposed to solar radiation [77]. long-term exposure to space conditions for a variety of eukary- otic organisms [73]. The lichen Xanthoria elegans, shielded Alongside AMINO, the ORGANIC experiment tested a from the solar UV, was once again found to achieve a full number of polycyclic aromatic hydrocarbons (PAHs) aboard (∼99.4 %) photosynthetic recovery upon reactivation. Other or- EXPOSE-R [78]. As the samples (in thin film form) were ganisms tested did not fair as well, with the lichen Rhizocarpon unshadowed for a 17 wk period, a radiation dose of ∼ 14 × geographicum attaining a particularly low 2.46 % of its photo- 109 Jm−2 has been estimated. Spectral measurements were Modelling panspermia in the TRAPPIST-1 system — 8/32 used to analyse the samples post-exposure. It was concluded ongoing, and so we merely provide details of the mission objec- that the PAHs behaved in a stable manner within the space tives and early findings. The ‘Biology and Mars Experiment’ environment tested on EXPOSE-R, with spectral changes as (BIOMEX) aimed to test various biomolecules, pigments and low as ∼ 10 %. films, alongside selected extremophiles, within space-like and In order to extend the results of the early Biopan missions, Mars-like environments [86]. Analyses have been performed samples of the cyanobacterium Synechococcus (Nageli)¨ and on two carotenoids contained in either the cyanobacterium Halorubrum chaoviator, a member of the Halobacteriaceae Nostoc sp. or the alga Sphaerocystis sp., exposed to space family (archaea), were exposed for a longer period aboard conditions [87]. Carotenoids are plant pigments that are re- EXPOSE-R as part of the OSMO experiment [79]. If kept sponsible for strong red, yellow and orange hues in fruits and shielded from solar radiation, the samples were found to have vegetables, and have been identified as potential biosignatures a 90 % survival rate. However, those samples that were sub- to be searched for on Mars. The carotenoids in both organ- jected to the solar UV had very low survival rates, particularly isms used remained detectable after the ∼15 mths exposure. when using cell growth as a gauge. Cosmic radiation doses Also part of the BIOMEX experiment, spores of the fungus for this experiment were measured to be within the range 225- Cryomyces antarcticus have been exposed to a variety of space 320 mGy. Also investigated previously using the Biopan facil- conditions [88]. Colonies were found to form even after the ity, Chroococcidiopsis cells were exposed to the space environ- highest radiation doses (10000 Jm−2), with survival rates up to ment under the protection of impact-shocked rock as part of the 72 % observed. In addition to the UV irradiation, the spores ENDO experiment [80]. This study found that encapsulation also had to deal with temperature extremes of -25 ◦C to +60 ◦C. within such rock provides adequate protection, allowing the Another constituent of the EXPOSE-R2 mission, the ‘Biofilm cyanobacteria to survive in a dessicated state over the ∼1.5 yr Organisms Surfing Space’ (BOSS) experiment aimed to test period. the effectiveness of growing micro-organisms as biofilms when The SPORES experiment aimed to simulate the expected considering their survivability in space [86]. The Biochip ex- conditions during lithopanspermia [81, 82]. Spores of B. sub- periment investigated the effects of temperature and cosmic tilis 168 were exposed either in monolayers or with different radiation aboard various biochip models, designed to detect concentrations of meteorite powder. The samples were sub- biomarkers in space-like environments [89]. jected to an overall UV fluence of 8.59×108 Jm−2 during the Still ongoing, the Tanpopo mission on the Japanese Kibo mission. Low viability rates demonstrated the damaging nature Module aboard the ISS is the most recent astrobiological mis- of the solar UV (>110 nm), and were found to be due mainly sion to take place [90, 91]. The mission, launched in 2015, aims to photodamaged DNA. Besides the bacterial samples, spores to collect cosmic dust particles using a low-density gel, known of the fungus Trichoderma longibrachiatum were also tested. as aerogel. Sample collection will continue until 2018 [92]. Survival rates of ∼30 % were found, if the spores were shielded against UV. Surprisingly, however, the UV-exposed samples 1.2.8 Cosmic rays had very similar viability levels following the exposure, imply- Thus far, our discussion has placed particular focus on the dam- ing that its effects may not be as deleterious for the fungus as aging effects of the solar UV. Whilst this has certainly been they are for the other micro-organisms tested. One explana- found to cause the most harm to micro-organisms in space, it tion for this could be that the spores clustered, thus offering a is important to also consider its counterpart, galactic cosmic greater degree of protection for those spores inside. In a similar radiation. GCR comprises predominantly of high-energy pro- experiment, named PUR, bacteriophage T7 and uracil were tons and light atomic nuclei, accelerated by supernovae either tested [83]. The solar UV was found to cause photolesions in galactic or extragalactic in origin. Nuclei heavier than alpha the uracil, once again exhibiting its damaging effects. particles make up ∼ 1 % of the GCR flux [9]. Referred to as Finally, the IMBP experiment investigated the exposure of HZE particles (high charge Z, high energy), they can cause a variety of bacterial and fungal spores, including B. subtilis, severe, localised damage. Their destructive nature is enhanced B. pumilus, Penicillium expansum and Aspergillus versicolor, by the production of radicals along their path through a cell, alongside a number of plant seeds, such as Arabidopsis thaliana leading to major mutations. and tomato seeds [84]. Bacterial spores exposed to the full ex- The GCR environment within a given planetary system can tent of the solar UV were found to be sterilised, whilst shielded be greatly influenced by the presence of magnetic fields. For samples showed viabilities of the order 10−2 −10−4 %. Fungal instance, the van Allen belts of the Earth arise from the inter- spores showed similarly high survival rates to those observed action between high-energy particles and the Earth’s magnetic for fungi tested in the SPORES experiment. Once again, it field. The belts exhibit high fluxes of cosmic rays due to the has been suggested that clustering of spores could provide an magnetic trapping of protons and electrons, with the magnetic insight as to why their viability remained high. Tomato seeds poles serving as mirrors which reflect the gyrating charged par- failed to survive the exposure, even when shielded from the so- ticles at each encounter. In the belts, electrons reach energies lar UV. Conversely, seeds of Arabidopsis retained the ability to up to 7 MeV, whilst protons reach up to ∼ 600 MeV [93]. In germinate and produce plants following the exposure, provided one region of the belts, the so-called ‘South Atlantic Anomaly’, adequate shielding was present. the proton flux is around 1000 times higher than in other parts The third of the EXPOSE missions to take place aboard of low Earth orbit [9]. However, the belts also provide a degree the ISS, EXPOSE-R2 facilitated the testing of numerous sam- of shielding from cosmic radiation, contributing to the overall ples of bacteria, fungi, lichens, archaea and anthropods [85]. habitability of the Earth. Further out in interplanetary space, the Mounted on the same module aboard the ISS as EXPOSE- GCR dose would be higher. Indeed, at the outskirts of low Earth R, EXPOSE-R2 launched in 2014, exposing the biological orbit, annual doses could reach as high as 10000 Gy, depending samples for between 12 mths and 18 mths. Analysis is still on both the altitude and amount of shielding in place [9]. Modelling panspermia in the TRAPPIST-1 system — 9/32

In conjunction with GCR, micro-organisms in the space however, that with pressures > 42 GPa, the impact led to the environment will also undergo exposure to stellar cosmic ra- destruction of the majority of the thalli, leaving behind only the diation, most notably in the form of stellar winds, flares and generative part of the lichen. coronal mass ejections. The former of these comprises of rel- High-speed projectiles have also been used to replicate the atively low-energy particles, emitted in a steady stream from pressures involved in large impact events. Burchell et al. (2004) the star. The other two contributors mentioned arise due to dis- varied the applied pressures from 1–78 GPa [98]. Over this turbances in the star’s magnetic stability, which can lead to the range, the survival rate of cells of Rhodococcus erythropolis ejection of particles with energies of the order of GeV [9]. In dropped from 10−4 to 10−7. It had previously been shown the case of the Earth, the magnetic field again provides adequate that the same cells can survive accelerations up to 5 kms−1, shielding from such events. Interplanetary transits, however, the escape velocity of Mars [99]. In the same study, survival will certainly be affected if a flare were to coincide with the rates for B. subtilis were found to be 10−5 when 78 GPa was path of the vessel. Flares are particularly common in M-dwarfs, applied. A similar survival rate for B. subtilis was found in a such as the focus of our study, TRAPPIST-1. Temperate planets subsequent study by Fajardo-Cavazos et al. (2009), for pres- around M-dwarfs orbit very close in; magnetohydrodynamic sures ∼60 GPa [100]. Whilst these rates are relatively low, they simulations have shown that the stellar wind pressures experi- are still finite, suggesting that colonies of micro-organisms can enced by such planets can be up to five orders of magnitude indeed survive hypervelocity impact and consequent ejection higher than those experienced by the Earth [94]. into space. A number of experiments have investigated the effect of cosmic radiation on micro-organisms, a review of which is 1.3.2 Atmospheric entry provided by Horneck (1992) [95]. Space missions orbiting If the target planet possesses an atmosphere, the life-carrying the Earth at altitudes ranging from 250 km to 530 km have meteorite must then make it through the layers of gas, all the been found to receive a GCR dose of 0.04–1.07 mGy day−1. time withstanding aerodynamic heating. Landing occurs on Of course, the dose will also vary depending on the degree a timescale of seconds to minutes. As this is very rapid, a of shielding involved; a study by Pavlov et al. (2002) found fusion crust forms on the surface layers of the meteorite, ef- a dose of 0.2 Gy yr−1 when shielded at a depth of 25 gcm−2 fectively protecting the inner layers from the intense heat [9]. in Martian regolith [96]. With this amount of shielding, it is With this protection, the inner regions of the meteorite are able estimated an exposure time of 300000 yrs would be necessary to maintain a relatively constant temperature throughout the to produce a kill fraction of 10−6 for B. subtilis spores [14]. process, as the ablation of surface material carries away the heat [101]. The eventual impact with the surface of the new 1.3 Micro-organisms in simulation experiments planet will occur at terminal velocity (∼50 ms−1 for Earth), The testing of micro-organisms in space has contributed vastly producing pressures much tamer than those during the ejec- to our understanding of how certain forms of life would fare dur- tion stage [16]. Furthermore, the impact will likely result in ing lithopanspermia, specifically the interplanetary transit stage. the meteorite breaking up into many pieces, distributing any As outlined in Sec. 1.1, the life-form must survive through two remaining viable micro-organisms across the nearby landscape. other key stages: impact-induced ejection from the original Depending on the nature of the planet, this could aid the initial planet and atmospheric entry of the target planet. Thus, a sig- spreading of life. nificant amount of research has been dedicated to simulating The atmospheric entry stage of lithopanspermia has been these events, such that a complete overview of the process may investigated using the STONE facilities aboard the Russian be understood. In this section, we provide a brief summary of Foton satellites, simultaneously used to host the Biopan experi- the findings so far in this area. ments. The STONE 5 and 6 facilities were mounted to the heat shields of the Foton-M2 and Foton-M3 recovery modules, re- 1.3.1 Planetary ejection spectively [102, 103, 104]. Spores of B. subtilis and the fungus In order for life to transfer from one planet to another, it must Ulocladium atrum, alongside cells of lichens and Chroococcid- first survive the extreme conditions induced by an impact event iopsis, were secured within rock samples during atmospheric upon its host planet. To obtain a ‘kick’ in velocity sufficient to entry. The entry speed was measured to be 7.7 kms−1, slightly escape the planet, pressures in excess of 50 GPa will likely be below what would be expected for a typical meteorite. In both required, leading to post-shock temperatures of over 500 ◦C[18, experiments, no viability was detected following retrieval of the 19]. A number of experiments have attempted to emulate these samples. It was found that the fusion crust in one sample had conditions, for the purpose of assessing the survivability of penetrated to ∼ 5 cm deep, raising the internal temperature such such an event with regards to extremophiles. that the Chroococcidiopsis cells were destroyed. This poses One option is to use high explosives to simulate the impact, an issue for photosynthetic organisms, since the efficiency of a technique favoured in a study by Stoffler¨ et al. (2007), where photosynthesis is greatly reduced at this depth; depths lower pressures ranging 5–50 GPa were applied to a thin (∼microns) than 5 mm are required to provide enough stellar energy [16]. layer of viable micro-organisms [97]. These pressures induced As such, the atmospheric entry stage may act as a barrier to the temperature spikes of 500–1000 ◦C within the layer. Spores of transfer of photosynthetic life via lithopanspermia [17]. B. subtilis were found to survive up to an impact pressure of A previous experiment made use of a sounding rocket to 42 GPa, where viability stood at 10−4. Cells of the cyanobac- subject granite samples containing B. subtilis spores to atmo- terium Chroococcidiopsis failed to survive pressures exceeding spheric entry at a velocity of 1.2 kms−1, with temperatures 10 GPa, where survival rates dropped from 10−3 to 10−6. On peaking at values of ∼150 ◦C[101]. Survival rates were found the contrary, cells of the lichen X. elegans exhibited similar sur- to be in the range 1–5 %, suggesting that some spores may be vival rates to those of the bacterial spores. It should be noted, able to pull through the final stage of lithopanspermia. Modelling panspermia in the TRAPPIST-1 system — 10/32 Reference(s) Dose et al. (1995) [ 51 ] Taylor et al. (1974) [ 47 ] Hotchin et al. (1968) [ 45 ] Lorenz et al. (1968) [ 106 ] Bucker et al. (1974) [ 108 ] Horneck et al. (1995) [ 52 ] Horneck et al. (1984) [ 48 ] Horneck et al. (1994) [ 50 ] Hotchin et al. (1967) [ 105 ] Spizizen et al. (1974) [ 109 ] Grigoryev et al. (1972) [ 107 ] inactivation Inactivation Inactivation inactivation, Survival, repair shielding by dust Inactivation, repair Long-term survival DNA strand breaks, Studied phenomena photoproducts, repair inactivation, mutation, UV action spectrum of UV action spectrum of UV action spectrum of 4.8 1.3 410 240– dose 4800 GCR (mGy) 295, > Solar UV 280, > Space vacuum, Space, solar UV Space, solar UV Space, solar UV 170, > Studied parameters 220,230,260,290) nm 110, solar UV (254,280) nm Space vacuum, solar UV Space vacuum, solar UV Space vacuum, solar UV > (163,206,254,306-20) nm 170,220,240,260,280) nm ( > ( Bacteria and bacterial spores tested in space. 6 d 9 d, 3 m time 1.3 h, 204 s 327 d 2107 d Vacuum Vacuum 6 h 24 m 5 h 17 m Exposure UV 19 m– Table 2. UV 10 min , , , , R1, , B. , , Bacteriophage spores spores 168 spores, HA 101, HA 101 spores HA 101, HA 101 spores, Aspergillus oryzae D. radiodurans Aeromonas proteolytica Micro-organisms Bacteriophage T1, Bacteriophage T1 Bacteriophage T1, Saccharomyces ellipsoides Zygosaccharomyces bailii B. subtilis thuringiensis B. subtilis F, TKJ 6312 spores B. subtilis B. subtilis F, TKJ 6312, TKJ 8431 spores, plasmid pBR322, plasmid pUC19, lettuce and pea seeds Saccharomyces cerevisiae T7, Rhodotorula rubra B. subtilis Penicillium Penicillium Chaetomium globosum Hydrogenomonas eutropha Key: Bacteria and bacterial spores – Archaea – Fungi and fungal spores – Lichens – Plant seeds – Bacteriophage/virus – Yeast – Animals 500 km ∼ 149 km Mission Rockets, altitude of 150 km Luster rocket, altitude of < Gemini XII, altitude of 300 km Cosmos 368 Apollo 16 lunar mission (MEED) Spacelab 1, altitude of 240 km (ES029) LDEF, altitude of EURECA (ERA) (Exostack) Year 1965 1966 1966 1970 1972 1983 1984– 1992– 1990 1993 Modelling panspermia in the TRAPPIST-1 system — 11/32 Reference(s) Sancho et al. (2007) [ 55 ] Cockell et al. (2007) [ 17 ] ´ anchez et al. (2014) [ 113 ] Saffary et al. (2002) [ 111 ] Rettberg et al. (2002) [ 53 ] Horneck et al. (1996) [ 49 ] Horneck et al. (2001) [ 54 ] Horneck et al. (2001) [ 54 ] Horneck et al. (2001) [ 54 ] Fajardo et al. (2005) [ 101 ] S Rettberg et al. (2004) [ 112 ] Mancinelli et al. (1998) [ 110 ] Branstatter et al. (2008) [ 104 ] by dust by dust Survival Survival inactivation, Ozone layer meteorite dust permafrost soil by dust or salts UV shielding by mutation, role of Martian regolith or Survival, mutations Studied phenomena photoproducts, repair, Survival, shielding by UV action spectrum of Survival, UV shielding Survival, UV shielding Survival, UV shielding 3.1 0.74 dose 6–74 4–30 5–28 GCR 37–49 (mGy) 314, entry > 400) nm > 317) NM 313, > atmosphere > 320, Space vacuum, > 316, 304, > > 280, Studied parameters solar EUV (30.4 nm) > 315, 190, > High-speed atmospheric Space vacuum, solar UV Space vacuum, solar UV Space vacuum, solar UV Space vacuum, solar UV Space vacuum, solar UV Space vacuum, solar UV > 170, (190,210,220,230,260,280, Meteorite entry into Earth’s > ( Bacteria and bacterial spores tested in space. 10 d 98 d 10 d, time 395 s 350 s 14.8 d 12.7 d 14.6 d 120 m UV 5 – Vacuum Exposure Table 3. , D. ¨ ageli) Bacillus sp. spores, spores B. R1, sp. cells sp. (N R1, plasmid sp. cells, sp. cells, Aspergillus niger 168 spores, HA 101, HA F, HA 101 spores, HA 101 spores HA 101, TKJ spores, spores, spores, , Aspergillus Micro-organisms B. subtilis radiodurans pBR322, ochraceus B. subtilis B. subtilis Bacteriophage T1, cells B. subtilis B. subtilis 6312 spores D. radiodurans B. subtilis amyloliquefaciens B. subtilis B. subtilis sp. PS3D Synechococcus Synechococcus TKJ 6312 spores, Rhizocarpon geographicum Xanthoria elegans Chroococcidiopsis Haloarcula Haloarcula Ulocladium atrum Key: Bacteria and bacterial spores – Archaea – Fungi and fungal spores – Lichens – Plant seeds – Bacteriophage/virus – Yeast – Animals 304 km Mission Spacelab D2 Foton 9 Foton 11 Foton 12 MIR-Perseus Terrier Black rocket, altitude < Terrier Mark 70 improved rocket Foton–M2 Foton–M2 (RD–UVRAD) (Biopan 1) (Biopan 2) (Biopan 3) (Exobiologie) (SERTIS) (Biopan 5) (Stone 5) Year 1993 1994 1997 1999 1999 1999 2004 2005 2005 Modelling panspermia in the TRAPPIST-1 system — 12/32 Reference(s) Billi et al. (2011) [ 117 ] Panitz et al. (2015) [ 82 ] Onofri et al. (2012) [ 73 ] ¨ onsson et al. (2008) [ 60 ] J Brandt et al. (2015) [ 120 ] Olsson et al. (2009) [ 114 ] Rabbow et al. (2012) [ 71 ] Raggio et al. (2011) [ 115 ] Dachev et al. (2012) [ 119 ] Westall et al. (2008) [ 116 ] Dachev et al. (2015) [ 122 ] Cockell et al. (2011) [ 118 ] Rabbow et al. (2015) [ 121 ] Novikova et al. (2015) [ 84 ] Neuberger et al. (2015) [ 81 ] Wassmann et al. (2012) [ 72 ] Mancinelli et al. (2015) [ 79 ] de La Torre et al. (2008) [ 59 ] Survival and pigments gene activation gene activation salt crystals, cortex Survival protection, Survival protection, Martian regolith and DNA photoproducts, DNA photoproducts, Studied phenomena Survival, shielding by dose 3–80 GCR (mGy) 400) nm 200 nm) 200) nm 110 nm), > > > > atmosphere 290, 110, Space vacuum, > atmosphere and > ( simulated Martian 200, Studied parameters solar UV ( > UV climate ( Space vacuum, solar UV Space vacuum, solar UV 110, Meteorite entry into Earth’s > ( Bacteria and bacterial spores tested in space. 1 yr 10 d time 1.5 yr ∼ ∼ Exposure Table 4. , , , , B. and , , , B. , B. , Xanthoria ´ ’ageli), D. , , (N Halococcus Xanthoria , Cryomyces Anabaena Richtersius Rhizocarpon , , Cryomyces , Milnesium Gloeocapsa , , spores, 168 spores, , B. licheniformis Aspicilia Chroococcidiopsis spores, , 168 spores, , , Antarctic , Micro-organisms B. subtilis radiodurans geographicum elegans fruticulosa tardigradum coronifer Rhizocarpon geographicum B. subtilis pumilis dombrowskii cylindrica elegans cryptoendolithic communities, antarcticus minteri Bacteriophage T7, subtilis pumilus aurantiogressium longibrachiatum Synechococcus Penicillium expansum Rosenvingiella Chroococcidiopsis Nostoc commune Halorubrum chaoviatoris Geomyces pannorum Trichoderma koningii Trichoderma Aspergillus sydowii Aspergillus versicolor Key: Bacteria and bacterial spores – Archaea – Fungi and fungal spores – Lichens – Plant seeds – Bacteriophage/virus – Yeast – Animals Mission Foton–M3 Foton–M3 ISS ISS (Biopan 6) (Stone 6) (EXPOSE–E) (EXPOSE–R) Year 2007 2007 2008– 2009 2009– 2011 Modelling panspermia in the TRAPPIST-1 system — 13/32

1.4 The TRAPPIST-1 system The recent discovery of multiple planets lying within the habit- able zone of TRAPPIST-1 has caused considerable excitement in the field of astrobiology. With seven planets orbitting within a radius much smaller than the Sun–Mercury distance, the system seems ideal for the process of lithopanspermia, with in- terplanetary transits likely to occur on much smaller timescales than in our Solar System. As such, panspermia in TRAPPIST-1 has already received a significant amount of attention [24, 123].

1.4.1 Discovery Previously thought to host three planets, all placed outside of the habitable zone, recent photometric measurements have revealed that TRAPPIST-1 possesses a further four planets, with at least three lying safely within the habitable zone of the star [124, 125]. Of course, the habitable zone merely takes into account the presence of liquid water, whilst a variety of other factors must be taken into account when considering the true Figure 4. Mass-radius plot for the TRAPPIST-1 system, along- habitability of the planets. These factors will be addressed later side a variety of mass-radius models for reference. Models were on in this section. sourced from Zeng et al. (2016) [131]. The mass for planet h Five of the planets have been deemed Earth-like in terms was taken from Quarles et al. (2017), whilst errors for this value of size, whilst the other two are slightly smaller than Earth. were based on a consideration of the range of masses obtained Follow-up investigations have suggested that six of the planets by different studies that have taken place since the discovery of have pure rocky compositions, similar to that of the Earth [126]. the seven planets [126, 127]. On the other hand, the composition of TRAPPIST-1f is best explained by a water envelope, which comprises ∼20 % of its overall mass. A mass-radius plot is provided in Fig.4, where it planets are at 1/8–1/2 their original distance from TRAPPIST-1, can be seen that TRAPPIST-1f is the best constrained, sitting depending on how rapidly they formed [129]. Another has sug- comfortably atop a model corresponding to 25 % H2O content. gested a formation mechanism consisting of pebble accretion It should be noted that the planetary masses have since been at the snow line, then gravitational feedback upon reaching updated, using data from NASA’s K2 mission [127]. These Earth-like masses, followed by Type I migration to their current new estimates are notably different. The original masses were configuration [130]. determined based on photometry from Spitzer, which was better positioned to observe TRAPPIST-1. However, only 20 days 1.4.2 Habitability of measurements were taken, compared to the 73.6 days with With the discovery of seven earth-size planets in a single system, K2. As such, better constraints on most parameters have been the natural question to ask is how likely are any of them to be obtained. For the purpose of this study, which concerns compact habitable? A number of factors can contribute to the habitability planetary systems in general, and merely considers TRAPPIST- of a system, such as the stellar spectrum, stellar flares, the 1 as a notable example, we will continue to use the values presence of atmospheres and, of course, the compositions of published in the discovery paper throughout. the planets themselves. Investigations into many of these factors A number of properties relevant to this study are provided will rely heavily on a detailed knowledge of the planets and their in Table 5. As can be seen, the inclinations are all close to properties. The majority of research thus far has therefore been 90◦, making the orbits essentially coplanar. This has enabled dedicated towards providing better constraints on the planetary transits for all planetary members to be detected via photomet- parameters. ric techniques. Even more remarkable, the system possesses TRAPPIST-1 is an ultra-cool M-dwarf star with a surface the longest resonant chain found to date. Originally thought to temperature of 2559 K. Although temperate, the habitable zones comprise of the inner six planets, follow-up studies have since of such stars can play host to extremely damaging radiation en- confirmed that the chain extends to planet h, making use of vironments. This is due to the close proximity of the habitable data from the K2 mission [128]. Orbital resonances arise when zone to the star itself, alongside higher frequencies and intensi- a regular, periodic gravitational influence is exerted between ties of stellar flares [132]. Lyman-α observations have revealed orbiting bodies, leading to integer (or close to) orbital period that the chromosphere of TRAPPIST-1 may be moderately ac- ratios. In the case of TRAPPIST-1, the ratios Pi/Pj, where Pi tive, potentially leading to the loss of planetary atmospheres is the orbital period of planet i (e.g. c), and Pj is the orbital within a few Gyr [133]. For comparison, the rotational period period of its inner neighbour (e.g. b), are (moving outwards): of TRAPPIST-1, determined via Fourier analysis of K2 data, [8/5,5/3,3/2,3/2,4/3,3/2]. This large chain of resonances con- has been used to predict an age range of ∼3–8 Gyr [128]. tributed to the large transit time variations observed for the Stellar irradiation of the planets in the TRAPPIST-1 system TRAPPIST-1 system [124]. covers the range 4.3 SEarth–0.13 SEarth, where SEarth refers to Due to the existence of this resonant chain, it is believed solar irradiation at a distance of 1 AU [124]. The habitable zone that the seven planets each formed at a greater distance from planets (e,f,g) receive a stellar flux of ∼0.66, ∼0.38 and ∼0.26 their star, migrating towards their current orbital radii as the times that at the Earth [132]. Cooler, redder stars are more system evolved over time. One study has estimated that the efficient at heating Earth-like planets, and so these fluxes would Modelling panspermia in the TRAPPIST-1 system — 14/32

Table 5. Key properties of the TRAPPIST-1 system, used throughout this study. Values for the periods and semimajor axes were sourced from the discovery paper, Gillon et al. (2017) [124]. This paper also provided the masses and radii, except for those of planet h, which were instead sourced from Krijt et al. (2017) [24]. Finally, the orbital parameters were taken from a stable configuration of the system presented by Quarles et al. (2017) [126].

b c d e f g h

Period (d) 1.51 2.42 4.05 6.10 9.21 12.35 20.00

Semimajor axis (AU) 0.0111 0.0152 0.0214 0.0282 0.0371 0.0451 0.0630

Eccentricity < 0.046 < 0.040 < 0.036 < 0.038 < 0.033 < 0.027 < 0.036

Mass (Mearth) 0.85 1.38 0.41 0.62 0.68 1.34 0.40

Radius (Rearth) 1.09 1.06 0.77 0.92 1.04 1.13 0.76

Inclination (deg) 89.46 89.28 89.78 89.93 89.67 89.67 89.36

Arg. of periapsis (deg) 352.2 104.2 86.0 232.9 15.6 180.0 256.1

Mean anomaly (deg) 206.7 239.1 171.6 185.6 98.0 106.7 16.9 result in surface temperatures comparable to those upon Earth, in Fig.4. As mentioned previously, TRAPPIST-1f appears given the presence of an Earth-like atmosphere. This is due to consistent with a model corresponding to 20–25 % water con- a decrease in the effectiveness of Rayleigh scattering towards tent [131]. It has been suggested that this water content could longer wavelengths, combined with an increase in near-IR ab- manifest itself in the form of an envelope surrounding an inner, sorption by H2O and CO2. A preliminary study of potential rocky planet [126]. The study showed, using thermal evolution planetary atmospheres in the TRAPPIST-1 system has found models, that the temperature at the bottom of such an enve- that the presence of a dense, Earth-like atmosphere, complete lope after 8 Gyr (approx. the age of the system) would exceed with protective ozone layer, would supply the planet with a sim- ∼1400 K. Moreover, the pressure would reach ∼130 kbar, over ilar UV surface environment to that on Earth. On the contrary, one hundred times that of the deepest regions of the oceans an eroded atmosphere, or one depleted in oxygen, would allow upon Earth. Hence, it would be impossible for liquid water much higher quantities of UV to reach the planetary surface, to exist on the surface of the planet, instead residing in the making it a much more hostile environment even to the most form of a high-pressure molecular fluid. Whilst liquid water radition-resistant extremophiles known on Earth [132]. could be present in the form of clouds in the upper atmosphere, these regions are likely to be much more heavily affected by Investigations into the X-ray/EUV irradiation of the planets the radiation activity of the star. For TRAPPIST-1f, therefore, have been undertaken by Wheatley et al. (2017) [134]. They it would seem that the planet is incredibly unlikely to host any deduced that TRAPPIST-1 is a relatively strong coronal X- form of life. ray source, with an X-ray luminosity similar to that of the quiet Sun, despite being intrinsically much fainter. Specifically, More recently, it has become clear that this conclusion −4 the study found a ratio LX/Lbol = 2–4 × 10 , where LX is could likely be made for most of the planets in the TRAPPIST- the X-ray luminosity and Lbol is the bolometric luminosity of 1 system. The update in masses, mentioned in the previous the star. With such a high flux at these wavelengths, it has section, has painted a very different picture compared to that been hypothesised that the planets could be stripped of their of the discovery paper. The inferred compositions of the four atmospheres and (potential) oceans on a timescale of Gyr [134]. outer planets now appear to be consistent with pure H2O con- As such, both the primary and secondary atmospheres of the tent [127]. It would seem that only TRAPPIST-1c could be planets are likely to have been affected, raising severe doubts terrestrial, with surface temperatures too high to permit the about their potential habitability. The planetary atmospheres presence of liquid water. One fairly optimistic possibility could in TRAPPIST-1 are also expected to alter frequently due to be a series of ocean worlds with liquid seas atop layers of persistent flaring events. Analysis of the K2 data found 42 high-pressure ice [136]. events with integrated flare energies in the range 1.26 × 1030– Taking all of these considerations into account, none of 1.24 × 1033 ergs [135]. Between the high X-ray/EUV flux the (known) TRAPPIST-1 planets are likely to be habitable. and the relentless bombardment from flares, the TRAPPIST- A combination of the aggressive radiation behaviour of the 1 planets are extremely unlikely to support atmospheres that star and the unfavourable compositions of the planets would would enable the harbouring of life. appear to make the system a fairly hostile environment. Despite Returning focus to the planets themselves, numerous ef- this, the compact nature of the system still makes it an enticing forts have been made to further constrain their compositions, prospect in the field of astrobiology. Whilst it would seem in order to more accurately infer the degree to which they are that transfer of life is unlikely to occur in the specific case of ‘Earth-like’. Initial analysis of the Spitzer lightcurve provided TRAPPIST-1, the system can still be used as a testing ground estimates of the masses of the inner six planets, as depicted for modelling panspermia in similarly compact systems. Modelling panspermia in the TRAPPIST-1 system — 15/32

Figure 5. Orbits of the seven planets (black dots) of TRAPPIST-1 (black star), shown in the xz-, xy- and zy-planes. The orbits have been plotted using the planetary parameters provided in Table 5. The compact nature of the system is clear here, with the outermost planet orbiting roughly 5 % of the Earth-Sun distance from its star. This implies that interplanetary transfer of material should be more efficient than in our own Solar System. Furthermore, the system is remarkably coplanar, such that all the planets orbit almost within exactly the same plane. One would expect this to further contribute to the speed at which lithopanspermia could hypothetically take place within the system. Dotted lines for each orbit lead to the respective pericentres. Values for the orbital elements were taken from a stable configuration presented by Quarles et al. (2017) [126]. Modelling panspermia in the TRAPPIST-1 system — 16/32

1.4.3 Motivation The TRAPPIST-1 system has caused much excitement in the fields of exoplanets and astrobiology. Its compact and highly- coplanar nature, clearly apparent in Fig.5, make it intuitively ideal for the fast transfer of material between planets. In this sense, the discovery marks an unprecedented opportunity to investigate the potential speed and efficiency of lithopanspermia within such a system. Thus far, the majority of research in this area has focused on panspermia within our Solar System, with an emphasis on interplanetary exchange between Earth and Mars [14, 137, 138, 139, 140]. Comparatively, lithopanspermia involving exoplan- etary systems has received much less attention, most proba- bly due to the inferior level of detail we are able to ascertain Figure 6. The Keplerian orbital elements ( f ,I,ω,Ω) are shown. relative to our own system. This aside, a number of studies The shaded ellipse represents the reference plane, whilst the have investigated the case of interstellar transfer between ex- blue line tracks out the orbit. The reference direction in this oplanetary systems, or between our Solar System and other diagram is along the X direction. systems [141, 142, 143, 144, 145]. It is only very recently, however, that investigations of lithopanspermia in the same exoplanetary system have begun to surface. One such study, 2.1 Equations of orbits by Steffen and Li (2016), was inspired by the discovery of 2.1.1 General orbits two planets in the Kepler-36 system by NASA’s Kepler satel- The current work makes extensive use of the formalism pre- lite [146]. Orbiting very close (a ∼ 0.1 AU) to a G-type sub- sented by Jackson et al. (2014) in their efforts to model gi- giant star, the two planets (a super-Earth and mini-Neptune) ant impacts of planetary embryos [147]. Similar concepts are far from having the potential to be habitable. It was instead were addressed in an earlier consideration of the specific case their close-proximity to each other (∼0.01 AU separation) that of a moon-forming impact, which considered circular orbits showed promise with regards to panspermia; the study therefore alone [148]. A thorough account of the necessary equations is concerns itself with hypothetical systems possessing similarly provided here for completeness. proximate planets within the habitable zone. We consider a planetary body that undergoes a collision In comparison, the TRAPPIST-1 system has sparked much with an impacting meteorite. In order to describe the dynamics more interest from an astrobiological point of view, with sim- of the debris that escapes, one must first consider the orbit of ilarly closely-packed planets and three placed within the hab- the planet itself. The ejecta will have a velocity relative to the itable zone of the star. Lingam and Loeb (2017) provide a planet, and we can use this ‘kick’ in velocity to relate the orbits relatively simple quantitative model for estimating the prob- of the escaping debris to that of the impacted body. ability of lithopanspermia, finding that the process is orders The elements (I,Ω,ω) describe the orientation of a general of magnitude more likely to occur in TRAPPIST-1 than in three dimensional orbit. We may, without loss of generality, the Earth-Mars case [123]. Krijt et al. (2017) instead use N- define the xy-plane to be that of the pre-kick orbit. Furthermore, body simulations to investigate the timescale of lithopanspermia we may set the pre-kick argument of pericentre to be zero, such within the system, concluding that it could occur 4–5 orders of that we have I = Ω = ω = 0. With this in place, the z-axis is magnitude faster than in our Solar System [24]. In the present defined as the direction of the initial orbital angular momentum work, we aim to extend the results of this latest study, by con- with the pre-kick particle orbiting counter-clockwise. sidering the radiant UV exposure of life-bearing rocks as they The semimajor axis and velocity v of the particle are related undergo lithopanspermia within TRAPPIST-1 and correlating via the expression: this with the astrobiological findings explored thus far. 1 2 v2 = − . (1) a r G(M∗ + m) 2. Theory Here, M∗ and m denote the masses of the star and particle, When viewed from an inertial frame, two orbiting bodies trace respectively, whilst r gives the radial position of the particle. out distinct trajectories, each with a focus at the common centre This relation will apply before and after the kick. Thus, we of mass. If instead we observe from a non-inertial frame, cen- may simply rewrite the expression with primes, representing tred on one of the bodies, solely the trajectory of the opposite the post-kick elements. body will be apparent. Keplerian orbital elements describe The post-kick velocity v0 can be expressed as the sum of the these non-inertial trajectories. pre-kick velocity v and the kick velocity ∆v. As such, we have 02 2 2 Throughout the following, we will adopt the standard no- a relation of the form v = v + ∆v + 2v.∆v. Combining this tation for the orbital elements. The argument of periapse ω, with both versions of Eq.1, we obtain an expression relating longitude of the ascending node Ω and inclination I are used to the pre- and post-kick semimajor axes, given by describe the orbital plane of the body in question, relative to a 1 1 ∆v2 + 2v · ∆v defined reference plane and direction. The semimajor axis a, = − . (2) a0 a G(M + m) eccentricity e and true anomaly f instead describe the position ∗ and motion of the body within its orbital plane. Four of these For simplicity, the same Cartesian system as that defined above elements are depicted in Fig.6. will be used to describe the kick velocity, ∆v. An overview Modelling panspermia in the TRAPPIST-1 system — 17/32

To proceed, we may write the post-kick angular momentum in the form h0 = h + R × ∆v. Using the expression for ∆v provided in Eq.3, we have that  S C  1 − e2 f θ R × ∆v = a ∆v −Cf Cθ . (7) 1 + eCf Sθ S(φ− f ) From this, we can derive an equation relating the pre- and post-kick angular momenta, given by

h02 (1 − e2)1/2 ∆v 2 = 1 + 2 Sθ S(φ− f ) h 1 + eCf vk Figure 7. Illustrative example of the difference between a pre- 2  2 1 − e ∆v  2 2 2  and post-kick orbit, the transition between which occurring + 2 Cθ + Sθ S(φ− f ) . (8) + eC
vk due to an impact event [147]. The solid black orbit is that 1 f of the impacted body, whilst the dotted orbit is that of the As given in Murray and Dermott (1999), and applied by Jackson ejected material that results, following the application of the et al. (2014), we may use the expression [147, 149] kick velocity ∆v. Also apparent is the coordinate system in use, −1/2 with the angles (θ,φ) shown relative to the normal Cartesian h0 h02  cosI0 = z , (9) coordinates. h h2 to determine the post-kick inclination. Thus, combining Eq.8 of the coordinates in use is given in Fig.7, with an angle with h and denoting H = h02/h2, we find θ between the kick direction and the z-axis, and an angle φ √ " 2   # between the projection of ∆v onto the xy-plane and the x-axis. 0 1 1 − e ∆v cosI = √ 1 + Sθ S(φ− f ) . (10) It is important to note that, with this choice of coordinates, we H 1 + eCf vk are defining the orientation of the kick in terms of the initial In order to determine the post-kick longitude of the ascend- orbit of the particle alone. As such, if one were to apply this ing node, we must consider the geometry of the problem at formalism to a problem with a preferred direction, one could hand. As is evident in Fig.7, the new orbit must pass through find the corresponding coordinates (θ,φ) via rotations. the application point of the kick. Since this point also lies on With this coordinate system in place, we may write: the original orbit, the new orbit must lie within the xy-plane.

From this consideration, we can infer the following: v = ξ −S f , e +Cf ,0 ,
( ∆v = ∆v SθCφ ,Sθ Sφ ,Cθ , f , θ ≤ π ; Ω0 = 2 (11) · ∆ = vS e +C
S − S C , π v ∆v ξ∆ θ f φ f φ (3) f + π, θ > 2 . π 0 2
1/2 It is important to note that, when θ = , Ω is undefined, since where ξ = vk/ 1 − e , with vk the Keplerian velocity at 2 the particle orbit remains confined to the xy-plane. As such, it orbital distance a. Note that the conventional notation Sx = sinx may be set arbitrarily to any physically acceptable value, hence and Cx = cosx will be used throughout this work for brevity. Substituting the third expression into Eq.2, undertaking some why the condition can be extended as in Eq. 11. Also from the trigonometric manipulation, and making use of the formula geometry of the problem, we find, modulo 2π v2 = G(M + m)/a, we find a relation which connects the pre- ( k ∗ 0, θ ≤ π ; kick semimajor axis to that of the ejecta, given by 0 0 2 ω + f = π (12) π, θ > 2 . a ∆v 2 ∆v √   0 = 1 − − Sθ S(φ− f ) + eSφ . (4) In order to determine the post-kick true anomaly, we once a vk − e2 vk 1 again draw upon the wisdom of Murray and Dermott (1999) [149], We may also find a relation between the pre- and post-kick making use of the following: eccentricities, via consideration of the eccentricity equation, a0 1 − e02
R.v0 sin f 0 = , h2 h0e0 r e2 = 1 − , (5) ! G(M + m)a 1 a0 1 − e02
∗ cos f 0 = − 1 . (13) e0 r again equally valid before and after the impact event. Here, we have introduced the magnitude of the angular momentum vector With use of Eq.6, alongside some additional manipulation, 2 2
1/2 a(1−e )
these expressions may be rewritten to give h = R×v = avk 1 − e zˆ, with R = 1+eC Cf ,S f ,0 the f √ position of the particle at the impact event. Combining the pre-     0 H 2
1/2 ∆v and post-kick versions of Eq.5, we find sin f = 0 eS f + 1 − e SθC(φ− f ) , e vk 0 h 2 a  0 1
e02 = 11 − e2
. (6) cos f = H(1 + eCf ) − 1 . (14) h2 a0 e0 Modelling panspermia in the TRAPPIST-1 system — 18/32

Throughout the derivation presented (as provided by Jack- Table 6. Useful properties of the TRAPPIST-1 planets. The son et al. [147]), we have assumed that the kick in velocity may Hill radius is given by Eq. 23, whilst the escape velocity can p be treated as impulsive. This assumption merely asserts that the be calculated using vesc = 2Gm/R, where m and R are the timescale of the kick must be smaller than the particle’s orbital mass and radius of the planet, respectively. Finally, as the mass period. Even when considering the close-packed nature of the of the star far exceeds that of the planet, we may calculate the p TRAPPIST-1 system, this assumption is valid; the planetary Keplerian velocity with the expression vk = GM∗/a. orbital periods are of the order of days, whilst typical impact Planet r (R ) v (kms−1) v (kms−1) timescales would instead be of the order of minutes–hours. This Hill ⊕ esc k assumption ensures that the change in velocity due to the orbital b 5.68 9.90 79.9 motion of the particle is negligible compared to the size of the kick. Mathematically, this condition can be expressed as [147] c 9.23 12.79 68.2 2 ∆v |v(t + ∆t) − v(t)| (1 + eCf ) ∆t d 8.67 8.15 57.5  = 2π 2 . (15) vk vk (1 − e2) P e 13.11 9.19 50.2 Here, the left-hand side corresponds to the fractional change in velocity due to the kick, whilst the right-hand side instead refers f 17.79 9.02 43.7 to the fractional change in velocity due solely to the particle’s g 27.11 12.20 39.6 motion in its initial orbit.

2.1.2 Circular orbits h 25.31 9.35 33.9 For the purpose of this study, which focuses on the TRAPPIST- 1 system, it is acceptable to assume that the planetary orbits are circular. This is because the orbital eccentricities are yet to be maximum√ value of ∆v/vk for which an object can remain bound precisely measured, whilst upper limits appear to place them is 2 + 1. These values√ arise due to the fact that the circular below the small value of 0.085 [124]. What’s more, tidal forces velocity vk is a factor of 2 less than the stellar escape velocity from host stars are thought to heavily damp eccentricities in at any given distance [147]. This study will concern itself such compact planetary systems [150]. with velocities ∆v/vk < 1, in order to minimise ejection from In making the assumption of circular orbits, we greatly the system. Krijt et al. (2017) found that lithopanspermia in simplify the equations presented in Sec. 2.1.1, since e = 0. TRAPPIST-1 is most efficient for velocities around or just above Furthermore, for an initially circular orbit, the true anomaly the planetary escape velocity [24]. For ease of comparison, we becomes arbitrary, and so we can choose to set it to zero. Geo- use identical ejection velocities in our simulations to those metrically, this means that we define the impacted planet to lie considered in their study. on the x-axis upon impact. With no dependence on e or f , the main equations reduce to the following: 2.2 Impacting the target We have established a formalism which enables us to describe a ∆v ∆v2 the initial ejecta orbits that result from a meteoric impact event; 0 = 1 − 2 Sθ Sφ − , (16) a vk vk the first stage of lithopanspermia. In order to simulate the h02 a  overall process, we must now consider the final stage, namely e0 = 1 − , (17) a collision with a planetary body. This can be with the original h2 a0 planet, a different planet, or indeed the star.     02 −1/2 0 ∆v h The Hill sphere of a planetary body refers to the region cosI = 1 + Sθ Sφ 2 , (18) vk h in which it dominates the gravitational attraction of satellites. For the purposes of this study, we will assume that any ejecta h02 ∆v ∆v2 = 1 + 2 S S + C2 + S2 S2
, (19) passing through the Hill sphere of a planet will eventually go h2 v θ φ v θ θ φ k k on to collide with that planet. For circular orbits, like those in  02 1/2   0 1 h ∆v TRAPPIST-1, the radius of the Hill sphere is given by sin f = 0 2 SθCφ , (20) e h vk  m00 1/3 1 h02  r00 = a00 , (23) cos f 0 = − 1 . (21) Hill 3M e0 h2 ∗ As a final act of simplification, we may combine the latter two where double-primes have been used to denote properties of the of these equations to obtain (credit: D. Veras) target planet. Table 6 lists the Hill radii for the seven planets of TRAPPIST-1, alongside other useful properties to be used later.   1 + ∆v sinφ cotφ Note that we are unable to define a collision with the star in the 0 vk same way. As such, we adopt the same approach used by Krijt tan f =   . (22) 2 + ∆v sinφ et al. (2017), setting the collisional radius of the star to equal vk two stellar radii [24]. With the underlying formalism in place, For the case of circular orbits, the minimum kick velocity the next section will provide an overview of the N-body code for which√ an eccentricity of 1 can be achieved is given by produced. Following this, we will discuss preliminary outputs ∆v/vk = 2 − 1. This represents the lowest kick velocity from of the simulations, before turning our attention to potential which particles can be ejected from the system by the kick. The extensions that can be made in future. Modelling panspermia in the TRAPPIST-1 system — 19/32

3. Simulation set-up Table 7. Example of the format used to store parameters in a csv file, entitled ‘system.csv’, where system refers to the 3.1 Simulation script desired planetary system. The file stores the mass m, semimajor A Python script, Panspermia.py, was created to simulate axis a, eccentricity e, inclination I, mean anomaly M, longitude the process of lithopanspermia within a compact exoplanetary of ascending node Ω, argument of periapsis ω and radius r of system, from an initial impact ejection event to eventual re- each body. The # symbol represents a value for the user to accretion. The user is asked to specify the following variables: insert, whilst blank cells do not require input. As the program is set up to deal with circular and coplanar orbits, e and I are • system – the name of the planetary system in question set to zero; Sec.5 will explore possible ways of extending this. (e.g. ‘TRAPPIST-1’); Objects are listed using their conventional identifiers: ‘A’ for the star, ‘b’–‘d’ for the planets in a 3-planet system. • planet – an identification of the impacted planet, from which ejecta will be released (e.g. ‘e’); Object m a e I M Ω ω r

• ratio – the desired value of ∆v/vk, the ratio of the A # # velocity kick to the the Keplerian velocity; b # # 0 0 # # # # • N testparticle – the number of test particles to be released from the specified planet. c # # 0 0 # # # # d # # 0 0 # # # # The first of these is subsequently used to identify a file of the form ‘system.csv’, containing the necessary parameters of the chosen system. An example of the format used is provided in Table 7 for reference. The program reads the relevant data from that the orbits remain well-defined in the circular, coplanar the file, before storing the parameters in a dictionary named limits appropriate for considerations of TRAPPIST-1. params for ease of access. In order to simulate the trajectories of ejecta from the im- In its current state, the program requires the eccentricities pacted planet, we make use of REBOUND, an N-body integrator and inclinations of the planetary bodies to be zero, such that package for Python. Specifically, we implement WHFast, an it can only simulate circular, coplanar orbits. As discussed unbiased Wisdom-Holman integrator [151]. The REBOUND in Sec. 1.4, these assumptions are justified in the case of the simulation deals with angles in the range [−π,π] (except for TRAPPIST-1 system. However, a relatively simple extension the inclination, which by convention lies in the domain [0,π]), will ensure that general orbits with e 6= 0 and/or I 6= 0 can also so a final check of the input parameters is made to ensure that be tested (see Sec.5). What’s more, the equations outlined in the relevant angles all lie in the required range. The final dictio- Sec. 2.1.2 require that I = Ω = ω = 0 for the impacted planet. nary of parameters is used to add the star and seven planets to In order to enforce this, the program checks for a non-zero the REBOUND simulation. value of each parameter and adjusts accordingly. For the preliminary runs of the code detailed in this work, As we are treating the orbits as coplanar, relative arguments a timestep of 100 s was deemed appropriate. This places the of pericentre are defined by their differences, so the planetary timestep just below 0.1 % of the orbital period of TRAPPIST- arguments of pericentre are updated by subtracting that of the 1b, the innermost planet. An in-depth consideration by Rein and impacted planet from each. A similar adjustment was chosen Tamayo (2015) revealed that timesteps too far below ∼ 0.1 % of for the mean anomalies; our comparison study, by Krijt and the shortest orbital period lead to larger errors due to an increase coworkers, found that the initial true anomalies of the planets in the number of floating point operations the integrator has had little impact on the results of their simulations, allowing to perform [151]. Future development of the code will look them all to be set to zero [24]. We instead choose to main- to incorporate the orbital period of the impacted planet when tain the relative differences of the values quoted by Quarles setting the timestep. et al. (2017), which were found to constitute a stable configu- The simulations to be presented in Sec.4 were run for 1000 ration [126]. Finally, we set Ω = 0 for all the planets as their orbits of the impacted planet. Checks for close encounters mutual inclinations are small [126]. were carried out on 106 occasions, or every thousandth of an Since we are considering circular orbits (e = 0), the true/ orbit. This was based on an appreciation of the Hill radii of mean anomaly becomes redundant as a parameter, as it is unde- the planets, introduced in Sec. 2.2. These were used to define fined for such a case. The argument of pericentre is similarly close encounters and, consequently, mergers with the planets ill-defined for circular orbits, so the program instead passes throughout the simulations. Mergers with the star were instead the mean longitude, λ = Ω + ω + M, the angular distance the defined as the test particle passing within two stellar radii. This object would have from the reference direction if it were travel- was also implemented for similar simulations carried out by ling at uniform speed. This remains well-defined for circular Krijt et al. (2017), where a timestep 1/15 of the TRAPPIST- orbits. However, a second issue arises due to the coplanarity of 1b orbital period was used [24]. Although this timestep was the system in question. For coplanar orbits, the line of nodes adaptive (such that is would get smaller in the vicinity of a becomes ill-defined, as does the argument of pericentre (since close encounter), it supports our justification of checking for it is referenced to measure from the line of nodes). As such, close encounters every one thousandth of an orbit. we also pass the longitude of pericentre, ω¯ = Ω + ω, the angle Following the integration set-up, the code adds the test par- from the x-axis to the pericentre. Passing these parameters to ticles to the REBOUND simulation. For this, the equations for the simulation instead of the individual orbital elements ensures circular orbits provided in Sec. 2.1.2 are implemented. For now, Modelling panspermia in the TRAPPIST-1 system — 20/32 we consider orbits that are coplanar with the orbital plane of the planets only (such that θ is fixed at 90◦), since these are most likely to undergo efficient lithopanspermia. Thus, we take N testparticle evenly-spaced values of φ in the range [0,2π] and feed them into the equations for the post-impact semimajor axis, eccentricity, true anomaly and longitude of pericentre. Note that the ejecta orbits can possess non-zero values of e, so the true anomaly is well-defined for these cases. Hence, the true/ mean longitude is no longer needed. The lon- gitude of pericentre, on the other hand, must still be passed, as we have I = 0, such that the ejecta orbits are coplanar. As detailed in Sec. 2.1, the true anomalies and longitudes of peri- centre for the ejecta orbits are set in such a way as to ensure that each one passes through the point of impact from which the ejecta fragments (test particles) originate. We choose to use Figure 8. The spectrum used to model that of TRAPPIST-1. We massless test particles because the gravitational influence of the extracted the model from the AMES-Dusty model atmosphere planets in such a compact system will far outweigh that of the grid, itself generated using the PHOENIX model atmosphere other ejecta. Furthermore, the use of test particles is much less code [154]. The spectrum corresponds to a brown dwarf with expensive computationally. an effective temperature 2600 K and surface gravity g given With all bodies added to the REBOUND simulation, the code by logg = 5.0. Note that this is the spectrum at the surface of then iterates through an array of timestamps, corresponding to the star; a normalisation factor dependent on distance from the the output times mentioned above (106 linearly-spaced times star must be applied to find the spectrum at a particular location in the range [0,1000 T ], where T is the orbital period planet planet within the system, as will be outlined in the text. of the impacted planet). For each timestamp, the simulation is integrated up to that time and is then checked for close encounters. This is done by iterating through the active bodies (star and seven planets), looping through the test particles for iterates through the merger times (again supplied by the relevant each one. The distance between the given active body-test ‘Merge info...’ file), performing a cumulative count for each particle pair at each step is calculated and compared to the Hill possible fate. It then displays merger trails, showing how the radius of that active body. If the distance is smaller, the test number of ejecta that have reached each fate changes through- particle is removed from the simulation, such that future loops out the simulation. will only consider the remaining particles. The FluxTracker.py script plots the irradiance received Issues involving particle tracking (indices will update each by a specified test particle (identified by index) throughout time a particle is removed) were solved by implementing a map- the simulation, within a waveband to be set by the user (e.g. ping system, whereby the updated indices are mapped to the 100–400 nm for UV). For this, we make use of the AMES- original ones. This enables the code to keep track of the proper- Dusty model atmosphere grid provided by Allard et al. (2001), ties of each particle, such as distance from the star (direct) and generated using the PHOENIX model atmosphere code [152, stellar irradiation (derived). The former of these is stored tem- 153, 154]. Specifically, the spectrum we use to model that of porarily in dictionary format, before being appended to a file TRAPPIST-1 corresponds to a brown dwarf with no irradia- of the form ‘d star planet ratio N testparticle.csv’ tion. The model does, however, account for dust opacity. Exact every 1000 iterations. This was found to give the best bal- details of the spectrum are supplied in the caption of Fig.8. ance when considering speed and memory use. The code, The spectrum is cut down to the waveband specified by the 2 in its current form, takes roughly 8 hours to run on a sin- user and subsequently normalised by a factor (R∗/d) , where gle core. Future development will look to improve the effi- R∗ is the radius of the star and d is the relevant distance from ciency, by reducing the number of outputs stored (this will the star. We then integrate the resulting curve, to convert from reduce accuracy when calculating irradiation but greatly im- spectral irradiance [ergs/cm2/s/nm] to irradiance [ergs/cm2/s]. prove the speed). At each merger event, the test particle index, The script repeats this process for each distance recorded during time and planet are appended to a separate file of the form the simulation for the particular particle in question, ultimately ‘Merger info planet ratio N testparticle.csv’. Ad- plotting the irradiance as a function of time. ditional Python scripts were written to manipulate the data Finally, the Fluence.py script may be used to calculate obtained from the simulations and produce the outputs to be the radiant exposure, or fluence, experienced by each test par- presented in Sec.4. ticle during the simulation. The process is very similar to that outlined above. However, upon finding the irradiance of the test 3.2 Additional scripts particle as a function of time, a second integration is performed The Python script, Fate.py, will extract data from the rele- to convert from irradiance [ergs/cm2/s] to radiant exposure vant ‘Merge info...’ file, perform a simple counting operation [ergs/cm2]. This is then repeated for each of the test particles and subsequently display a bar chart showing the final desti- in the simulation. Currently, this code takes around three days nations of the ejecta particles for the simulation in question. to run; future alterations will look to improve upon this, possi- User input is fairly consistent throughout the scripts used, in bly by taking a random selection of particles, or reducing the order to identify the correct files from which to extract infor- number of distances used to calculate the irradiance. The next mation. Another useful script is MergerTrails.py, which section will summarise preliminary testing of the code. Modelling panspermia in the TRAPPIST-1 system — 21/32

4. Results and Discussion In this section, we present the results of eight preliminary runs of our N-body simulation code. We aim to compare the outputs to those found by Krijt et al. (2017) in their own simulations, be- fore extending these results by considering the UV irradiation of the ejecta as they undergo lithopanspermia. As such, our chosen cases mirror those tested previously, simulating ejection from planet e at velocities ∆v/vk = {0.184,0.276,0.552}, ejection from planet f at velocities ∆v/vk = {0.205,0.307,0.610} and ejection from planet g at velocities ∆v/vk = {0.450,0.900}. In each simulation, a thousand massless test particles are re- leased from the selected planet at time t0 = 0. The system then integrates forward in time, routinely checking for close encoun- ters between the test particles and the planets. When a close encounter occurs, such that the particle-planet distance drops below the Hill radius of the planet (or the particle-star distance drops below two stellar radii), the particle is removed from the simulation and the ‘merger’ is recorded. This continues until all particles have decided their fate, or until t = tmax. For these preliminary tests of the code, we set tmax = 1000 Tplanet, where Tplanet refers to the orbital period of the impacted planet. This provided sufficient time for the vast majority (> 98 %) of ejecta particles to either merge or escape the system. In Sec. 4.1, we examine the final destinations of the ejecta particles and inves- tigate the timescales over which lithopanspermia can occur in the TRAPPIST-1 system. We then proceed to track the UV irradiation experienced by the ejecta throughout the simula- tions, before assessing the survivability from an astrobiological perspective, in Sec. 4.2. Finally, we discuss the implications of our results, alongside potential future avenues of exploration, in Sec. 4.3 and Sec.5, respectively.

4.1 Timescale and fate Let us first consider low-velocity ejection from planet e, the innermost planet thought to lie within the habitable zone of TRAPPIST-1. As is apparent in Fig.9, we find that re-accretion onto the same planet is the most common eventuality, with 37 % of the ejecta particles making their way back to planet e. Whilst the majority of particles undergo inward panspermia (mainly to planets c and d), a healthy 24 % reach one of the other habitable zone planets (f and g). These findings are consistent with those of Krijt et al. (2017), although they observed that ∼ 10 % more particles returned to planet e. Similar results were found for low-velocity ejection from f and g. Generally speaking, the majority of test particles (25– 37 %) re-merge with their original planet after low-velocity ejection. With low ∆v, transfer of material between habitable zone planets is fairly efficient, particularly so in the case of ejection from planet f. Here, we observe ∼ 46 % of parti- Figure 9. The final destinations of ejecta from (a) planet e, cles merging with planets e or g. Whilst this overall figure is in (b) planet f and (c) planet g of TRAPPIST-1, following 1000 agreement with the previous study, we see an even split between orbits of the impacted planet. By this time, the vast majority of the two planets, whilst Krijt and coworkers recorded a prefer- particles had either collided or escaped. Target ‘A’ refers to a ence towards outward transport, with g having twice as many collision with the star, whilst ‘b’–‘h’ refer to a collision with mergers as e. It is reassuring to note that escaping√ particles are a planet. Escape from the system was defined as reaching one only recorded for kick velocities obeying ∆v/vk > 2−1, as is Earth-Sun distance from the star and denoted ‘X’. Finally, a to be expected from the theory outlined in Sec. 2.1.2. Strangely, target of ‘-’ denotes particles that are yet to collide or escape. however, we record a much higher proportion of escapes for the The velocity ratio refers to ∆v/vk, for a kick velocity ∆v and low-velocity g simulation. This could be due to the positioning Keplerian velocity vk. All ejecta orbits were coplanar with those of the ‘impact event’. In its current form, the code places the of the planets. The ejecta particles were emitted isotropically test particles at a point located a few planetary radii away from within that plane. Modelling panspermia in the TRAPPIST-1 system — 22/32 the selected planet. Whilst this works sufficiently for prelimi- in the comparison study, it is likely that far fewer orbits were nary testing, future development of the code will look to place coplanar with those of the planets. Hence, we can infer that the test particles along a circle surrounding the planet; this will the mergers we observe are probably consistent with the results ensure that a premature collision with the chosen planet does presented in Krijt et al. (2017), however, they will correspond not occur. to only the first few particles that undergo collisions in their We now consider ‘medium’-velocity ejection from the plan- simulations. As such, it is difficult (indeed, uninformative) to ets e and f (only two simulations were run due to time con- compare the two sets of simulations in this way. straints). This refers to ejection at one and a half times the For low-velocity ejection, one would intuitively expect escape velocity of the impacted planet. In both cases, we again mergers to occur initially with the neighbouring planets, before see that the majority of particles return to their original host. We migrating further afield to the innermost or outermost planets also observe more of a balance between inwards and outwards in the system. We observe this pattern for low-velocity ejection transportation of material; a trend which, for the most part, is from planet f; after f itself, the neighbouring planets d, e, g in agreement with previous findings [24]. As for low-velocity and h are the next to receive material, followed by planet c. ejection, exchange of material within the habitable zone is fairly In the case of low-velocity ejection from planets e and g, on common with 26–37 % of test particles making their way to the the other hand, ejecta reach planet c very early on, within less other two planets. In their equivalent simulation of medium- than half an orbit. This could owe itself to the starting con- velocity ejection from planet f, Krijt and coworkers instead find figuration of the planets; from Fig.5, it is clear that planet c that the majority of ejecta merge with planet g. This disagree- is relatively close to both e and g, whilst f is slightly further ment may also be due to the starting positions of the particles; away at time t0. It would be interesting to investigate different premature mergers with planet f could have shifted the majority starting configurations for the planets and confirm this. from g to f. In every case tested, 98–100 % of ejecta particles decide Finally, we investigate ejection at much higher velocities: their fate by the end of the simulation. For a clearer perspective three times the escape velocity of the impacted planet. In of time, the simulations lasted {16.7, 25.2, 33.8} yrs for ejection accord with the comparison study, we observe a much broader from planets e, f and g. respectively. From the trails, it would distribution of mergers, in some cases extending to all possible seem that a higher ejection velocity leads to faster accretion destinations. This is due to the fact that higher ejection speeds of material when considering coplanar ejecta orbits. This may will lead to ejecta orbits with higher eccentricities, enabling change when considering non-coplanar orbits, something to the particles to reach further inwards or outwards. We find a be investigated after further development of the code. It is marked decrease in the number of particles returning to their important to remember, however, that a large proportion of the original planet, alongside fewer exchanges between habitable particles in such high-velocity cases end up escaping, and so zone planets. As expected, we see a sharp rise in the proportion are rendered useless when considering lithopanspermia within of ejecta particles escaping the system, with almost half the the system. population lost following high-speed ejection from planet g. To summarise, we observe similar fate distributions to those Overall, we find similar merger distributions to those pre- of Krijt et al. (2017), though are unable to make a decent com- sented in Krijt et al. (2017), with only minor discrepancies [24]. parison with regards to the timescale on which mergers take In combination with the aforementioned issue regarding the place. This is due to the fact that our study only concerns itself starting positions of the particles, another contributor to these with ejecta orbits coplanar with those of the planets, whereas differences could be linked to the coplanarity of our ejecta the previous work considers ejection that is isotropic in x, y orbits. The comparison study released ejecta from a sphere, and z. As such, our simulations place a focus on the most simulating isotropic ejection that wasn’t solely confined to the efficient interplanetary exchange of material, occurring in the plane of the planets. We aim to extend this aspect of the code earliest stages of those run by Krijt and coworkers. For these in future; this will be further discussed in Sec.5. coplanar orbits, we find that low-velocity ejection (at or just Throughout each simulation, we also keep track of the times above the escape velocity of the impacted planet) leads to the taken for the particles to merge or escape (if at all). This al- most efficient transfer of material to planets in the habitable lowed for cumulative merger trails to be plotted, as displayed in zone, whilst high-velocity ejection results in a broader distribu- Fig. 10. In the following discussion, it is important to remember tion of destinations, spanning the entire system in some cases. that the current starting position of the test particles leads to Whilst further development of the code is required to reinforce many colliding with the impacted planet straight away. This our findings, the preliminary runs imply that lithopanspermia will be rectified in future, but for now it means a sharp rise in could occur on a timescale as small as days in the TRAPPIST-1 the merger count occurs very early on, after only a fraction of system, were the planets to be habitable (see Sec. 1.4.2). an orbit of the impacted planet. This aside, we also see rapid transfer of material between the impacted planet and neigh- 4.2 Radiant exposure and survivability bouring planets, for the most part occurring within the first We now proceed to extend previous investigations of pansper- orbit. Interestingly, we see much faster exchange of material mia in TRAPPIST-1, by tracking the flux throughout our simu- with other planets than was observed by Krijt and coworkers. lations and assessing the survivability from an astrobiological They found that ∼ 10 % of ejecta reached other planets within perspective. As outlined in Sec.3, the distance from each test 100 yrs [24]. We instead see this happening on a timescale of a particle to the star is recorded throughout the simulation. Flux few orbits of the impacted planet. The most likely explanation is governed simply by an inverse-square law, thus we are able for this is that we have confined our study to ejection in the to examine the amount of energy imparted to each particle by plane of the planets, whilst ejecta were released isotropically the star. To do so, we make use of a stellar spectrum model, in the other set of simulations. Despite the use of 104 particles generated using the PHOENIX model atmosphere code [152]. Modelling panspermia in the TRAPPIST-1 system — 23/32

Figure 10. Cumulative merger trails for the eight preliminary runs of the N-body simulation code. The trails show how the number of particles that have undergone each fate (merger with a planet, merger with the star or escape from the system) changes throughout the simulation. A key is provided in the top right corner. Labels for each plot give the impacted planet and ∆v/vk, the ratio of the kick velocity to the Keplerian velocity. Times are given in units of the impacted planet. For convenience, the orbital periods are {6.10, 9.21, 12.35} days for planets e, f and g, respectively. Modelling panspermia in the TRAPPIST-1 system — 24/32

the lack of this destructive component. However, given that the lowest fluences experienced in our simulation are three orders of magnitude higher, this revelation is unlikely to make too much difference with regards to the survivability of the journeys. It is apparent, therefore, that some form of shielding would need to be in place for lithopanspermia to have some chance of being successful within the system. Fortunately, numerous missions have tested a wide range of shielding materials and their properties; we discuss them here. The HA 101 strain of B. subtilis was exposed to vari- ous wavebands of UV during the ER 161 experiment of the ERA aboard EURECA [52]. When protected by 5 % glucose, the spores exhibited survival fractions as high as 10−3 after a dose of 3 × 104 Jm−2. Higher doses, on the other hand, were observed to be much more damaging, with a fluence of 3 × 106 Jm−2 reducing this to just 10−6. These results would imply that, with some form of shielding in place, a small yet sig- Figure 11. The irradiance received by a test particle (φ = π) nificant proportion of bacterial spores should be able to survive following low-velocity ejection from planet e. A waveband of radiant exposures of ∼ 106 Jm−2. This would cover ∼ 65 % of 100–400 nm was used for integration, corresponding to stellar the ejecta journeys in our simulation. UV irradiation. As is to be expected from the elliptical nature In the Biopan missions, bacterial spores and other micro- of the orbit, the graph is sinusoidal. This particular segment of organisms were subjected to doses of ∼ 107 Jm−2 [54]. Sam- the overall time series shows a clear variation in the orbit of the ples of unshielded B. subtilis spores showed survival fractions ejecta particle; a significant drop in the average irradiance is of 10−6 following 15 days of exposure. The samples had been observed ∼ 33 orbits into the simulation, suggesting a change subjected to a UV fluence of 1.7 × 107Jm−2, the space vacuum in the semimajor axis. and cosmic radiation at a dose rate of 5 Gy day−1. The UV fluence quoted for the Biopan experiments exceeds almost all of those determined for the particles in our N-body simulation. Specifically, we use the spectrum corresponding to a brown Only six of the thousand ejecta experienced a higher dose of dwarf with effective temperature 2600 K and surface gravity UV, two of which were yet to undergo a merger by the end of logg = 5.0, shown in Fig.8. These properties closely match the allotted time. As such, a finite survival fraction calls for a those of TRAPPIST-1 [125]. We then use the recorded dis- certain degree of optimism regarding the survivability of the tances to normalise the spectrum. Integration of the spectrum, journeys. It is thought that the inactivation of surface spores constrained by a given waveband (e.g. UV), yields the irradi- may have formed a protective layer, attenuating the UV before ance at a particular distance. An example of the irradiance of a it reached the remaining layers beneath. These results imply test particle as a function of time is provided in Fig. 11. that multi-layer endolithic bacterial spores residing close to the We are subsequently able to integrate the resulting time se- surface of an ejecta fragment may stand a decent chance of ries, in order to estimate the overall radiant exposure (fluence) survival through interplanetary transit within the TRAPPIST-1 the particle is subjected to throughout the simulation. As an ex- system. ample, we consider the first of our simulations, corresponding Survival rates showed a vast improvement when the spores to low-velocity ejection from planet e. The radiant fluence is were mixed with meteorite powder for protection. Some sam- determined for each test particle as a function of merger time ples, exposed during the Biopan 2 mission, exhibited survival and angle φ; the resulting plot is shown in Fig. 12. For refer- fractions as high as 10−1. As part of the Biopan 5 mission, ence, we place markers for a number of UV dose measurements the lichen X. elegans exhibited an impressive resilience against taken during a variety of astrobiological missions. Whilst a the solar UV, with ∼ 70 % of the thalli still viable after expo- detailed summary of these experiments is provided in Sec. 1.2, sure [155]. A much smaller, yet finite, proportion of a sample of we re-discuss them here in the context of our own findings. tardigrades survived exposure to the full space environment as Aboard Spacelab 1, it was found that the surviving fraction part of the Biopan 6 mission [60]. Whilst extremely damaging, of B. subtilis spores dropped by 95 % after just 10 s of exposure these experimental findings appear to suggest that the stellar to the full extent of the solar UV, with no shielding. The radiant UV can be survived (in small, though significant quantities) by exposure from this experiment was ∼ 150 Jm−2. However, a wide range of micro-organisms. Given the sizeable range of Horneck (1992) calculates F10 values (fluence needed to reduce fluences experienced by our test particles, it would be justifi- survival to 10 %) as low as 5 Jm−2 [95]. This is far below the able to assume that at least some journeys must be survivable. fluences recorded for our ejecta particles. It is thought that the Specifically, if we consider the results of the Biopan missions, inactivation owed itself mainly to the UV-C part of the spectrum it would seem that any journey below ∼ 106 − 107 Jm−2, given (< 295 nm), known to cause most damage to DNA [9, 95]. the right level of shielding, should be survivable, at least for An interesting point to note is that the TRAPPIST-1 spectrum small fractions of the initial microbial population. shows fairly negligible flux in this range, as is made clear in Finally, the LDEF and EXPOSE missions have provided Fig.8. One could therefore argue that the bacterial spores the longest exposures to the space environment thus far. All would be more capable of surviving radiant exposure to the UV of the missions have measured total UV fluences in excess of band within TRAPPIST-1 than in our own Solar System, due to ∼ 109 Jm−2, eclipsing the radiant exposures determined for Modelling panspermia in the TRAPPIST-1 system — 25/32

Figure 12. The radiant exposure (UV, 100–400 nm) experienced by each ejecta particle as a function of merger time (x-axis) and ejection angle φ (colour bar), for the case of low-velocity ejection (∆v/vk = 0.184) from planet e of TRAPPIST-1. It is clear that the vast majority (∼ 95 %) of particles collide within the first 2 years of the simulation. Markers have been placed to signify UV doses that have been measured in a variety of astrobiological experiments. Modelling panspermia in the TRAPPIST-1 system — 26/32 our simulation. Despite this, some positive results have still dessication, leading to lower survival rates. On the other hand, been obtained. In the LDEF experiment, B. subtilis spores over 80 % of the ejecta particles in our simulation merge within were exposed to selected conditions of the space environment the first year; for these journeys, the space vacuum will have for a total of 6 yrs [50]. Unshielded samples, exposed to the much less of an impact on the micro-organisms, especially with full extent of the solar UV, showed survival fractions of 10−6, shielding or a source of rehydration in place. Overall then, still very significant considering the very high fluence involved. whilst we cannot draw upon experimental evidence for vacuum Once again, the samples were exposed in multi-layers, so it is pressures typical of interplanetary space, we can attempt to thought that the top layers formed a protective crust throughout extrapolate results obtained in low Earth orbit, inferring that the duration of the experiment, attenuating the radiation. We it wouldn’t pose too much of a threat to the vast majority of can deduce from the LDEF results that bacterial spores, exposed journeys undertaken by ejecta particles in our simulation. in multi-layers near the surface of an ejecta fragment (mixed in We now consider the effect cosmic radiation could have with the surface layers) will likely show a finite survival rate, on the survivability of the ejecta journeys. In our treatment with the innermost members of the population pulling through. of the space vacuum, we assumed that the effect would vary The series of EXPOSE missions recorded radiant exposures linearly in time. This was a reasonable assumption, since the comparable to those of NASA’s LDEF. The amino acid glycine vacuum mainly leads to severe dessication, an effect that will showed particular resilience, achieving ∼ 70 % survival when intuitively get worse as time passes. We may use the same exposed to 1.04 × 109 Jm−2 of UV as part of the AMINO assumption for cosmic radiation; the effect is measured as a experiment [76]. In a separate experiment, a variety of plant dose (typically in units of Gray [Gy]). As such, we can simply seeds underwent a similar degree of exposure; certain samples extrapolate quoted values from tested environments in order were found to have > 40 % survival rates when considering to gain an appreciation of the severity of cosmic radiation in post-exposure germination [74]. the context of our simulation. The dose from cosmic rays can vary quite dramatically depending on the given location and 4.3 Discussion corresponding environment. In low Earth orbit, for example, From our review of the astrobiological literature, it is clear the annual dose can be anything in the range 1–10000 Gy, with that, given the right conditions (e.g. positioning, shielding, the highest doses arising in the Earth’s radiation belts [9]. Once presence of protective substances), a wide variety of micro- again, the LDEF mission offers the best estimate of a long- organisms can survive UV fluences much higher than those term effect, due to it being the longest exposure mission to experienced by the ejecta particles in our simulation of material date. The bacterial spores received a cumulative GCR dose of exchange in TRAPPIST-1. Thus far, however, we have merely 4.8 Gy during the 6 yr mission. Specifically, this amounts to taken into account the stellar UV. As discussed previously, a a daily dose of 2.28 × 10−3 Gy day−1. Applying this to the number of other factors will contribute to the destructive nature ejecta particles, we find doses in the range 0.0021–13.91 Gy. of the space environment. Most notably, these include the space The upper limit here corresponds to the particles yet to un- vacuum, cosmic radiation and temperature extremes. In general, dergo a merger, although these represent only 0.2 % of the samples that have been exposed to solely the space vacuum and population. As such, we can take ∼ 14 Gy as the highest dose cosmic radiation tend to fair much better than those exposed for our simulation. This is still relatively feeble; spores of B. to the full space environment, suggesting that the stellar UV subtilis are known to have D10 values (doses leading to 10 % radiation has the most deleterious effect on micro-organisms. survival) up to 1500 Gy, whilst lichens have been shown to However, these factors still cause damage of their own, so it is exhibit ∼ 50 % survival when exposed to doses in the range important to take them into consideration when conducting an 90–480 Gy [54, 156]. Remarkably, tardigrades have an even investigation into lithopanspermia. more impressive radiation tolerance, surviving up to ∼ 5000 Gy It is particularly difficult to comment in any detailed fashion even when in an anhydrobiotic state [157]. Of course, we have on the effect of the space vacuum, as astrobiological experi- neglected a number of factors in our treatment here. Stellar ments to date have taken place in low Earth orbit, where the flares, particularly common in dwarf stars like TRAPPIST-1, vacuum pressure is around seven orders of magnitude higher can significantly increase the dose [135]. The habitable zone than it is in interplanetary space [52]. In the LDEF mission, B. of TRAPPIST-1 is situated very close to the star; it will con- subtilis spores that were exposed to the space vacuum alone sequently host much higher stellar winds than those present exhibited a survival rate of 1–2 %. The micro-organisms were around the Earth [94]. Planetary magnetic fields in the system situated in a vacuum of 10−6 Pa, which is the closest to inter- could have very different configurations to those in our Solar planetary space of the astrobiological experiments reviewed System. Whilst these factors could certainly have an effect on here. We therefore choose to use this as the benchmark for our the survivability of our ejecta journeys, further modelling will discussion. The LDEF mission lasted 6 yrs, whilst our simula- be required to provide any quantitative detail. These aside, for tion involving ejection from planet e lasted just over 16 yrs. If now, it would seem that cosmic radiation has a fairly negligible we assume that the effect of the space vacuum varies linearly effect in our simulation. as a function of time, then we can estimate that 16 yrs of ex- Another factor to take into account is the temperature varia- posure would lead to a survival fraction of ∼ 10−5. Protective tion within the system. The innermost and outermost planets substances can greatly improve matters. When equivalent B. span an effective temperature range of 400–173 K [125]. Sam- subtilis spores were immersed in 5 % glucose, up to ∼ 70 % ples of the species M. tardigradum have shown > 90 % survival survival was observed. We could then estimate that ∼ 30 % of following exposure to temperatures of 373 K for over an hour, spores would survive the 16 yrs of our simulation, if exposed whilst other species are known to survive at temperatures far to the vacuum alone. Of course, the much lower vacuum pres- below that of the outermost planet in TRAPPIST-1 [61]. Of sure of interplanetary space would likely induce more severe course, many of the ejecta orbits will pass either one of these Modelling panspermia in the TRAPPIST-1 system — 27/32 boundaries, into a hotter or colder environment for certain pe- deeper understanding of the shielding properties of meteorites riods of time. Due to time constraints, investigations into the will be paramount in assessing the survivability of material effective temperature were unable to take place; this will be transport in the system. addressed in future development of the code. Finally, an area that remains relatively unexplored is the From our preliminary results, however, it would seem that impact of asteroid rotation on lithopanspermia. This coincides lithopanspermia can occur very efficiently within the TRAPPIST- well with our aim to investigate the temperature extremes expe- 1 system. Our review of the astrobiological literature would rienced by ejecta throughout their journey. It seems intuitive suggest that a large proportion of the ejecta journeys could be that rotation would have a large say in the kinds of environ- survivable, given the right conditions. Further development of ments micro-organisms would find themselves in. For example, the code will be required to account for other damaging aspects slower rotation of an asteroid would lead to longer periods of of the space environment, including stellar flares, stellar winds exposure to direct starlight. This may or may not be benefi- and the large temperature extremes which the micro-organisms cial to the survival of the micro-organisms, depending on the could be subjected to. This will then allow for a more quantita- asteroid’s distance from the star. Specifically, we could inves- tive estimate of survivability to be made. tigate the YORP (Yarkovsky–O’Keefe–Radviesvki–Paddack) effect [158]. This is the spinning up or down of a body due 5. Further work to anisotropic absorption and radiation of light [159]. The timescale involved to significantly alter the rotational speed As mentioned in Sec. 4.3, we would like to further develop of an asteroid of size ∼ 1 m is of the order ∼ 106 yrs [160]. the code to account for a number of additional contributors to As such, the effect will be negligible for material transfer in the deleterious nature of the interplanetary environment, such compact systems like TRAPPIST-1. However, such timescales as stellar flares, stellar winds, planetary magnetic fields and are indeed applicable to investigations of panspermia in our extreme temperature conditions. Beyond this, however, we Solar System; the spin up of an asteroid could alter its ability discuss here potential future avenues of exploration. to host life over the duration of its journey from one planetary Firstly, it will be useful to extend the code such that it can body to another. The change in the asteroid’s spin s with respect consider more generalised planetary orbits. As for the circular, to time may be expressed as [159, 161] coplanar limits currently in action, this would be solved simply ds Y  f  by extending the equations of the ejecta orbital parameters to = √ , (25) those provided in Sec. 2.1.1. In their current format however, dt 2πρR2 a2 1 − e2 the elements (I,Ω,ω) for the impacted planet must be set to where R is the mean radius of the asteroid, ρ is the density zero in order for the equations to apply. Fortunately, suitable and Y is a constant dependent on the physical properties of the forms for non-zero values are provided by Jackson et al. (2014), asteroid. The force f takes into account the incident stellar obtained via rotations [147]. Here we give a brief outline for light pressure, the asteroid’s albedo and other factors relating to reference. A new Cartesian basis is introduced by moving to absorption and radiation. Whilst the YORP effect would only the I,Ω.ω 6= 0 frame. New spherical polar angles θ1 and φ1 are be applicable to considerations of material transfer in planetary introduced, which are related to the previous definitions θ and systems like our own, the rotation of asteroids is likely to have φ by the following: an impact on lithopanspermia in all kinds of systems, including TRAPPIST-1. This could be an intriguing area to look into. cosθ = C C − S S S , θ1 I θ1 I β To summarise, we present preliminary outputs of an N-body Sθ (S CICω −C Sω ) +Cθ SICω integration code designed to investigate lithopanspermia in the tanφ = 1 β β 1 , (24) compact, coplanar TRAPPIST-1 planetary system. We find that Sθ1 (SβCISω +CβCω ) +Cθ1 SISω exchange of material between the habitable zone planets (e, f, g) where β = φ1 − Ω. These new definitions of θ and φ can is very efficient, with ejecta reaching another planet in a matter then be plugged into the existing equations for the post-impact of days following release. By tracking the radiant exposure of semimajor axis a0, eccentricity e0 and true anomaly f 0. Things each ejecta particle throughout the simulation, we find fluences are more complex for I0,Ω0 and ω0, as these will still be defined in the range 103 −107 Jm−2 in the case of low-velocity ejection relative to the orbital reference frame, instead of our new frame. from planet e of TRAPPIST-1. We subsequently use an in-depth Suitable equations are provided in Appendix A of Jackson et review of astrobiological literature to provide a commentary al. (2014) [147]. on the survivability of the ejecta journeys, taking into account It would also be interesting to take into account attenuation the stellar UV, space vacuum and cosmic radiation. In future of radiation by the ejecta fragment, simulating the effect of development of the code, we aim to extend the current format shielding. The contribution of shielding to the survival of micro- to allow consideration of more generalised planetary systems organisms during panspermia has been investigated in great (with non-circular, non-coplanar orbits). We will also look to detail [14]. Thus far, however, this work has focused solely on track temperature profiles for each ejecta particle, comparing to the Earth-Mars case. Comparatively little attention has been known tolerances of micro-organisms to establish some appre- paid towards shielding from the much higher stellar winds ciation of survivability. Future avenues for exploration include and more frequent flares exhibited within dwarf star systems. studying the impact of ejecta rotation on these temperature pro- Combining our code with stellar magnetohydrodynamic models files. Finally, it would be interesting to investigate the level of of such compact systems would provide more of an insight into shielding required to protect micro-organisms against the harm- the effects of cosmic radiation on the survivability of ejecta ful radiation environment of TRAPPIST-1’s habitable zone, in journeys. Furthermore, the high X-ray fluxes experienced by particular accounting for the high stellar winds and frequent the ejecta particles as they orbit TRAPPIST-1 imply that a stellar flares hosted by the system. Modelling panspermia in the TRAPPIST-1 system — 28/32

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