Modelling Panspermia in the TRAPPIST-1 System

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] Universe, and can propagate from one location to another. This interpretation owes itself predominantly to the works of William Thompson (Lord Kelvin) and Hermann von Helmholtz in the latter half of the 19th Century. Indeed, Thompson provided an excellent summation of the theory in 1871 [1]: Contents 1Introduction 11.1 Mechanisms for panspermia . . . . . . . . . . . . . . . 2 Radiopanspermia •Lithopanspermia Pseudopanspermia • Other mechanisms •Directed panspermia •“We must regard it as probable in the highest degree that there are countless seed-bearing meteoric stones moving about through space. If at the present instance no life existed upon this Earth, one such stone falling upon it might, by what we blindly call natural causes, lead to its becoming covered with vegetation.” 1.2 Micro-organisms in low Earth orbit . . . . . . . . . . 3 Early missions •Spacelab •LDEF •EURECA •MIR-Perseus •Biopan • EXPOSE • Cosmic rays 1.3 Micro-organisms in simulation experiments . . . . 9 Planetary ejection • Atmospheric entry 1.4 The TRAPPIST-1 system . . . . . . . . . . . . . . . . . 13 Discovery • Habitability • Motivation 2Theory 16 With these premises in place, panspermia has since been championed by a number of prominent scientists, with notable con- 2.1 Equations of orbits . . . . . . . . . . . . . . . . . . . . . . 16 General orbits • Circular orbits tributions from Svante Arrhenius [ 2<ul style="display: flex;"><li style="flex:1">], Francis Crick [ </li><li style="flex:1">3], Fred </li></ul>2.2 Impacting the target . . . . . . . . . . . . . . . . . . . . . 18 Hoyle [ ] and Chandra Wickramasinghe [ 44, 5,6]. Naturally, this level of support for the theory has ensured that numerous branches of thought have developed over the years, to be discussed further in Sec. 1.1. Panspermia would, however, remain conjecture until the 1980s, a decade which welcomed the ability to meticulously test the theory by exposing micro-organisms to low Earth orbit environments. Such experiments, both pioneer- ing and present-day, will receive attention in Sec. 1.2. A particular surge of interest came in the 1990s as a result of the Martian meteorite, ALH84001, which was found to possess structures that could indicate the presence of terrestrial nanobacteria. Careful testing of ALH84001, shown in Fig. 1, has alluded to the presence of amino acids and polycyclic aro- 3Simulation set-up 19 3.1 Simulation script . . . . . . . . . . . . . . . . . . . . . . . 19 3.2 Additional scripts . . . . . . . . . . . . . . . . . . . . . . . 20 4Results and Discussion 21 4.1 Timescale and fate . . . . . . . . . . . . . . . . . . . . . . 21 4.2 Radiant exposure and survivability . . . . . . . . . . 22 4.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 5Further work 27 28 28 Acknowledgments References matic hydrocarbons [7]. Whilst these can be an indication of life, most experts agreed that the compounds most likely formed abiotically from organic molecules, or via contamination during extraction. The past year has sparked a similar rise of interest in panspermia, due to the discovery of seven Earth-sized planets in TRAPPIST-1, a system whose compact nature provides an unprecedented testbed for the process. This constitutes the motivation for our study. 1. Introduction The concept of panspermia (‘seeds everywhere’) has existed since the ancient writings of the Greek philosopher Anaxagoras (500 BC–428 BC), although his notion differs from that of the current theory. In modern nomenclature, panspermia refers to the hypothesis stating that seeds of life exist across the entire Modelling panspermia in the TRAPPIST-1 system — 2/32 of lithopanspermia as a mechanism remains speculative, certain 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 temperature increases. A combination of petrographic studies 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 Figure 1. The ALH84001 Martian meteorite is and has been found to contain organic carbon compounds. ∼9 cm across 1.1 Mechanisms for panspermia launch, alongside post-shock temperatures in the range ◦<ul style="display: flex;"><li style="flex:1">100–600 C [18 </li><li style="flex:1">,</li><li style="flex:1">19]. Sec. 1.3 will provide further de- </li></ul>Whilst we have so far referred solely to the general theory of panspermia, numerous branches have developed over the last couple of centuries, each proposing a separate mechanism for the transfer of life throughout the Universe. Although there exists no conflict between the various mechanisms, and they could all be at work in relative harmony, it is important to distinguish between them. tails of these experiments, alongside others investigating the effect of hypervelocity impact on the wellbeing of micro-organisms. Whilst these impacts are undoubtedly energetic in nature, a small proportion of the resulting ejecta will not exceed the lower limit of 100 ◦C. These lower temperatures result from an ejection originating in the ‘spall’ zone, referring to the targetted surface of the impacted planet. Here, the shock wave from the impact is effectively cancelled by the reflected shock wave from the surface [20]. It has been estimated that more than a billion fragments of rock have been ejected from Mars at such low temperatures throughout the history of the Solar System, with ∼5 % having the potential to land on Earth. What’s more, the Earth’s crust has been found to be inhab- 1.1.1 Radiopanspermia Proposed originally by Arrhenius in his 1903 work, The Distri- bution of Life in Space, radiopanspermia is the hypothesis that micro-organisms can propagate through space, driven by stellar radiation pressure alone [8]. His assertions were driven by the knowledge that interplanetary space within our Solar System is littered with micron-sized dust particles. Such particles, below a critical size of 1.5 microns, would be driven away by the radiation pressure of the sun, potentially transporting microbial stowaways to other planetary systems. <ul style="display: flex;"><li style="flex:1">ited by a number of micro-organisms [21 22]. To many, </li><li style="flex:1">,</li></ul>these findings have strongly supported the lithopanspermia hypothesis. 2. Transit in space – if the microbial life is able to survive the ejection process, it must then withstand the interplanetary transfer that follows. The scale of this journey will depend on the system in question, and whether the lithopanspermia is occuring between planets of the same system, or an entirely separate system. One specific case that has been studied in great detail is exchange of ejecta An intriguing theory, though one which quickly loses effectiveness as the size of the particle increases. In this sense, radiopanspermia as a mechanism for transporting life holds solely for very small particles, at most hosting single bacterial spores [9]. Furthermore, Shklovskij and Sagan asserted that it is extremely unlikely that micro-organisms would be able to withstand the ‘lethal’ concoction of the solar UV and cosmic radiation, whilst more recent studies have highlighted the dele- <ul style="display: flex;"><li style="flex:1">between Mars and the Earth [ </li><li style="flex:1">9, </li><li style="flex:1">14, 23]. Here, estimates </li></ul>of journey times lie within the range of 1–20 million years, although simulations appear to suggest that a small proportion of meteorites could reach Earth within the first few years of travel [23]. This is the primary reason why the seven recently discovered planets of the TRAPPIST-1 system (see Sec. 1.4) have generated such excitement within the fields of panspermia and astrobiology. The terious nature of the space vacuum [10 11, 12]. Experiments looking into the effect of radiation on DNA stability have provided a final nail in the coffin for radiopanspermia, concluding ,that boulder-sized rocks ( provide effective shielding of bacterial spores from galactic cosmic radiation [13 14]. These findings instead support a ∼1 m in diameter) are required to ,mechanism involving much larger transportation vessels, such 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 the system is likely to occur on a much shorter timescale when compared with the Earth-Mars case above. Simula- tions performed by Krijt et al. (2017) found that ejecta released from planet f could reach the other ‘habitable’ 1.1.2 Lithopanspermia Lithopanspermia refers to the hypothesis that micro-organisms shielded by rocks can travel from planet to planet through either interplanetary or interstellar space. Whilst the viability Modelling panspermia in the TRAPPIST-1 system — 3/32 planet g within ∼80 years [24]. The present investigation aims to extend these results, as detailed in Sec. 4. A number of experimental facilities have placed a focus on assessing the response of microbial life to the testing environments of outer space, making use of low Earth nucleic acids stand alongside lipids, proteins and carbohydrates to constitute the four main macromolecules necessary for life. More recently, in 2012, scientists at Copenhagen University discovered signatures of glycolaldehyde, a sugar which holds im- portance in the production of RNA, around a solar-type young star that resides in the binary system IRAS16293-2422 [33]. The following year, researchers found traces of cyanometha- <ul style="display: flex;"><li style="flex:1">orbit satellites [ </li><li style="flex:1">9, </li><li style="flex:1">15]. These will be outlined further in </li></ul>Sec. 1.2. 3. Atmospheric entry – the final stage of lithopanspermia to test involves hypervelocity entry from space through the atmosphere of the target planet. As will be discussed in Sec. 1.3, numerous experiments have subjected rock samples containing micro-organisms to temperatures and pressures typically experienced by meteorites undergoing atmospheric entry [15, 25]. nimine in a giant ISM gas cloud ∼25000 ly from Earth. This molecule is known to produce adenine, a nucleobase that contributes to the ladder-like structure of DNA. The same project also discovered the presence of ethanamine, thought to have a part to play in the formation of alanine, one of twenty amino acids found in the human genome [34]. Finally, in 2015, NASA announced that the compounds uracil, cytosine and thymine, all key components of either DNA or RNA, had been successfully produced within the laboratory, subject to conditions remanis- cent of outer space [35]. 1.1.3 Directed panspermia In 1973, Crick and Orgel argued that life may have been pur- posely spread throughout the Universe by an advanced extrater- 1.1.5 Other mechanisms restrial civilisation [3]. Referred to as ‘directed panspermia’, A variety of other hypotheses exist regarding possible mechanisms for panspermia. Belbruno and coworkers have shown that low-energy transfer of rocks among protoplanets in orbit around young stars should be fairly common, supporting the theory of lithopanspermia [36]. Space probes are pushing new frontiers, reaching further than ever before. This naturally raises the question of whether such probes could cross-contaminate throughout the Solar System and beyond. Whilst numerous protection policies have been implemented, recent research has found that certain microbes may still be immune to clean-room procedures [37]. A rather alternative interpretation was proposed by Dehel, who suggests that magnetic fields ejected from the Earth’s magnetosphere could lift bacterial spores from the atmosphere, and propel them towards other stellar systems [38]. Despite the wide range of theories that exist, the present work will focus predominantly on the lithopanspermia hypothesis, with particular emphasis on interplanetary transfer within the same stellar system. this theory asserts that life may have deliberately been sent to seed the Earth. Naturally, this also concerns the seeding of other habitable worlds from the Earth, a feet that is fast becoming plausible due to developments in engineering (solar sails), astronomy (exoplanets, astrometry) and biology (microbial genetic engineering). Indeed, studies by astroecologists have deduced that a number of key nutrients could be obtained from materials typically found within asteroids [26]. In order to prove the hypothesis, it has been suggested that a distinctive ‘signature’ may be present in the genetic code of early microbial life on Earth, reminiscent of the parent civilisation [27]. Although evidence for such a signature is yet to be found, the theory continues to raise interesting questions as modern advancements add to its plausibility. 1.1.4 Pseudopanspermia Another form of panspermia, most notably championed by Chandra Wickramasinghe, is pseudopanspermia. According to this hypothesis, the solar nebula was able to draw in organic molecules that were already present in space during its formation. As the planets condensed from the nebula, these molecules would have been incorporated and subsequently distributed among the planetary surfaces. It is thought that these organic compounds then evolved to form life, via a process known as abiogenesis [28]. Wickramasinghe originally suggested formaldehyde, CH2O, as the main organic component of interstellar dust, although many other ideas were raised [29]. Organic molecules in the interstellar medium are most com- monly formed when an inorganic molecule becomes ionised, usually due to interactions with cosmic rays [30]. Electrostatic attraction then ensures that a reactant is drawn towards the charged ion. Interstellar dust plays a key role by shielding the newly formed organic molecules from UV light, which can further ionise them [31]. 1.2 Micro-organisms in low Earth orbit The continued development of space flight has enabled numerous micro-organisms to be exposed directly to the harsh conditions of outer space. Prior to testing, such conditions were thought to be extremely hostile to life. Most notably, the vacuum of up to 10−14 Pa is an impenetrable barrier for the bio- logical processes of growth, metabolism and reproduction [39]. What’s more, micro-organisms in space will undergo exposure to a complex concoction of radiation. Whilst the solar UV poses a particularly severe threat, bombardment by high energy particles originating either from the star (e.g. stellar wind) or from galactic/extragalactic space (cosmic rays) will also induce adverse effects. A summary of the space environment measured for various low Earth orbit missions to be discussed in this review is provided in Table 1. To date, a number of intriguing discoveries have been made in support of pseudopanspermia. In 2008, radiometric dating of Despite the hostile environment, certain life forms possess 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 the most notable molecules investigated was uracil, one of the four nucleobases that make up the structure of ribonucleic acid (RNA), the counterpart of deoxyribonucleic acid (DNA). The organisms that are able to form it. The cores of bacterial spores exhibit extremely low enzyme activity, thought to be due to their notable lack of water content. This contributes to their 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]. <ul style="display: flex;"><li style="flex:1">Space parameter </li><li style="flex:1">Spacelab </li><li style="flex:1">LDEF </li><li style="flex:1">EURECA MIR-Perseus </li><li style="flex:1">Biopan </li><li style="flex:1">EXPOSE </li></ul>Vacuum pressure (Pa) <ul style="display: flex;"><li style="flex:1">∼ 10−4 </li><li style="flex:1">∼ 10−6 </li><li style="flex:1">∼ 10−5 </li><li style="flex:1">∼ 10−4 </li><li style="flex:1">∼ 10−6 </li><li style="flex:1">∼ 10−4 </li></ul>Irradiance (Wm−2 UV fluence (Jm−2 ))<ul style="display: flex;"><li style="flex:1">1365 </li><li style="flex:1">∼1370 </li><li style="flex:1">1367 </li><li style="flex:1">1370 </li><li style="flex:1">∼1370 </li><li style="flex:1">∼ 1370 </li></ul>Undetermined <ul style="display: flex;"><li style="flex:1">≤ 103 </li><li style="flex:1">∼ 109 </li></ul>(>) 50, 170 4.8 <ul style="display: flex;"><li style="flex:1">≤ 3×103 </li><li style="flex:1">∼ 107 </li><li style="flex:1">∼ 109 </li></ul><ul style="display: flex;"><li style="flex:1">(>) 110, 170, </li><li style="flex:1">(>) 110, 170, </li><li style="flex:1">(>) 110, 170, </li></ul>200, 290, 400 <ul style="display: flex;"><li style="flex:1">Spectral range (nm) </li><li style="flex:1">(>) 110 </li><li style="flex:1">(>) 110 </li></ul><ul style="display: flex;"><li style="flex:1">290, 300 </li><li style="flex:1">280, 295 </li></ul>Cosmic ionising radiation dose (Gy) <ul style="display: flex;"><li style="flex:1">0.001 </li><li style="flex:1">0.2–0.4 </li><li style="flex:1">0.037–0.049 </li><li style="flex:1">0.004–0.074 </li><li style="flex:1">0.1–0.2 </li></ul>Temperature (K) Gravity (g) 243–290

Modelling Panspermia in the TRAPPIST-1 System

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