Europa Quad Echo (EQE)

Home , Conamara Chaos, Pwyll (crater)

Sub-cryospheric investigation into the habitability of Europa

Editors: - Kolja Hanke - Micha Nebel - Benjamin Heikki Redmond Roche

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Contents Page

Chapter 1 - Introduction To The Exploration Of Europa (Page 2)

Chapter 2 - The Structure Of Europa (Page 3)

Chapter 3 - The Required Parameters For Life (Page 5)

Chapter 4 - The Zones Of Europa (Page 8)

Chapter 5 - Indicators For Life (Page 10) 5.1 - Indicators of Astrobiology & Instrumentation (Page 10) 5.2 - Sensitivity of Instruments (Page 12) 5.3 - Amino-Acids as potential Indicator of Life (Page 13) 5.4 - Sterilisation (Page 15)

Chapter 6 - Target Zones For Exploration (Page 17) 6.1 - Cryogenic Morphology Of Europa (Page 17) 6.2 - Target Zones Of Europa (Page 24)

Chapter 7 - Discussion & Mission Options (Page 29)

Chapter 8 - Reference List (Page 33)

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1. Introduction

On the 8th of January 1610, Galileo Galilei pointed his telescope at Jupiter and made a ground- breaking discovery; changing humanity’s perspective of the Earth, the universe and its own place within it. He had discovered the four moons of Jupiter: Io, Ganymede, Callisto and Europa. In the following few centuries Europa, the smallest Galilean moon, was disregarded until exploration occurred in the 1970s with the Pioneer (10 & 11) and Voyager (1 & 2) flybys; and subsequently the Galileo Mission (1995-2003), which undertook detailed analyses of the Galilean network for the first time. The data collected by these missions revealed the Europan surface consists of water ice and seems to be very young, according to the small number of visible craters (Carlson et al., 2009). Moreover, the Europan magnetic field, measured during the Galileo Mission, yielded captivating data that indicated the existence of a saline ocean beneath the icy crust (Carlson et al., 2009). In 2013 the Hubble Space Telescope observed plumes, rising from the surface of Europa supporting the theory of a subsurface ocean (NASA, 2017). The high probability of a subcryospheric ocean existing on Europa results in this being the most likely place in the Solar System (that we currently know of) to harbour life, with a potential environment analogous to the Archaean and Proterozoic oceans (NASA, 2017). The main objective of this paper is to develop a theoretical mission, which could be able to detect possible traces of these extra-terrestrial life, that may have developed in a subsurface ocean of Europa. Therefore, we first look at the structure of Europa, define the requirements life needs to develop and flourish and identify the zones in which possible life could exist. After that we focus on which indicators life leaves behind, how and with what instrumentation to measure these traces. The final part of this paper proposes suitable target zones on Europa, as well as the three different mission options ranging from what is possible now (Technology Readiness Level = TRL 9) to what will be possible in the near future (TRL 3), to prove the existence of life on the Jovian moon.

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2. The Structure of Europa

During the eight years of the Galileo mission (1995-2003), the spacecraft explored the Jupiter system including the moon Europa (NASA, 2017). Thanks to its findings, today we know a great deal more about the gravitational force, electromagnetic characteristics and the shape of Europa (Schubert et al., 2009). With the collected data, it is possible to create simple models for the inner structure of Europa. These models indicate that Europa consists of a metallic core, surrounded by a silicate mantle which is covered with an outer shell of liquid water/ice (Fig.4A). The concept that there may be an ocean beneath Europa's icy surface first arose after measurements of the electromagnetic induction signals of Europa by the magnetometer of Galileo (Schubert et al., 2009). The collected data lead to the conclusion that only a salty ocean could explain the disturbances in Jupiter's magnetic field, which could be observed around Europa (NASA, 2017). According to the models of Europa's inner structure the thickness of the ice is estimated to be ~25 kilometres and protect the underneath laying ocean from incoming radiation (Paranicas et al., 2007). The temperatures in the ice vary between -183°C (top layer) and -3.15°C (bottom layer) (Kargel et al., 2000). The huge differences in salinity (Table 2.1) within the ice is due to convection movements within the ice: relatively warm ice rises from the bottom layers to the cooler ones above in a diapiric manner (see Chapter 6). Below the ice, the supposed ocean reaches depths of potentially up to ~100 kilometres. The temperatures in the ocean increases from top to bottom (0-100°C). This increase in temperature is assumed to be generated from hydrothermal vents in the on the seafloor. These hydrothermal vents may heat the surrounding area up to ~250°C similar to deep sea vents on earth (Table 2.1). The plumes spotted by the Hubble Space Telescope in 2013 may possibly be formed by, and therefore, indicative for the existence of these hydrothermal vents at the seafloor (NASA, 2017). The geological explanation for the formation of these vents is attributed to plate tectonics generated by tidal heating caused by Jupiter's gravity. This tidal energy, which stretches the Europan surface, most likely turns into mechanical energy, resulting in friction and heat and as such providing the mechanism required for temperatures conducive to subcryospheric liquid ocean (Europa Clipper, 2017). The boundary layer, estimated to be around 25 kilometres deep, is followed by the silicate mantle, in which the temperatures further increase from 250°C - 850°C by a depth of 250 kilometres (Table 2.1).

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Table 2.1: The structure of Europa Zone Temp. (°C) pH Pressure Radiation Chemical Conditions

Surface (Top Ice) -183.15 - 0.1 µPa 7.0 to 8.1 H, C, N, O, Na, Mg, Upto 10 meters depth =10-12 bar a MeV/cm^2 s Al, Si, S, Cl, K, Ca, → highest Fe (average mass fluxes → exogenic) values at

equator b Salinity: 240 - 400 g/kg d

Mid. Ice -93.15 - - Between 1 - Salinity: 240 - 400 (10 m to 10 km) 10 meters g/kg depth no radiation left b

Bottom Ice (up to 25 km -3.15 - - - Salinity: 240 - 400 depth) g/kg

Ocean (0km depth - 0 - 100 Alkaline 0 MPa - Salinity: For a beneath 25km ice layer e pH 8 - 12 d e MgSO4-rich ocean to 125 km) ** (Eutectic → estimated to be temperature in range of 1 - 16 238 K d) g/kg f

Boundary layer (Plume 100 – 250 - 0 - 80 MPa - - height/Fluid peroclation Depth into mantle) → 100 - 125km depth

Silicate Mantle (125 - 250 - 850 - 81 - 500 - - 250 km) MPa a(McGrath, 2000) b(Paranicas et al., 2009 c(Cooper et al., 2001) d(Kargel et al., 2000) e(Vance et al, 2009) f(Hand et al., 2007)

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3. Required parameters for life

While searching for life in space it is fundamental to initially define the parameters required for life to exist and copulate. Therefore, all of the important parameters regarding forms of life have been investigated and listed in Table 3.1, so that the ranges in which life as we know it can exist are clear. In combination with parameters of Europa´s interior (Chapter 2) Zones of a habitable environment on Europa have been defined (Chapter 4). As stated in Table 3.1 there are two relevant radiation parameters, ultraviolet (UV) radiation and ionizing radiation. Whereby life can stand a UV radiation up to 5,000 J/m2 and a ionizing radiation of up to ~6000 Grays (Pappalardo et al., 2009). The six most abundant elements (Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorous and Sulphur) in biomolecules as we know them must be present to form life. For

-3 Nitrogen, e.g. Aerobic microorganisms need at least 1–5 × 10 atmospheres N2, for fixation (Pappalardo et al., 2009) (Life, 2017) (McKay, 2014). A certain environment is habitable for life so long as the salinity values are no higher than 25 to 33%. The range of pH-values is wide (0 to 12.5) and covers almost the whole spectrum (0 to 14) (Pappalardo et al., 2009). Therefore, the pH-values are probably not a significant restricting factor for life forms. Similarly, pressure is going to be of limited significance as the upper limit is 1680 MPa and Earth´s surface has a pressure value of 0.1 MPa. For liquid water, as in the case of the Europan ocean, temperature is more likely to be a limiting parameter before pressure (Pappalardo et al., 2009) (Life, 2017). Temperatures in which forms of life can exist ranging from - 20 to 122 °C (McKay, 2014; Table 3.1). With regard to these values and the conditions which can be observed on earth, it seems likely that many different environments are habitable for life. However, the probability of finding forms of life in space is much more remote as conditions are significantly more challenging (see Chapter 2). The zones of Europa which are the most conducive to life are discussed in more detail, particularly regarding the possibility of finding forms of life, in Chapter 4.

Table 3.1: Requirements for known carbon based lifeforms PARAMETER RANGE REMARK

TEMPERATURE -20 - 122°C; 253.15 - 395.15 K Upper limit solubility of lipids in water/protein stability, Psychrophiles live up to -20 °C but can survive in a ‘dormant’ mode at much lower temperatures ~196 °C. c pH 0 - 12.5 Life known to survive at pH: (0) Cyanidium, Archaea -

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Natronobacterium, Protists – know to survive in >10.5. a

ENERGY Chemical redox from: Geothermal flux can arise from (i) Chemoautotrophic Life: the planet cooling off from its - geothermal processes gravitational heat of formation, (ii) - Methane, Sulphur(?) required for decay of long-lived radioactive chemosynthesis elements, or (iii) tidal heating for a Photoautotrophic Life: close-orbiting world or moon. - light from central star 0.01 μmol m-2⋅s-1 Both chemoautotrophic and photoautotrophic microorganisms. minimum amount for photosynthesis obtain their energy and produce - Carbon/Oxygen required for photosynthesis their nutrients from simple inorganic compounds such as carbon dioxide. Chemoautotrophs do so through chemical reactions, while photoautotrophs use photosynthesis. b

PRESSURE Upper limit of 1680 MPa Earth’s surface pressure 0.1 MPa - N.B. temperature is likely to be a ~2 GPa limitation before the pressure (for liquid water). a

UV RADIATION Significant ultraviolet radiation if Up to 5,000 J/m2 no atmosphere. a

IONIZING Upper limit of ~6000 Grays Microorganisms capable of RADIATION An exoplanet does not require a magnetic field to withstanding very high levels of be habitable radiation. a

LIQUID H2O Liquid H2O should be present but some Life could exist on a planet with (SURFACE) halophilic organisms live in high (4-5 Mol) NaCl ‘supercritical’ carbon dioxide i.e. existing as a gas and a liquid. a,c

NITROGEN Aerobic microorganisms require a minimum of 1– In the reducing conditions of the -3 (CHNOPS) 5 × 10 atmospheres N2 for fixation. outer Solar System N is present as ammonia which is also biologically usable. b,c

C,H,N,O,P,S Must be present, to form biomolecules as we The 6 most abundant elements: know them - essential for transfer of energy from Carbon, Hydrogen, Nitrogen, cells for metabolism etc. Oxygen, Phosphorous and Sulphur are the 6 most abundant elements in all biomolecules. a,b

SALINITY Up to 25-33% Microbes and algae, in saturated sodium chloride. a

a(Pappalardo, McKinnon and Khurana, 2009)

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b(Life, 2017) c(McKay, 2014) 4. Zones of Europa

The investigations concerning the required parameters for life and the parameters of the interior structure of Europa (Chapter 3 & Chapter 2) mark the key values for this section. By combining the information of Table 2.1 and Table 3.1 a simplified categorisation of zones can be implemented. In Table 4.1 these zones are listed with relevant parameters and their values for Europa. The colour scale shows the probability to find life in the defined zones based on the researched values. Where red means that life as we know it cannot survive in such an environment; yellow means that life could survive but with a medium likelihood (or life exists in a hibernated state); and green means that life may exist in such an environment. As shown the surface (top ice layer), the middle ice layer and the silicate mantle are marked in red due predominantly to their extreme temperatures. The boundary layer between ocean and silicate mantle is marked in yellow due to the relatively wide temperature range which could allow life in the lower temperature range. The bottom ice zone and ocean zone is marked in green, thus are the most promising zones for searching for life on Europa. Apart from temperature all the other known parameters are depicted in the green category. This leads to the assumption that temperature is probably the most limiting parameter for life on Europa. Table 4.1 has been illustrated in Fig. 4.1.

Table 4.1: The characteristics of the different zones of Europa with the degree of habitability superimposed Zone (depth) T pH Pressure Radiation Salinity

Surface (Top Ice) -183.15 °C a - 0.1 µPa f 7.0 - 8.1 < ocean Up to 10 m MeV/cm2/s d salinity

Middle Ice - 93.15 °C a - 0 MPa -? 0 MeV/cm2/s < ocean (10 m to 10 km) salinity

Bottom Ice (10 - 25 km) -3.15 °C a - 0 MPa -? 0 MeV/cm2/s < ocean salinity

Ocean (25 - 125 km) 0 - 100 °C b 8 - 12 c 0 - 100 MPa b 0 MeV/cm2/s 1 - 16 g/kg e

Boundary layer (Plume 100 - 250 °C b - 0 - 80 MPa b 0 MeV/cm2/s - height/Fluid peroclation Depth into mantle) (100 - 125 km)

Silicate Mantle 250 - 850 °C b - 80 - 500 MPa b 0 MeV/cm2/s - (125 - 250 km)

a Han & Showman (2005) b Vance et al., (2007) c Zolotov and Shock (2001) d Paranicas et al. (2007)

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e Hand and Chyba (2007) f McGrath et al., (2017)

Fig. 4.1: The 7 differentiated zones of Europa with varying probabilities of life: Improbable, Possible, Probable. Edited from: Solarsystem.nasa.gov, (2017)

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5. Indicators of Life

5.1 Indicators of Astrobiology & Instrumentation

The researched parameters of the previous chapter give a simplified overview of the zones where life may potentially be observed (Table 4.1). This chapter focuses on the indicators of astrobiology and secondly, how to measure them. Researched and summarised substances that can be used as Indicators such as: Amino Acids, Cells, DNA & RNA, Isotopes, Lipids and possibly Methane, along with the appropriate instruments for measurements are listed in Table 5.1. Since these Substances can only be produced by living organisms (with some exceptions, see chapter 5.3) they were chosen to distinguish life from other chemical non-biotic compounds. The likelihood of measuring these different substances in certain areas, as well as the desired properties to be measured and the reasoning for that, are also stated in Table 5.1. The detection of complex chemical or biochemical compounds is necessary as simpler compounds are also produced by inorganic processes. It seems to be promising to focus on Amino Acids, hence that will be discussed in chapter 5.3.

Table 5.1: The indicators of astrobiology and the instrumentation required to search for them Substances Properties Reasoning Instruments Area Likelihood (Plume Samples)

Amino Amount Known CE (Capillary Plume High acids Type lifeforms use electrophoresis sample Chirality amino acids of (amount and (Zone PL) only one chirality))e With chirality GCMS GCMS (Gas chromatograph y– mass spectrometry (amount)) f

LCMS (Liquid chromatograph y– mass spectrometry (chirality))f

Cells/Organi Amount Direct Microscope, Plume Medium sms Type detection of LCMS samples lifeforms (Zone PL)

DNA/RNA Amount Direct sign of Spectrography Plume Low life samples

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Immunoassay (Zone PL) Water PCR (Zone 3-5) (Polymerase chain reaction)

Hi-prec MS (High precision mass spectrometry) f

Isotopes Ratio C isotopes in irMS (Isotope- Plume Low/Medium biogenic ratio mass Sample methane have spectrometry) (Zone PL) different ratio than in CRDS (Cavity abiogenic C. Ring-Down Spectroscopy)

Lipids Amount Lipids are GCMS (Gas Plume Medium/High Length of essential for chromatograph Sample carbon chain known y– mass lifeforms spectrometry)

Methane Amount Many forms of irMS (Isotope- Plume High Isotope life produce ratio mass Sample ratios methane. spectrometry) C isotopes in biogenic CRDS (Cavity methane have Ring-Down different ratio Spectroscopy) g than in abiogenic C. a McKay and Smith, (2005) b Strobel (2010) c Clark et al., (2010) d (NASA/JPL, 2017) e (NASA Mars rover + mission info, 2017) f (Astrobiology.nasa.gov, 2017) g (Picarro.com, 2017)

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5.2 Sensitivity of Instruments

Table 5.2 summarises the sensitivity required for each relevant instrument, along with their current technology readiness level (TRL). For the purpose of different mission outlines the most suitable instruments will be selected (refer to chapter 7).

Table 5.2: The required instrument sensitivity to detect astrobiological signatures Instruments Sensitivity TRL

CE b <200 attomoles/ 80 zeptomoles f 6 n

j CRDS 10−29 cm/molecule g 6

GCMS [c] < 0.036 ng 9 k

Hi-prec MS attomolar range (10-18) (MS)h 9 k

irMS 0.01‰ (mass resolving power: 20000, 4 sensitivity: 3x10-12) e a (similar instrument on Curiosity TLS; TRL 9 i

LCMS d 5 ppm 5 (2012)

Microscope 30 microns/pixel (Mars Rovers), 200 nm for 9 o standard optical microscopes o

PCR Any part of single DNA segment (in theory) 8 m 1 pg - 1 μg per 50 μl of sample (in practice) i a (Assets.thermofisher.com, 2017) b (Agilent.com, 2017) c Cao et al., (2014) d Hines et al., (2015) e Eiler et al., (2013) f Mansfield et al., (1996) g Liu et al., (2009) h (Thermofisher.com, 2017) i (Biolabs, 2017) j Wang, (2010) k Palmer and Limero, (2001) l (Ssed.gsfc.nasa.gov, 2017) m (Biolabs, 2017) n (Rsc.org, 2017) o (NASA, 2017).

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5.3 Amino-Acids as potential Indicator of Life

As stated in Table 5.1 Amino Acids are potential Indicators of Life. However, there are 20 different Amino Acids which are used by life as we know it on Earth, and the majority can also be produced by abiogenic processes (Table 5.3; marked in red). This is based on findings from meteorites and findings of abiogenic laboratory experiments (Wolman et al., 1972; Zenobi et al., 2003; Garguad et al., 2011; Parker et al. 2010; Parker et al., 2011; Kobayashi K., 2010; Kojo, 2015; Koga & Naraoka, 2017). For three Amino Acids (Arginine, Lysine and Tryptophan; marked in green) currently no abiogenic formation pathway is known, and they have not been found on meteorites. Therefore, these Amino Acids are assumed to be exclusively biogenic (Garguad and Amils, 2011). Thus, these three Amino Acids are going to be focal concerning measurements on Europa. The number of Carbon elements (Table 5.3) in an Amino Acid chain shows the complexity of an Amino Acid. Looking at the biogenic Amino Acids, two of them (Arginine and Lysine) include six Carbon elements which is a medium chain length compared to all 20 Amino Acids. Whereas Tryptophan is significantly more complex with a Carbon chain consisting of 11 Carbon elements. However, Complexity seems unlikely to be a clear indicator because there are a few long chain Amino Acids (e.g. Phenylalanine, 9 Carbon-Elements; Kobayashi K., 2011) which were found on meteorites. Whereas the Isotope ratio of Carbon could be an indicator for organic processes, if distinct carbon isotope ratios were found in Amino Acids (e.g. enriched in 13C) in company of the same Amino Acid with a distinct different carbon isotope ratio. In this way, the different Isotope ratios could be attributed to organic or inorganic forming processes due to their most likely underwent fractionation processes. Furthermore, a promising way to distinguish biogenic and abiogenic Amino Acids is the chirality. A chiral molecule, in this case a certain Amino Acid, is non-superimposable on its mirror image. There are left handed Amino Acids (L-Amino acids) and right handed Amino Acids (D-Amino acids). Where the L-Amino Acids are known as ‘proteinogenic’ (protein-building) amino acids and the D-Amino Acids are known as uncommon in live organisms (Bruice, 2004). The Chirality of Amino Acids can be detected with Capillary Electrophoresis (CE) (see Table 5.2 and Table 5.3).

Table 5.3: The table contains the 20 amino acids that life on Earth uses. Amino acids marked in red are known to be formed abiogenically, i.e. they are found on meteorites or in abiogenic laboratory experiments. For the amino acids marked in green, no abiogenic formation pathway is currently known, and they have not been found on meteorites. Amino acid Abiogenic (red) / biogenic (green) Carbon number

Alanine Abiogenic (Alpha, Beta) b 3

Arginine Biogenic, not found on meteorites f 6

Asparagine Abiogenic, but unstable? c 4

Aspartic Acid Abiogenic b 4

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Cysteine Abiogenic, but not found on meteorites d 3

Glutamic Acid Abiogenic b 5

Glutamine Abiogenic, but unstable? 5

Glycine Abiogenic b 2

Histidine Abiogenic e 6

Isoleucine Abiogenic f 6

Leucine Abiogenic f 6

Lysine Biogenic, not found on meteorites f 6

Methionine Abiogenic f 5

Phenylalanine Abiogenic f 9

Proline Abiogenic a b 5

Serine Abiogenic f 3

Threonine Abiogenic a h 4

Tryptophan Biogenic, not found on meteorites i 11

Tyrosine Abiogenic f 9

Valine Abiogenic b 5 a Parker et al., (2010) b Wolman et al., (1972) c (Atlasofscience.org, 2017) d (Kobayashi et al., 2010) e (Zenobi et al., 2003) f Kobayashi, (2011) g Parker et al., (2011) h (Koga and Naraoka, 2017) i Kobayashi, (2011)

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5.4 Sterilisation

The process of sterilisation is an inherently important aspect of any space mission, due to the possible influence of earth based microbes that could contaminate the samples, or influence the measurements taken by the instruments on the spacecraft. If these prevention methods are not taken carefully, possible indicators for (in our case) life could be obscured or even destroyed by the microbes brought along from Earth. Particularly in the case of Europa, sterilisation is fundamental since the globally connected subsurface ocean could harbour life and therefore contamination could affect the entire global habitat (National Research Council, 2000). In 2012 Russian scientists probed the water of Lake Vostok, a subsurface lake in the Antarctic, which has been a closed system, isolated from atmospheric influences for several million years. During this procedure, drilling fuel contaminated the samples and thus the lake itself. This preventable contamination obscured the real chemical composition of the lake and with it possible traces of life, which could have developed under the ice, in an analogous environment to the Europan subsurface ocean (Alekhina et al., 2017). To prevent contamination, like in the case of Lake Vostok, on other planets different methods have been developed to sterilize spacecrafts. These methods can be split in physical and chemical methods (Table 5.4). An example for the physical methods could be dry and wet heating. These are used to sterilize the spacecraft interior and exterior. For further sterilization UV, Gamma and Beta radiation can be used as well. Chemical sterilisation could be carried out with hydrogen peroxide plasma and alcohol to wipe off the surfaces of the spacecraft.

Table 5.4: Sterilisation methods, implementation types and effectiveness. Adapted from: National research Council, (2000) METHOD & TARGET IMPLEMENTATION EFFECTIVENESS (estimation)

Physical

Dry heating - exterior/interior 105 - 180 °C for 1 to 300 hours High - Problems caused by thermomechanical incompatibility between materials can lead to the failure of electronic components.

Wet heating - exterior/interior 120 - 134 °C for 3 to 20 minutes High - Problems can be caused by steam (e.g., corrosion and water absorption).

UV irradiation - exterior surfaces 5,000 to 20,000 J/m2 High - Problems arise because unexposed surfaces remain untreated.

Gamma radiation Typically, 2.5 Mrad High - exterior/subsurface - Problems encountered include optical changes in glasses and damage to electronics and solar cells.

Beta radiation 1 to 10 MeV High - exterior/near-surface - Problems arise because of limited

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penetration.

Chemical

Alcohol wipes - exterior surfaces Isopropyl or ethyl alcohol swabbing High - Problems arise because interior and encased surfaces (e.g., electronic components) are inaccessible.

Hydrogen peroxide plasma 6 mg/l H202 concentrated at 58% High - exterior/internal exposed - Problems can be encountered surfaces because the unexposed surfaces remain untreated.

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6. Target Zones for Exploration

6.1 - Cryogenic Morphology Of Europa

Surface conditions on Europa are extremely cold at -180 °C (95 K) resulting in very solid ‘hard’ ice in the top few kilometres below the surface. Beneath this there is a middle layer of ice that extends down to ~20 km / ~10 km according to models (Fig. 6.1) a and c respectively, with an ice temperature of -90 °C (180 K). Beneath this is the ‘warm ice’ zone where the ice gradually warms with depth until the base of the ice sheet at ~50 km depth, where the ice temperature is -3 °C (270 K). Diapiric upwelling occurs due to the relative buoyancy of the ‘warm’ bottom ice that is driven by convection, resulting in diapirs of 270 K ice reaching up to 20 km, or possibly as much as 10 km depth according to models a and c. Above these diapirs there is a very small vertical offset in the cryogenic surface topography, with double ridges each up to 40 km wide straddled by valleys that run parallel 40 m below (Pappalardo, McKinnon and Khurana, 2009).

Fig 6.1: (a), (c): Temperature (b), (d): Dynamic topography from simulations of thermal convection in a 50km thick Europan ice shell. Han & Showman, (2004).

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Fig 6.2 is a Surface image of a double ridge, much steeper than Fig. 6.1 with a prominence of some >200 m over the median valley. The ridge appears to be subject to shear stresses associated with dextral ‘strike slip tectonics’ of the ice layer, acting analogously to lithospheric plates on Earth. Whether this shear stress is the driving force for ridge formation is unknown, but it may be a strong factor, possibly in association with diapiric upwelling that may be involved in the heating of the subsurface ice layers (Pappalardo, McKinnon and Khurana, 2009).

Fig. 6.2: (a): Topography of a prominent double ridge form Galileo mission with several hundred-metre ridge prominence (b): High resolution (~12m/pixel) of the ridge (c): Perspective view of the double ridge illustrating shear heating for ridge formation as a result of strike slip motion. Gaidos & Nimmo, (2000)/ (2002).

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Fig. 6.3 illustrates that diapirism can result in double ridges with vertical offsets between ridge crest and valleys up to ~400 m, however they tend to occur over much wider horizontal distances than in the ridges possibly formed by faulting as seen in Fig. 6.2. Thermodynamic processes results in the salt poor ‘warm’ buoyant ice to rise through the ice column, bringing -23 °C (250 K) ice to ~5 km depth - whereas the ice surrounding the diapir at this depth is -163 °C (~110 K) (Pappalardo, McKinnon and Khurana, 2009).

Fig. 6.3: Diapir cross section. (a): Topography (b): Composition - black dots are tracers marking the locations of salt poor, low density ice, that was originally near the bottom of the ice shell and subsequently has experienced diapirism (c): Temperature. Han & Showman, (2005).

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Europa experiences subglacial convection cycles that have a timespan of some ~50 - 100 ka in the cryosphere, resulting in buoyant ‘warm’ ice taking this period of time to rise to the ~5 km depth as seen in Fig. 6.3. Below the surface there may well be pockets of water where partial melt of the ‘molten’ ice layer has occurred. Above these diapirs it is hypothesised that ‘Chaos Terrains’ occur, where surficial features appear enmeshed together with no order, believed to be caused by the subsurface water layer potentially associated with diapirs (Fig. 6.4). However, this is only one hypothesis for diapir formation (Schmidt et al., 2011).

Fig. 6.4: ‘Chaos’ Zone above a diapiric upwelling centre, indicative of the subglacial convection cycle. Liquid water near Europa’s surface a rarity. (European Planetary Science Congress, 2012).

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The Europan ice shell experiences thickness oscillations of: 15km - 50km over a 150 Myr period - primarily due to the coupled thermal orbital behaviour of Io. It is possible that the surface of Europa may undergo massive surficial changes over this period, perhaps resulting in ‘repaving’ events when it is at its thinnest. There are 6 hypothesised models of ‘Chaos Terrain’ formation (Fig. 6.5). In a, assuming the ice layer is at its thinnest - the subglacial ocean results directly in ‘melt through’ causing the chaotic surface as the liquid water is refrozen by the extreme surface temperatures. In b, the relatively ‘warm’ ice that is brought to within 5 km of the surface via diapirism (see Fig. 6.4) results in the formation of ‘Chaos Terrain’ due to thermodynamic processes above. c and d consider the effects of brine mobilisation associated with either diapirism (as seen in b) or melt through (as seen in a) respectively. In e, it is envisaged that a liquid sill has formed at a shallow depth below the surface resulting in the surface features. In f, an impact has caused total melt through of the icy layer resulting in chaotic melting of the surface ocean, however this may be ruled out due to the total lack of resemblance to known impact craters on Europa. Schmidt et al., (2011), Head & Pappalardo, (1999), Pappalardo, McKinnon and Khurana, (2009).

Fig. 6.5: ‘Chaos Terrain’ formation models: (a): Melt through (b): Diapirism (c): Brine mobilisation driven by diapirism (d): Brine mobilisation driven by partial melt through (e): Sill formation (f): Impact. Head & Pappalardo, (1999).

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Spectroscopic analyses of the Europan surface indicate that non-ice substances exist on the surface, with more recent ‘Near Infrared Mapping Spectrometry’ aboard the Galileo spacecraft confirming these non- ice substances to be sulphur compounds. The mechanism of sulphuric deposition on the surface is not fully understood, but there appears to be a tight coupling of ridges and darker areas of sulphuric compounds - as well as dark areas on ‘Chaos Terrains’ as seen in Fig. 6.2. The three models in Fig. 6.6 (of the focus area a) hypothesise different mechanisms of how subglacial material is transported to the surface, assuming that the sulphuric compounds are subcryospherically derived: b indicates the same shear heating associated with

‘strike slip’ tectonics resulting in H2O sublimation from ice melt and hydrate/sulphurous material precipitation, with sulphurous material being deposited from some intra-ice source, such as brine rich diapirs, coupled with cosmic dust. c indicates the shallow ocean is able to reach the surface where cryovolcanism occurs and oceanic sulphuric brine is precipitated onto the surface. d Suggests that tidal forces pushes the icy shell open and closed allowing for the periodic flooding of the ocean onto the surface, with the sulphur rich fluids freezing around the cracks. Thus; b suggests that melted ice has been transported to the surface and the sulphuric compounds are chiefly diapir related coupled with cosmic dust; whereas c and d suggest ocean water to have escaped to the surface and deposited sulphuric precipitates. Johnson and McCord, (1971); Carlson et al., (2002); Chela-Flores, (2006)

Fig. 6.6: (a): Galileo image with red indicating hydrated material whilst blue indicates more pure water ice and frost (b): Thermal heating of the surface by regions of the Linea, sublimation H20 and leaving higher concentrations of hydrate and darker, possibly sulphurous material (c): Thin shell volatile driven explosive cryovolcanism depositing oceanic brine (d): Effusive emplacement of near surface liquid by tide induced opening and closing of a crack. Carlson et al., (2005).

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The two ice shell models: thin ice a and thick ice b which vary over a 150 Myr period from 15 - 50 km are shown below (Fig. 6.7). What they indicate is that the potentially habitable zones can be found at different environments above the lithosphere: the rock - ocean boundary; the ice - ocean boundary and; in the ‘molten’ diapir ice column in temperatures above -20 °C (253 K). Potential biosignature locations may be found in the sulphuric compounds found at the double ridges and associated valleys (such as Fig 6B) and in the ‘Chaos Terrain’ potentially either from the ‘dry theory’ or ‘wet theory’ (refer to page x). Pappalardo, McKinnon and Khurana, (2009).

Fig. 6.7: (A): Thin ice shell model (B): Thick ice shell model. In both cases, the regions expected to be most conducive to life are the chemically rich interfaces at the seafloor - ocean boundary and the ice - open ocean boundary. Regions in the ice shell itself may also be habitable. Pappalardo, McKinnon and Khurana, (2009).

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6.2 - Target Zones Of Europa

The Europan Shell varies in age from ~0 - 180 Ma (Schenk et al., 2004). As Fig. 6.8 indicates; the oldest surface of the shell is the ridged plain terrain, which generally has a very small vertical offset of tens of metres over tens of kilometres (Fig. 6.1). This area is crosscut by stratigraphically younger ridges and cracks, which separate the ridged plain terrain into large ‘cryospheric’ plates analogous to lithospheric plates on earth. These ridges are much more topographically pronounced with a prominence of >200m over valleys over a few kilometres; appearing to dynamically respond to ‘strike slip tectonics’ once formed (Fig. 6.2). The impact craters on Europa, by very definition, vary in age from ~180 - 0 Ma. Two of the largest craters Pwyll (24 km diameter) and Tyre (40 km diameter) are particularly interesting due to their different morphologies, with Pwyll most likely to be ~2 Ma and Tyre also considered ‘geologically young’ (Fig. 6.9 and Fig. 6.11). The geologically youngest surface of Europa, i.e. water that has most recently frozen, are the ‘chaos terrain’ areas. There are several hypothesised methods of formation (Fig. 6.5), with more extensive discussion on diapir formation in Fig. 6.3 and Fig 6.4. The detection of Sulphurous compounds on the Europan surface is indicative of a sub cryospheric source, which is promising in the search for traces of astrobiology; the proposed transport mechanisms of subglacial ocean to the surface are illustrated in Fig. 6.6 (tile c and d). The target zones for exploration must be geologically relatively young, with anticipated sulphurous compounds present on the surface, as well as a hypothesised transport mechanism for subglacial ocean bio signatures to be present at the surface (as indicated in Fig 6.7). Considering these parameters, the three proposed target zones for exploration are: Tyre Crater, the Conamara Chaos Terrain associated with the Androgeos Line and possibly the Pwyll Crater. All three have sulphurous compounds associated with them, particularly the Chaos Terrain. Schenk et al., (2004); Pappalardo, McKinnon and Khurana, (2009); Harriss and Burchell, (2017) Wendel, (2017).

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Fig. 6.8: (Top) Satellite images of Europa (Top). (Bottom) Geological map of Europa with clearly zoned ‘Chaos Terrains’, impact craters cross cutting ridges and ridge plains. Edited from: Wendel, (2017).

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The Tyre impact crater located at 34°N, 144°W is one of the biggest craters on Europa, with a diameter of some 40 km. The unique concentric ring structure and very flat morphology of Tyre is anomalous to the usual formation of impact craters, with a lack of a continuous rim and central peak. This lack of surface features is indicative of the Tyre impact occurring into relatively thin ice above a subsurface ocean and penetrating through the ice. Indeed, Europan ice thickness is thought to vary between ~15 - 50km thickness over a 150 mya cycle, and so the structure of both Tyre and the less studied, but morphologically similar Callanish crater, indicate that the Europan shell was thinner at the time of impact. Therefore, Tyre is an excellent zone for exploration due to the high probability of subsurface ocean derived bio signatures having been transported to the surface or near surface environment. Moore et al., (2001); Harriss, K. and Burchell, M. (2017); Pappalardo, McKinnon and Khurana, (2009); Photojournal.jpl.nasa.gov, (2017).

Fig. 6.9: Mosaic of the Tyre Impact Crater multiring structure, with a crater diameter of 40 km, in the core of a much large structure. The 5 - 7 observable concentric rings are formed of ridges and troughs - indicative of the presence of subsurface liquid water at the time of impact. Location 1 in Fig. 6.8. Photojournal.jpl.nasa.gov, (2017)

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The Conamara ‘Chaos Terrain’ located 9°N, 87°E (1), is just south of the intersection of the Asterius Linea (2) and the (3) Agave Linea. The Linea are formed from one of the hypothesised mechanisms in Fig. 6.6, which are potential zones for bio signatures. The ‘Chaos Terrain’ is particularly interesting, as it is potentially indicative of a thin ice layer above a body of water, such as the subsurface ocean. The hypothesised mechanisms of ‘Chaos Terrain’ formation is illustrated in Fig. 6.5: with tiles a, d and e proposing a ‘wet theory’ whereby liquid water is near the surface, resulting in the formation of ice blocks analogous to icebergs. Tiles b and C propose a ‘dry theory’ whereby brine mobilisation is the chief driver for the ‘Chaos Terrain’ formation (Fig. 6.2), alongside a ‘slow diaipir’ where relatively warm ice rises due to its relative buoyancy (Fig. 6.1). The ‘wet theory’ is more conducive to identifiable bio signatures being present in the surface layer, and is supported by the raised elevation of the Conamara ‘Chaos Terrain’ which may well be a thin ice layer above the subsurface ocean. However, even in the diapir model of the ‘dry theory’ bio signatures may be identifiable, or potentially even in a dormant state. Schmidt et al., (2011); Head & Pappalardo, (1999); Pappalardo, McKinnon and Khurana, (2009).

Fig. 6.10: Conamara ‘Chaos Terrain’ of jumbled icy blocks, with potentially sulphuric compounds visible in red/brown. Immediately south of the intersecting Linea with visible potentially sulphuric compounds on ridges parallel to axial valley. Edited from: Svs.gsfc.nasa.gov, (2017)

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The Pwyll impact crater located at 26° S, 271°W is 26 km in diameter, and diametrically different in morphology to the Tyre crater: with both a raised central peak and continuous rim. Dark ejecta is. Finely fragmented ejecta in the form of discontinuous rays can be located up to 1000 km away. Pwyll is geologically young with an assumed age of ~2 Ma. Craters on Europa with a diameter <30 km all appear to adhere to this typical impact crater morphology, with an excavation depth of a few kilometres, whereas diameters of >30 km present signs of penetration to the subsurface ocean. Ejecta from the impact has an intra-ice source which may yield bio signatures from diapiric upwelling or liquid lenses in the ice. The sulphurous compounds associated with the ejecta is interesting for bio signature exploration as well as the ejecta that is in the Dyfed and Dyfed Regio Zones of ‘Chaos Terrain’ to the northwest and northeast respectively. Jpl.nasa.gov, (2017) Lpi.usra.edu, (2017) Harriss, K. and Burchell, M. (2017); Pappalardo, McKinnon and Khurana (2009).

Fig. 6.11: The Pwyll impact Crater has a 26km diameter, with observable raised central peak and continuous rim. Location 3 in Fig 6H. Lpi.usra.edu, (2017)

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7. Discussion & Mission Options

The fundamental objective of this mission is to identify traces of microbial astrobiology, either by: direct detection of astrobiological life in the subsurface of Europa, or, the detection of geochemical bio signatures indicative of astrobiology on Europa. The subsurface ocean is the most likely place to harbour life, with geological activity (volcanism) from tidal forcing resulting in the release of sulphurous material at the seafloor. This release of sulphurous material results in an ocean enriched in minerals, that is possibly conducive to chemosynthetic life, analogous to deep marine ecosystems propagating around black smokers on Earth. The superheated fluids rise and drive convection currents, resulting in mixing of ocean waters. Similarly, brine rich fluids form at the base of the ice shell and these may be another environment suitable to microbial life. Thus, for direct detection of astrobiology it may be necessary to penetrate through the thick (up to ~50 km) Europan cryospheric shell. Diapirism in the ice shell may bring oceanic material from the ocean/ice interface nearer to the surface of the ice, resulting in the potential identification of bio signatures in this elevated material - Diapir ice may be within ~5 km of the surface as seen from tomographic images. The formation of ‘Chaos Terrain’ is still debated, but the ‘wet theory’ would also indicate that ocean material has been brought to the surface or near surface and identifiable bio signatures may be found there. Similarly, the double ridges may be associated with subsurface material as there may be periodic volatile driven cryovolcanism and/or effusive emplacement of subsurface liquid driven by tidal opening of the cracks. ‘Chaos Terrain’ and ridges appear associated, as both have surficial deposits of sulphuric compounds which is potentially derived from the subsurface. The atypical crater structure of large impact craters on Europa (>30 km diameter), such as Tyre is indicative of the impact having occurred on ice situated above a liquid layer - be it the subsurface ocean, or subsurface lenses of water. These refrozen fluids now composing the surface multiring basins may potentially bear traces of subsurface astrobiology.

The first proposed mission is to place a spacecraft into orbit and engage in advanced spectral imaging and higher resolution mapping of the Europan surface, focusing particularly on the Conamara Chaos region and the Tyre crater and associated ejecta. In addition, the orbiter will attempt to confirm the existence and locate plumes to directly search for traces of life if possible by flying through them. For this mission option the applied sterilisation methods (Chapter 5.4) do not have to be in a high cleanliness level due to the fact the orbiter is not in physical contact with Europa. A standard procedure of sterilisation for spacecraft exterior is proposed.

The second proposed mission entails the deployment of a lander onto the Europan surface in addition to the orbiting spacecraft. The lander will be deployed to a relatively flat section of either special areas of interest: the Conamara Chaos Terrain or the Tyre Crater. It will then use the small telescopic drill to geochemically analyse samples from below the heavily radiated surface (1 - 10 metres). For this mission option the proposed sterilisation procedure should result in a high cleanliness level. A combination of chemical and physical methods (Chapter 5.4) should be applied to sterilise the lander, focusing specifically upon the drilling device to prevent contamination of the Europan surface and the acquired samples.

The third proposed mission entails the deployment of a lander equipped with clean deep drilling capabilities and a remotely operated vehicle (ROV), in addition to the orbiting spacecraft. The lander will be deposited to a relatively flat section of the Conamara Chaos Terrain, where it will drill through the relatively thin ice using a sterilised Icemole/laser hybrid. Upon reaching the base of the cryosphere the Icemole hybrid will detach the ROV Eel that will be equipped with a camera and mass spectrometers for geochemical and visual analyses. As the ROV Eel explores the brine deposits at the base of the ice, and most importantly the areas local to the hydrothermal activity at the seafloor, it will communicate via a ‘quad echo relay’: 1. ROV

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EEL - Icemole, 2. Icemole - Lander, 3. Lander - Orbiter, 4. Orbiter - Mission Control. For this mission option the proposed sterilisation procedure should achieve an equally high cleanliness level as that in the second mission option. A combination of chemical and physical methods should also be applied to sterilise the lander, the drilling device and especially the ROV Eel. For the the ROV Eel the sterilisation should be as high as possible in both the exterior and interior, due to the probability that the Eel will not be retrieved from below the cryosphere.

Table 7.1: The three proposed missions MISSION OBJECTIVES TRL LIKELIHOOD OF MISSION SUCCESS TIMESCALE

Orbiter Spectral 8 HIGH 0-2 YEARS Analysis, Plume Sampling

Lander Lander Surface 6 MEDIUM 5-10 YEARS Sampling

Subsurface ROV Subsurface ROV 3 LOW 20+ YEARS Eel Exploration

Table 7.2: Proposed instruments for the Orbiter Mission option ORBITER MISSION INSTRUMENTS TRL

PIMS (Plasma Instrument for Magnetic Sounding) 8

ICEMAG (Interior Characterization of Europa using 8 MAGnetometry)

MISE (Mapping Imaging Spectrometer for Europa) 8

ICEMAG (Europa Imaging System) 8

REASON (Radar for Europa Assessment and 8 Sounding: Ocean to Near-surface)

E-THEMIS (Europa THermal Emission Imaging 8 System)

MASPEX (MAss SPectrometer for Planetary 8 EXploration/Europa)

UVS (Ultraviolet Spectrograph/Europa) 8

SUDA (SUrface Dust Mass Analyzer) 8

The focus of the orbiter mission is the mapping of the Europan surface with several imagining systems and additionally radar and magnetometers, for remote subsurface exploration (Table 7.2). Even though plume samples may include more interesting geochemical information on the subsurface ocean, it is improbable to predict the exact timing and location of a plume, particularly with the fact that it currently takes 5 years to get

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to Europa. Due to the TRL (8) of these instruments of the orbiter mission could proceed within the next two years. (Jpl.nasa.gov, 2017)

Table 7.3: Proposed additional instruments for the Lander Mission option ADDITIONAL INSTRUMENTS FOR LANDER TRL MISSION

irMS (Isotope-ratio mass spectrometry) 4* (Analogous instrument used on Mars Rover = 9)

CRDS (Cavity Ring-Down Spectroscopy) 6 ISOTOPES

GCMS (Gas chromatography– mass spectrometry) 9 LIPIDS

CE (Capillary electrophoresis) 6

MICROSCOPE 9

TELESCOPIC SHALLOW DRILL (1-10 m) 7

CAMERA 9

In addition to the observations made by the orbiter, the lander mission will focus on the composition of the surface ice (1-10m) and particularly on the sulphurous compounds. The instruments will focus on the detection of isotopes, lipids and amino acids which may potentially indicate life as well as potentially dormant organisms. The mission has a TRL of 6 which means it could be achieved in the next 5-10 years.

Table 7.4: Proposed additional instruments for the Deep Drilling Mission option ADDITIONAL INSTRUMENTS FOR DEEP TRL DRILLING MISSION

irMS (Isotope-ratio mass spectrometry) 4* (Analogous instrument used on Mars Rover = 9)

CRDS (Cavity Ring-Down Spectroscopy) 6 ISOTOPES

GCMS (Gas chromatography– mass spectrometry) 9 LIPIDS

CE (Capillary electrophoresis) AMINO ACIDS 6

MICROSCOPE 9 DRILL - (ICE MOLE / LASER) 4* ROV EEL - (CE / MS) 3

ROV EEL - (CAMERA) 9

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The third and most complex mission includes a combination of an orbiter, lander, Icemole and an ROV. in addition to the previous missions, the focus is on the conditions of the subsurface ocean. To detect possible traces of life, or even directly detect active astrobiology, a sterilised drilling/melting device will be deployed to reach the subsurface ocean. Upon reaching the ocean the ROV will deploy, searching for amino acids and isotopes as the main priority in the ocean. Additionally, the ROV could search for lipids and organic material to increase the chances of finding life. The “quad echo relay” means that the hole in the ice does not have to remain constantly open. Furthermore, the ROV Eel is powered via its electrodynamic ‘power scavengers’ that harvest energy from the locally changing magnetic fields. This results in the timespan of the mission being dependent on the amount of time the lander and orbiter can withstand the extreme radiation of the Jovian system, which is promising, as the Galileo Orbiter lasted ~8 years. The TRL of this mission is 3 and therefore will take in excess of 20 years to proceed. (NASA, 2017); (The Verge, 2017); (Moore, 2017); (Jpl.nasa.gov, 2017).

These three missions will go some way toward answering the question of whether there is life on Europa. The Europan subsurface ocean is fundamentally the most promising location to finding astrobiology, both in active forms and through traces of life. The first two missions will focus upon the detection of traces left by life. Sampling plumes is a possible option, since it could contain traces of life that might have developed around the hydrothermal vents at the bottom of the ocean that has subsequently been transported to space via cryovolcanism. Probing the first few meters of ice could be another way to detect these traces, which might have been deposited by the plumes or been trapped in the ice at the ocean surface and transported upwards by the diapiric upwelling within the ice layers. The third mission in addition to the detection of traces of life, could identify active life forms. If astrobiology does indeed exist, or did exist in the subsurface ocean of the Galilean Moon Europa, these missions have a very high likelihood of success to prove their existence. .

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8. Reference List

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Wolman, Y., Haverland, W. and Miller, S. (1972). Nonprotein Amino Acids from Spark Discharges and Their Comparison with the Murchison Meteorite Amino Acids. Proceedings of the National Academy of Sciences, 69(4), pp.809-811.

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Gaidos, E. and Nimmo, F. (2002). Strike-slip motion and double ridge formation on Europa. Journal of Geophysical Research, 107(E4).

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December 2017

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Chapter 7

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