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Search for on the of

Leonie Schunk Mikko Pöntinen

Planetary , Autumn 2017 University of Helsinki

Lecturer: Tomas Kohout Course assistant: Juulia-Gabrielle Moreau

Search for life on the

Table of Contents

1. Introduction 3

2. Parameters for a habitable environment 5

3. Life conditions on the moons 7 9 9 10

4. Evaluation of geological maps of the moons 11 Maps of Enceladus 11 Map of Dione 12 Maps of Titan 13 Map of 14 Map of 14 Map of 15 Map of 15

5. Detecting life 16

6. Prospects of a successful mission to different moons 20

7. Mission options and instrumentation 21

8. Preliminary mission plan 23

9. Conclusions 24

References 25

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1. Introduction

In 1997 a , including a probe from NASA (this was a flagship from NASA) and a from ESA, was launched to study Saturn and its system. This - mission was the first one to enter its and only the fourth one that was sent to visit the . It spent 13 years orbiting Saturn, after a which took around seven years with four assists on , and , and a of . The mission ended in September 2017, when Cassini burned up in Saturn’s . The Huygens lander was able to on Saturn’s biggest Titan on January 14th 2005 with a parachute, which was very unique since it was the first time a landing on a moon other than our own was successful. The Cassini probe’s mission was extended several times and it also managed to fly through the .

The Cassini-Huygens mission was the only mission which focused on Saturn. Since Saturn has 62 moons, from which only 53 have names and 13 have a larger than 50 km, there are still a lot of open questions. The major moons, from largest to smallest, are Titan, Rhea, Iapetus, Dione, Tethys, Enceladus and Mimas. All of them have an ellipsoidal shape but only Titan and Rhea are at and the six moons besides Titan only constitute to 4% of the in orbit around the planet. Most of what we know about the Saturnian system is from this single mission.

Several of the Saturnian moons are known or suspected of having subsurface liquid . On Earth, everywhere there is water, there is life. One of the most important questions in is whether that is the case elsewhere in the too.

Since there is not enough information, especially no returned samples from the Saturnian system, our mission proposal will on searching for evidence of possible life on the icy moons, and in order to do that, collect more information about the system in general. During the course of the space mission, we will investigate some of the most interesting objects and evaluate the chance of finding certain indications of water-based life. The Cassini-Huygens mission was not equipped to find evidence for or complex organic compounds.

In order to plan the mission, stats and facts about the moons have to be collected. The biggest moon Titan is the second largest one besides Jupiter's moon and larger than the smallest planet . It has a radius of 2575 km, it 1,200,000 km above Saturn (~20 Saturn radii) and is the only moon with a dense atmosphere as well as the only object with stable bodies of surface liquid besides Earth.

Another moon is Enceladus, a relatively small one with a radius of 250 km, orbiting 240,000 km above Saturn (~4 Saturn radii). It is only 1/10 the radius of Titan and the sixth largest moon of Saturn. Enceladus is mostly covered by fresh, clean which makes it one of the most reflective bodies in our . It has a subsurface water .

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Dione is the fourth largest moon of Saturn, with a 561 km radius and orbiting 377,400 km above Saturn (6 Saturn radii) which is roughly the same distance from Earth to its moon. ⅔ of its mass is water ice and the rest is the dense core, probably silicate . Dione’s average surface is -186°C, so the ice on the surface behaves like a rock, same as on the other icy moons. Dione also likely has a water ocean beneath the icy surface.

Mimas only has a radius of 198 km which makes it the smallest of the seven major moons, orbiting above Saturn the closest out of the moons, with 186,000 km and because of its small it is not able to hold a perfectly round shape. The moon has a low , which suggests that the moon only consists mostly of water ice. Its most distinguishing surface feature is a giant .

Rhea i​s the second-largest of Saturn's moons. The distance to Saturn is 5​27,040 km and Rhea has a radius of 764 km. The average surface temperature is -174°C. The moon has a typical heavily cratered surface and, similar to Dione, a wispy on one of the hemispheres and two large impact basins. Rhea’s composition, its and surface structure in general is similar to that of Dione which probably means they went through the same phases of development.

Iapetus is Saturn’s third-largest moon with a radius of 736 km and it orbits the furthest away from the massive planet at a mean distance of 3,561,000 km. The larger distance could explain why the moon has not been affected as much by melting processes like the moons closer to Saturn. Its unique due its high dark and contrast in the albedo.

Tethys, with a radius of 533 km, is similar to Rhea and Dione except for its surface features, since it does not nearly have as many impact craters as the other two moons. It is Saturn’s fifth-largest moon and is supposed to consist mainly of water ice and a small part of silicate rock because of its low density and high albedo.

Figure 1: The Saturnian system. The major moons are shown, except for Titan and Iapetus, which fall outside the image.

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2. Parameters for a habitable environment

In order for known forms of life to exist, certain conditions need to be met. The most important restricting parameters are shown in Table 1, which focuses on requirements of known microbes. Temperature has to be within a certain range, namely between around -20 and +122°C. Microbes can be in hibernation in lower , but a temperature higher than -18°C is needed for active and reproduction. Larger multicellular organisms, which can regulate their body temperature, can survive in even lower external temperatures. Since pH, pressure and salinity have a high survivable range for simple organisms, they are not as limiting factors as temperature and . More concerning is the radiation issue, especially UV radiation, and if a planet or moon does not have an atmosphere, said radiation can get relatively high on the surface and requires special adaption from possible life forms. Known life needs so called CHNOPS-elements, in other words , , , , and sulphur. All known life forms also require liquid water, which is the main parameter to look for.

Table 1: Most important parameters regarding the survival of microbial life. PARAMETER RANGE REMARK

TEMPERATURE -20 - 122°​C​ Upper limit solubility of lipids in water/ stability, Psychrophiles live up to -20 but can survive in a ‘dormant’ mode at much lower temperatures ~196°​C​ . [​C] pH From 0 to >11 Life known to survive at pH: (0) Cyanidium, Archaea - Natronobacterium, Protists (>11). [​A, D]

ENERGY Chemical redox from: Geothermal flux can arise from Chemoautotrophic Life: (i​)​ the planet cooling off from - geothermal processes its gravitational heat of - M​ ​required for formation, (i​i)​ decay of chemosynthesis long-lived radioactive Photoautotrophic Life: elements, or (i​ii)​ - light from central 0​.01 for a close-orbiting or μmol m​-2​⋅s​-1 ​minimum amount moon. for photosynthesis Both chemoautotrophic and - Carbon/Oxygen required for photoautotrophic photosynthesis . obtain their energy and produce their nutrients from simple inorganic compounds such as carbon

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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 ~2 GPa likely to be a limitation before the pressure (for liquid water). [A]

UV RADIATION Up to 5,000 J​/​m​2 Significant radiation if no atmosphere. [​A]

IONIZING RADIATION Upper limit of ~6000 Grays Microorganisms capable of An does not require a withstanding very high levels to be habitable of radiation. [​A]

LIQUID H2O (SURFACE) Liquid H2​​O should be present Life could exist on a planet but some halophilic organisms with ‘supercritical’ carbon live in high (4-5 Mol) NaCl dioxide i.e. existing as a and a liquid. [​A, C]

NITROGEN Aerobic microorganisms In the reducing conditions of (C​ HNOPS)​ require a minimum of 1–5 × the outer Solar System N is -3 ​​ 10​ N2​​ for present as which is fixation. also biologically usable. [​B, C]

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

SALINITY Up to 25-33% Microbes and algae, in saturated chloride. ​[A]

Reference shortcuts used in Table 1: [A​ ]​ P​ appalardo, McKinnon and Khurana, 2009 [B​ ]​ h​ttps://www.space.com/26189-alien-life-requirements-exoplanet-search.html [C​ ]​ M​ cKay, 2014 [D]h​ttps://nai..gov/media/medialibrary/2015/10/NASA_Astrobiology_Strategy_2015_151 008.pdf ​pp 59.

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3. Life conditions on the moons

Of the Saturnian moons, Enceladus, Dione and Titan show the most promise for a habitable environment.

Enceladus is a small moon, with a radius of only around 250 km. Nevertheless, it has a global liquid water ocean beneath its ice shell (Thomas et al. 2016). The ocean is likely in direct interaction with a rocky core, increasing the potential for energy and nutrition sources in the ocean. With the current models, possibly the only limiting factor for life is the possible high pH of the ocean, which could be quite close the upper limits of known Earth-based life forms . It has a variety of surface features such as tectonically deformed , and old, heavily cratered regions and has a liquid water ocean beneath its 5-30 km cover of ice. The existence of the ocean is known through gravity, shape and data from the Cassini mission. Enceladus has also plumes of water flying into space from its southern polar region, and these plumes are the source of Saturn’s E ring. Around 20 gigawatts of heating power is required to keep the ocean liquid and generate the plumes, and according to recent models, most of the heat could be generated by tidal heating in a porous rocky core of Enceladus, which has water circulating within the core (Choblet et al. 2017).

From gravity and shape measurements, Dione is also suspected of having a liquid water ocean, but it would be beneath an ice layer of approximately 100 km thick (Beuthe et al 2016). On the other hand, the pressure would not be so high that high-pressure ice starts to form on the bottom, so interaction between the ocean and a rocky core is a possibility. The moon also has very prominent large craters all over its surface, up to 100 km in diameter.

From shape, topography, gravity anomalies and tidal deformations, it is derived that Titan has a liquid water ocean too, but like on Dione, beneath up to 100 km of ice (Mitri et al. 2014). A more serious problem is the very high pressure in the Titanian ocean, which causes formation of high-pressure ice in the bottom of the water layer, severely limiting or completely removing the interactions between the ocean and the rocky core. This aspect reduces the potential energy and nutritional sources in the ocean, making it less likely for water-based life to survive. Titan has a thick atmosphere and also and consisting of and ethane located at the polar areas. The surface temperature is around -180°C, which means no known life forms can survive there. However, the possibility of some exotic methane-based life in the has been suggested (McKay & 2005, Stevenson et al. 2015). The of Titan is composed of water ice and rock. The atmosphere consists mostly of nitrogen, and methane and ethane which conclude to a methane cycle which is analogous to the on Earth. As a result, there are seasons, as well as huge methane and the atmosphere has a thick orange of . The surface features include dried up riverbeds, lakes, impact craters, mountains and several possible cryovolcanoes.

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Figure 2: After the Huygens probe collected information about Titan’s atmosphere it revealed parallels and contrasts with that of Earth, since both are nitrogen-dominated but the differences are due to Titan’s much lower temperature and the methane in its atmosphere. (C​ redit: )

Tables with more details about the possibly habitable zones on the aforementioned three moons are below. The parameters outside the range of known life forms are marked in r​ed,​ parameters around the limits of the acceptable values are marked in y​ellow,​ and parameters clearly within the hospitable range for life are marked in g​reen.​ According to the values of separate parameters, a suggestive general judgment is given to the layers, in the form of color red, yellow or . For example, the surface of Enceladus is marked in red, because it is too for life, the pressure is 0 and there is no liquid available. The ice crust of Enceladus is marked in yellow, because although the values are generally within acceptable ranges, the potential for energy sources and liquid water are uncertain. The actual Enceladian ocean is marked green, because all the values are within favorable ranges, with the possible exception being the pH, which could in some models be close to the upper edge of habitable. According to models by Glein et al. (2015), the pH of the ocean could be between approximately 11-12, but it should be noted that other models have suggested pH ranging from 5.7-6.8 (Marion et al 2012), 8.5-9 (Postberg et al 2009), 8.5-10.5 (Hsu et al. 2015) and 10.9 (Zolotov 2007), so the certainty of the actual pH is not very high. All in all, according to the tables, the most promising places to look for water-based life are in the oceans of Enceladus and Dione.

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Enceladus Radius 250 km Ice thickness 5-30 km [​1] Semi-major axis 238 000 km Ocean depth 20-70 km [​1] 1.370 d Core radius 150-225 km Gravity 0.113 m/s​2 Mean density 1.609 g/cm​3 Zone T pH Energy Pressure Radiation Salinity Liquid

Surface -240 - - 0 0-5% No -130°C

Ice -240 - - Chemosynthesis 0-30 ~0 0-5% Some crust 0°C water?

Water ~0°C 11-12 Chemosynthesis 5-100 bar ~0 0-5% Water ocean (6-12) [3] [2]

Rocky 0-1000°C - Chemosynthesis >100 bar - Some core water? Table 2: Zones of Enceladus and the known parameters.

Dione

Radius 560 km Ice thickness ~100 km Semi-major axis 377 396 km Ocean depth ~50-100 km Orbital period 2.736 d Core radius ~350-400 km Gravity 0.232 m/s​2 Mean density 1.478 g/cm​3 Zone T pH Energy Pressure Radiation Salinity Liquid

Surface -220°C - Sun 0 0-5% No

Ice -220 - - Chemosynthesis 0-200 bar ~0 0-5% Some 0°C water?

Water ~0°C Not Chemosynthesis 200-400 ~0 0-5% Water ocean known bar

Core >0°C - Chemosynthesis >400 bar - Some water? Table 3: Zones of Dione and the known parameters.

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Titan Radius 2575 km depths 0-200 m Semi-major axis 1 221 870 km Ice thickness ~100 km [​4] Orbital period 15.945 d Ocean depth ~250 km [​4] Gravity 1.352 m/s​2 Ice V/VI thickness 100-250 km [​4] Mean density 1.8798 g/cm​3 Core radius 2000-2150 km [​4] Zone T pH Energy Pressure Radiation Salinit Liquid y

Atmosphere -180°C - Chemo- 0-1.5 bar - Some synthesis, ethane, Sun methane

Lakes -180°C - Chemo- 1.5-3 bar ~0 - Ethane, synthesis methane

Ice -180 - - Chemo- 1.5 - ~0 Some -20°C synthesis >1000 bar ethane, methane, water?

Water -20°C Not 1000 - ~0 Up to Water ocean known 4000 bar 35%

High - 4000 - ~0 No pressure ice 8700 bar

Silicate core - Chemo- 8700 - - No synthesis 49000 bar Table 4: Zones of Titan and the known parameters.

Reference shortcuts used in tables 2,3 and 4: [1] Van Hoolst et al. 2016 - The diurnal libration and interior structure of Enceladus [2] Glein et al. 2015 - The pH of Enceladus' ocean [3] Beuthe et al. 2016 - Enceladus’s and Dione’s floating ice shells supported by minimum stress isostasy [4] Forter 2012 - Titan’s internal structure and the evolutionary consequences

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4. Evaluation of geological maps of the moons

The following maps show the surfaces of the three most promising moons Enceladus, Dione and Titan, and as a reference the maps of the rest of the major Saturnian moons Mimas, Tethys, Rhea and Iapetus are shown as well.

Maps of Enceladus

Geologically interesting places are the uncratered areas, which are . The ice is thinnest on the pole, which also has water plumes coming through the ice and flying to space from the “tiger stripe” (light blue lines in maps) areas. For landers and flybys, the south polar region is the area to focus on.

Figure 3: Surface map of Enceladus. (h​ttp://www.unmannedspaceflight.com/index.php?act=attach&type=post&id=26483)​

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Figure 4: Maps of Enceladus viewed from (left) and (right). (h​ttps://rightbasicbuilding.files.wordpress.com/2015/01/enceladus_foldableglobe_clark2015.j pg)​

Map of Dione

Dione has a density of 1.48 g/cm3​​, and it has a rocky core. Most of the surface is cratered and old, but certains areas appear younger, possibly due to cryovolcanism. According to shape and gravity measurements, Dione probably has an ocean underneath 100 km of ice.

Figure 5: Surface map of Dione. (h​ttp://stevealbers.net/albers/sos/sos.html)​

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Maps of Titan

Interesting places on Titan are the liquid lakes, which are mostly located on the northern polar region. The south polar region also has lakes, but not as large ones.

Figure 6: map of Titan. (h​ttps://saturn.jpl.nasa.gov/system/resources/detail_files/7492_PIA20713_browseimage.jpg)​

Figure 7: Maps of Titan viewed from north pole (left) and south pole (right). The seas and lakes are marked in red. The blue lines mark a lake that visibly changed between two observations a year apart. (h​ttps://commons.wikimedia.org/wiki/File:Titan_2009-01_ISS_polar_maps.jpg)​

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Map of Mimas

Mimas has a density of only 1.15 g/cm3​,​ which means it is mostly ice and has little rock. The surface is filled with craters, which means it is very old. It seems unlikely that Mimas could have an ocean.

Figure 8: The surface map of Mimas. ​(h​ttp://stevealbers.net/albers/sos/sos.html)​

Map of Tethys

Tethys has a density of only 0.98 g/cm3​,​ which means it’s almost completely water ice. The surface is cratered and old, but there are some less cratered areas, suggesting younger surface on those areas. Tethys could have had an ocean at some point, which then froze and expanded, creating a large , named Ithaca .

Figure 9: The surface map of Tethys. (h​ttp://stevealbers.net/albers/sos/sos.html)​

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Map of Rhea

Rhea has a density of 1.24 g/cm​3,​ which means it has around 25% rock, but it is unclear whether Rhea has a rocky core or not. Surface is mostly cratered and ancient. A subsurface ocean is unlikely.

Figure 10: The surface map of Rhea. (h​ttp://stevealbers.net/albers/sos/sos.html)​

Map of Iapetus

Iapetus has a density of 1.09 g/cm​3,​ so it is mostly water ice. Surface is strikingly two-toned due to different albedo rates. An ocean is unlikely.

Figure 11: The surface map of Iapetus. (h​ttp://stevealbers.net/albers/sos/sos.html)​

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5. Detecting life

When going to to search for water-based life, there are certain substances a mission can look for and those indicate which instruments should be used. In Table 5, the properties of the different substances show what they consist of and then an explanation why they could potentially be important. Table 6 shows the sensitivities and technical readiness levels of the instruments. The amino acids which are used by life forms on Earth can be divided into abiogenic and biogenic and these are shown in Table 7.

Table 5: List of biomarkers and signs of life, what they show and with which instruments the substances can be detected. Substance Properties Reasoning Instrument

Methane Amount Many forms of life -ratio Isotope ratios produce methane. C in Cavity Ring-Down biogenic methane [g] have different ratio than in abiogenic C.

Hydrogen, Amount Best source of Measured already by Cassini, energy for life on depletion of both hydrogen and

(Titan) Titan is C2​​H2​ + 3H2​ acetylene was found [​b]​ [​c] [d]

-> 2CH4​​, which depletes hydrogen and acetylene [​a]

Amino acids Amount Known lifeforms Capillary electrophoresis Chirality use amino acids of (amount and chirality) [​e] only one chirality Gas chromatography– mass spectrometry (amount) [​f]

Liquid chromatography– mass spectrometry (chirality) [​f]

Raman spectrometry

DNA/RNA Amount Direct sign of life Spectrography

Immunoassay

Polymerase chain reaction

High precision mass spectrometry

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[f]

Raman spectrometry

Lipids Amount Lipids are essential Gas chromatography– Length of for known lifeforms mass spectrometry [​f] carbon chain Raman spectrometry

Cells/Organisms Amount Direct detection of Microscope Type lifeforms

Reference shortcuts used in Table 5: [a] McKay & Smith 2005 - Possibilities for methanogenic life in liquid methane on the surface of Titan [b] Strobel 2010 - Molecular hydrogen in Titan's atmosphere: Implications of the measured tropospheric and thermospheric mole fractions [c] et al. 2010 - Detection and mapping of hydrocarbon deposits on Titan [d] h​ttps://www.jpl.nasa.gov/news/news.php?release=2010-190 [e] h​ttps://.nasa.gov/news/a-new-test-for-life-on-other-planets/ [f] h​ttps://astrobiology.nasa.gov/research/life-detection/ladder/ [g] h​ttps://www.picarro.com/technology/cavity_ring_down_spectroscopy

Table 6: The sensitivities and technological readiness levels (TRL) of the instruments Instruments Sensitivity TRL

CE [b] <200 attomoles/ 80 zeptomoles (gefärbte DNA) [f] 6 [n]

CRDS 10​−29 ​cm/molecule [g] 6 [j]

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, sensitivity: 4 3x10​-12)​ [e], [a] (similar instrument on Curiosity TLS. Level 9 [l])

LCMS [d] 5​ ppm 5 (2012)

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

PCR Any part of single DNA segment (in theory) 8 [m] 1 pg - 1 μg per 50 μl of sample (in practice) [i]

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Reference shortcuts used in Table 6: [a]h​ttps://assets.thermofisher.com/TFS-Assets/CMD/brochures/BR-30293-IRMS-253-Ultra-B R30293-EN.pdf ​In Isotope Ratio MS the result of a single analysis is basically meaningless. The information hidden in isotope ratios can only be accessed in comparison to other isotope ratios [b] h​ttps://www.agilent.com/cs/library/primers/Public/5990_3777EN.pdf ​(page 30?) [c] Cao et al. 2014 (h​ttp://www.tandfonline.com/doi/abs/10.1080/19440049.2014.980855)​ [d] Hines et al. 2015 (h​ttp://clinchem.aaccjnls.org/content/61/7/990.full)​ [e] h​ttps://ai2-s2-pdfs.s3.amazonaws.com/8f57/d82b920f79d9ed8f01c1d5ac743ff5f16feb.pdf [f] h​ttp://genome.cshlp.org/content/6/9/893.full.pdf [g] h​ttp://www.sciencedirect.com/science/article/pii/S0022407309001630 [h]h​ttps://www.thermofisher.com/de/de/home/life-science/protein-biology/protein-biology-lear ning-center/protein-biology-resource-library/pierce-protein-methods/overview-mass-spectrom etry.html [i]h​ttps://www.neb.com/protocols/1/01/01/taq-dna-polymerase-with-standard-taq-buffer-m027 3 [j] WANG 2010 h​ttp://ascelibrary.org/doi/abs/10.1061/41096%28366%29149 [k] PALMER et al. 2001 https://www.sciencedirect.com/science/article/pii/S1044030501002495 [l] h​ttps://ssed.gsfc.nasa.gov/sam/samiam.html [m] NICHOLS 2016 https://www.neb.com/tools-and-resources/feature-articles/first-successful-pcr-experiment-per formed-in-space [n] KIM et al. 2014 h​ttp://www.rsc.org/images/loc/2014/PDFs/Papers/011_0515.pdf [o]h​ttp://an.rsl.wustl.edu/mer/help/Content/About%20the%20mission/MER/Instruments/Micro scopic%20Imager.htm

Table 7: 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 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, and thus are potential markers for life. Abiogenic (red) / biogenic (green) Carbon number

Alanine Abiogenic (, Beta) [b] 3

Arginine Biogenic, not found on meteorites [f] 6

Asparagine Abiogenic, but unstable? [c] 4

Aspartic Acid Abiogenic [b] 4

Cysteine Abiogenic, but not found on meteorites 3 [d]

Glutamic Acid Abiogenic [b] 5

Glutamine Abiogenic, but unstable? 5

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Glycine Abiogenic [b] 2

Histidine Abiogenic [e] 6

Isoleucine Abiogenic [g] 6

Leucine Abiogenic [h] 6

Lysine Biogenic, not found on meteorites [i] 6

Methionine Abiogenic [j] 5

Phenylalanine Abiogenic [k] 9

Proline Abiogenic [a] [b] 5

Serine Abiogenic [l] 3

Threonine Abiogenic [a] [m] 4

Tryptophan Biogenic, not found on meteorites [n] 11

Tyrosine Abiogenic [o] 9

Valine Abiogenic [b] 5

Reference shortcuts used in Table 7: [a] h​ttp://www.pnas.org/content/108/14/5526.full [b] h​ttp://www.pnas.org/content/69/4/809.full.pdf [c]h​ttps://atlasofscience.org/l-amino-acids-key-for-the-evolution-of-life-came-from-extraterrest rial-space/ [d] h​ttps://link.springer.com/referenceworkentry/10.1007%2F978-3-642-11274-4_382 [e] h​ttp://www.sciencedirect.com/science/article/pii/S1387380603003051 [f] h​ttps://link.springer.com/referenceworkentry/10.1007/978-3-642-11274-4_104 [g] h​ttps://link.springer.com/referenceworkentry/10.1007/978-3-642-11274-4_821 [h] h​ttps://link.springer.com/referenceworkentry/10.1007/978-3-642-11274-4_876 [i] h​ttps://link.springer.com/referenceworkentry/10.1007/978-3-642-11274-4_911 [j] h​ttps://www.ncbi.nlm.nih.gov/pubmed/21063908 [k] h​ttps://link.springer.com/referenceworkentry/10.1007/978-3-642-11274-4_1180 [l] h​ttps://link.springer.com/referenceworkentry/10.1007/978-3-642-11274-4_1428 [m] h​ttps://www.ncbi.nlm.nih.gov/pmc/articles/PMC5428853/ [n] h​ttps://link.springer.com/referenceworkentry/10.1007%2F978-3-642-11274-4_1619 [o] h​ttps://link.springer.com/referenceworkentry/10.1007/978-3-642-27833-4_1624-5

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6. Prospects of a successful mission to different moons

As seen from Table 8, in the Saturnian system Enceladus is the best option for studying and finding the potential due to its best habitable zone and ease of access to ocean samples even from orbit via geysirs. Dione is another place where known life forms might be able to survive, but getting access to them is unlikely due to the 100 km thick ice cover over the ocean, even with a lander. Titan is interesting and it is easy to land there with a heat shield and parachutes through the thick atmosphere. The problem is that survival aspects of water-based life are very slim due to the cold surface temperature and the isolation of the ocean between two layers of ice. Titan could, theoretically, sustain exotic, methane-based life forms on its surface and its lakes, but their existence is more speculative and less likely. Also, if searching for hypothetical, unknown forms of methane-based life, it is quite unclear what parameters to actually measure.

Table 8: Prospects of a mission sent to the different moons of Saturn. Coloring follows the same logic as before, with green marking positive, yellow mediocre or speculative and red negative parameters. Habitable Geysirs Young Likelihood of Likelihood of environment surface reaching reaching material with material in orbit lander

Mimas (r = 200 km) No No No Low Low

Enceladus (250 km) Ocean Yes Yes High High

Tethys (530 km) No No Medium Low Low

Dione (560 km) Ocean No Medium Low-Medium Low

Rhea (760 km) No No No Low Low

Titan (2575 km) Speculative No Yes Speculative Low

Iapetus (735 km) No No No Low Low

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7. Mission options and instrumentation

For the reasons mentioned in chapter 6, Enceladus is the main target of our mission proposal. It has probably the best habitable conditions and getting access to the ocean water is relatively easy due to the plumes. It is possible to acquire ocean samples from the space above Enceladus’ south polar region without needing a lander.

Targeting the plumes gives us basically two options: 1) Elliptical orbit around Saturn, from which multiple flybys of Enceladus are conducted 2) Direct orbit around Enceladus

The flyby option from Saturnian orbit requires less ∆v, which makes it possible to either use a smaller launch vehicle, have a heavier spacecraft with more instruments, take a more direct and faster route from Earth to Saturn, or even go for a sample return mission. The negative side of a flyby-style mission is the higher relative velocity to Enceladus. On its Enceladus flybys, Cassini had velocities ranging from approximately 6 km/s to 18 km/s. These high velocities make it hard to acquire samples for the on-board instruments. Collecting samples with a sample collector for a sample return mission is a bit easier, and, for example, Stardust mission managed to collect samples from 81P/Wild-2 with a flyby velocity of 6.1 km/s with an aerogel sample collector. Nevertheless, a lower velocity would be preferred in order to limit the impact damage experienced by the samples. Cassini was in an orbit with high eccentricity, which caused the high relative velocities. Lowering the flyby velocities to somewhere below 6 km/s is possible by lowering the apoapsis of the spacecraft in the Saturnian orbit.

Getting into an orbit around Enceladus makes collecting samples much easier due to the low orbital velocity of approximately 150 m/s. With this velocity it’s possible to get virtually undamaged samples for the on-board instruments. The downside is the higher ∆v requirement, which makes a sample return mission quite impracticable to conduct.

In short, conducting flybys of Enceladus from an orbit around Saturn is a better option for a sample return mission but worse for in-situ measurements. Correspondingly, getting into an Enceladian orbit is a better choice for in-situ measurements, but not very feasible for a sample return mission.

The chosen instruments for either type of mission are shown in Table 10. Measuring the amino acid profile of the samples is one of the main goals, because detecting an excess of one chirality of amino acids or the presence of arginine, lysine or tryptophan would be a very strong sign of life. Another strong sign would be a clearly different carbon isotope ratio in methane compared to some other carbon based molecule, such as , or a bimodal distribution of carbon isotopes, because this would indicate that the methane in question would be produced by biological processes. Of course, a direct detection of DNA or

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even a complete organism would be a direct sign of life, but they are less likely to be found from the acquired samples.

Table 9: General information of the possible missions to the Saturnian system. TRL highly depends on which objects the lander would try to land, on Titan it’s up to 9 due to the already successful Huygens probe, on the other moons much lower. Lander is here just for theory, since our concentrates on an flyby/orbiter mission. Mission Objectives TRL Likelihood of Timescale success

Orbiter/flyby Plume samples for 6-9, depending on Medium-High 0-5 years in-situ analysis or instruments sample return

Lander Surface samples (3-9) Low-Medium 7-10 years

Table 10: Chosen instruments for the flyby/orbiter mission and information about the functionality and Technical Readiness Level. Instruments for Function TRL flyby/orbiter mission

High Precision Mass Detect amounts of amino acids, lipids and even 9 Spectrometer more complex molecules like DNA

Laser Spectrometer Detect amounts of isotopes of carbon 9

Capillary electrophoresis Detect amounts and chiralities of amino acids 6

Microscope Detect organisms or their remnants directly 9 (although not very likely)

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8. Preliminary mission plan

Start of the mission: - Launch - Depending on the launch vehicle and mission profile, either direct transfer to Saturn (~6 years transit time), or with Venus/Earth/Mars/Jupiter gravity assists (>6 years transit) - Getting to elliptic orbit around Saturn

Flyby option: - Flybys of Enceladus from Saturnian orbit, collecting plume material above south polar area with both sample return collector and on-board instruments - Flybys of other moons - Either the whole spacecraft or only the sample collector is sent to Earth for detailed study - In the case of only the sample collector sent to Earth, the rest of the spacecraft is collided with Saturn, in order not to contaminate Enceladus with Earth-based life - The sample collector or whole spacecraft arrives to Earth, where it might be first studied in in order not to contaminate Earth with possible Enceladian life

Orbiter option: - Flybys of other moons - Getting to orbit around Enceladus - Collecting plume material and conducting in-situ measurements - The spacecraft is collided with Saturn or with some of the dead moons such as Tethys, depending on the available ∆v, in order to avoid contaminating Enceladus with Earth-based life

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9. Conclusions

The Saturnian system has several moons with potentially habitable environments, namely Enceladus, Dione and Titan. All the three moons are strongly suspected of having global liquid water oceans beneath their surfaces, with the Enceladian ocean being most reachable, while the oceans of Dione and Titan are much deeper and more isolated. Titan has also some speculative chance of exhibiting unknown, exotic methane-based life forms on its hydrocarbon lakes.

Our mission proposal focuses on Enceladus and its plumes, which make it possible to collect samples from the subsurface ocean without a need for a lander. There are two different options for how to investigate the samples, the first one being a sample return mission. This would involve several flybys over the south polar region of Enceladus from an elliptic orbit around Saturn, collecting samples with an aerogel sample collector. The flyby velocities would need to be 6 km/s or preferrably much less, in order to minimize the impact damage experienced by the samples during collecting. After collecting, the samples would be sent back to Earth for detailed study. The second mission option is an in-situ mission, which would have the spacecraft on an orbit around Enceladus, where samples would be collected with a much more manageable orbital velocity of around 150 m/s and then studied with the on-board instruments.

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