Exobiology in the Solar System & The Search for Life on Mars - ESA

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I.5 Potential Non-Martian Sites <strong>for</strong> Extraterrestrial <strong>Life</strong> <strong>The</strong> outer regions of <strong>the</strong> icy satellites have water (albeit frozen) <strong>in</strong> abundance. One of <strong>the</strong> necessary conditions <strong>for</strong> life is thus met. We can conceive of life exist<strong>in</strong>g on or <strong>in</strong> an icy satellite provided <strong>the</strong>re are sites where nutrients and an energy source are available. Hot vents on Earth’s deep ocean floors (see I.3.2.1) are sites where bacteria and bacteria-like organisms (<strong>the</strong> hyper<strong>the</strong>rmophilic archea) feed off <strong>the</strong> supply of chemical energy, and are entirely <strong>in</strong>dependent of energy from <strong>the</strong> Sun. <strong>Life</strong> on Earth may have begun <strong>in</strong> such a sett<strong>in</strong>g, perhaps at bubble membranes between alkal<strong>in</strong>e spr<strong>in</strong>g-water and acidic sea-water (Russell & Hall, 1997). Many of <strong>the</strong> icy satellites probably once hosted comparable environments, and a few may still do so today. Given an adequate source of heat with<strong>in</strong> an icy satellite, we would expect water to be drawn down <strong>in</strong>to <strong>the</strong> rocky core, to emerge elsewhere as a hot sal<strong>in</strong>e fluid bear<strong>in</strong>g many constituents dissolved from <strong>the</strong> rock. <strong>The</strong>se could act both as nutrients and as an energy source to susta<strong>in</strong> life, <strong>in</strong> <strong>the</strong> same way as <strong>the</strong> effluent at hydro<strong>the</strong>rmal vents on <strong>the</strong> Earth’s deep ocean floors. <strong>The</strong>re is a grow<strong>in</strong>g body of evidence that life was established on Earth no more than 700 million years after <strong>the</strong> planet’s <strong>for</strong>mation (Mojzsis et al., 1996; Schopf, 1993), and so <strong>the</strong> limited duration of comparable conditions on most icy satellites cannot be used as an objection to <strong>the</strong> development of life <strong>in</strong> those sett<strong>in</strong>gs. Photosyn<strong>the</strong>sis is a less credible energy source of life <strong>in</strong> icy satellites, <strong>for</strong> two reasons: 1) <strong>the</strong> distance from <strong>the</strong> Sun means that each square metre of Jupiter’s satellites receives only 4% <strong>the</strong> amount of solar power <strong>in</strong>cident on Earth; 2) it would be viable only <strong>in</strong> <strong>the</strong> uppermost few metres of ice, a long way from any easily replenishable source of nutrients (unless <strong>the</strong>se could be supplied by cometary <strong>in</strong>fall). <strong>The</strong> latter drawback is least serious <strong>in</strong> those bodies undergo<strong>in</strong>g (or have recently undergone) resurfac<strong>in</strong>g by processes capable of br<strong>in</strong>g<strong>in</strong>g fresh nutrients from below (e.g. <strong>in</strong> <strong>the</strong> <strong>for</strong>m of m<strong>in</strong>erals dissolved <strong>in</strong> br<strong>in</strong>es), so whe<strong>the</strong>r search<strong>in</strong>g <strong>for</strong> hydro<strong>the</strong>rmally-susta<strong>in</strong>ed or photosyn<strong>the</strong>tic life it is <strong>the</strong> active or recently active icy satellites that are <strong>the</strong> primary targets. I.5.1.1 Europa It is very likely that Europa is heated by tidal <strong>for</strong>ces at present. Europa’s 2:1 orbital resonance with Io is <strong>the</strong> cause of <strong>the</strong> tidal heat<strong>in</strong>g that makes Io <strong>the</strong> most volcanically active body <strong>in</strong> <strong>the</strong> <strong>Solar</strong> <strong>System</strong>, and we expect a smaller, but far from negligible, rate of heat<strong>in</strong>g with<strong>in</strong> Europa. Dissipation of tidal energy with<strong>in</strong> Europa’s rocky layer would require heat to be transported to <strong>the</strong> surface through its ice/water shell. <strong>The</strong> Voyager images showed very few impact craters on Europa’s surface, confirm<strong>in</strong>g recent (probably cont<strong>in</strong>u<strong>in</strong>g) resurfac<strong>in</strong>g by cryovolcanic and tectonic processes. Galileo images have revealed more small impact craters, but too few to demonstrate that resurfac<strong>in</strong>g has ceased (Belton et al., 1996). Europa has <strong>the</strong> th<strong>in</strong>nest icy carapace of any large icy satellite. In <strong>the</strong> absence of seismic data, its thickness cannot be determ<strong>in</strong>ed with certa<strong>in</strong>ty. Density arguments depend on <strong>the</strong> size of Europa’s metallic core and <strong>the</strong> extent of hydration of <strong>the</strong> outer part of <strong>the</strong> rocky layer. An estimate based on <strong>the</strong> first two Galileo encounters with Europa has an ice (or ice plus water) shell 100-200 km (Anderson et al., 1997). <strong>The</strong> existence of a global water ocean between Europa’s ice and <strong>the</strong> rocky <strong>in</strong>terior is controversial, but Galileo’s high-resolution images (Figs I.5.1.1/1 and I.5.1.1/2) make it very hard to deny at least <strong>the</strong> temporary occurrence of localised melts with<strong>in</strong> or below <strong>the</strong> icy layer. <strong>The</strong> f<strong>in</strong>al scale of <strong>the</strong> texture of some of <strong>the</strong> ridge and groove patterns can be used to argue <strong>in</strong> favour of th<strong>in</strong>-sk<strong>in</strong>ned tectonics (with a de<strong>for</strong>m<strong>in</strong>g I.5.1 <strong>The</strong> Icy Satellites 65
SP-1231 Fig. I.5.1.1/1. A 34×42 km area of Europa imaged by Galileo, where ice floes have apparently drifted apart, ei<strong>the</strong>r float<strong>in</strong>g on a liquid sea or lubricated at <strong>the</strong>ir bases by a mushy icy layer (P-48526, NASA). 66 layer of <strong>the</strong> order of 1 km thick), but clearly <strong>the</strong> ice is generally much thicker than this. <strong>The</strong> most likely sites <strong>for</strong> extant life would be at hydro<strong>the</strong>rmal vents below <strong>the</strong> most recently resurfaced areas. To study <strong>the</strong>se directly would require mak<strong>in</strong>g a borehole through <strong>the</strong> ice <strong>in</strong> order to deploy a robotic submersible. <strong>The</strong> technology <strong>for</strong> such submersibles already exists, but <strong>the</strong> means of penetrat<strong>in</strong>g ice of unknown thickness poses a greater challenge. However, biological processes <strong>in</strong> and around hydro<strong>the</strong>rmal vents could produce biomarkers that would appear as traces <strong>in</strong> cryovolcanic eruptions and <strong>the</strong>reby be available at <strong>the</strong> surface <strong>for</strong> <strong>in</strong> situ analysis or sample return. M<strong>in</strong>eral nutrients delivered through cryovolcanic eruption would make <strong>the</strong> same locations <strong>the</strong> best candidates <strong>for</strong> photosyn<strong>the</strong>tic life. I.5.1.2 Ganymede Large tracts of Ganymede have been resurfaced by some k<strong>in</strong>d of tectonovolcanic process. Galileo images reveal that <strong>the</strong> ridge and groove patterns are repeated on a scale an order of magnitude smaller than that of those discovered by Voyager (Belton et al, 1996). However, <strong>the</strong> density of superposed impact structures even of Ganymede’s youngest terra<strong>in</strong>s <strong>in</strong>dicates that this activity ceased long ago (probably more than 1 Gyr). Moreover, Ganymede’s ice is probably more than 1000 km thick, so m<strong>in</strong>eral nutrients are likely to be very deeply buried. Thus, although hydro<strong>the</strong>rmally susta<strong>in</strong>ed life at <strong>the</strong> ice-rock <strong>in</strong>terface could have been viable <strong>in</strong> <strong>the</strong> distant past, <strong>the</strong> prospects of f<strong>in</strong>d<strong>in</strong>g biomarkers <strong>in</strong> <strong>the</strong> present surface ice are much poorer than <strong>for</strong> Europa.
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SP-1231 Exobiology
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Cover Fossil coccoid bacteria, 1 µ
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4 I.5 Potential Non-Martian Sites <
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6 6.2 Imaging of F
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The Exobio
Page 11 and 12: pyrolysis, or similar techniques, f
Page 13 and 14: Ignasi Casanova, Institute
Page 15 and 16: I AN EXOBIOLOGICAL VIEW OF THE SOLA
Page 17 and 18: SP-1231 18 surface pressure of CO 2
Page 19 and 20: SP-1231 20 10 bar befor</st
Page 21 and 22: SP-1231 22 kilometres in</s
Page 23 and 24: SP-1231 24 Laboratory Investigation
Page 25 and 26: I.3 Limits of Life
Page 27 and 28: temperatures of 95-106ºC. This sug
Page 29 and 30: (mainly found <str
Page 31 and 32: cannot exclude fin
Page 33 and 34: Responses to Cosmic Radiation <stro
Page 35 and 36: acid to identify the</stron
Page 37 and 38: Horneck, G. (1993). Responses of Ba
Page 39 and 40: SP-1231 Fig. I.4.2.2/1. Size distri
Page 41 and 42: SP-1231 Fig. I.4.2.2/4. Evaporite w
Page 43 and 44: SP-1231 Fig. I.4.2.3/1A (left). Net
Page 45 and 46: SP-1231 48 A detailed chemical anal
Page 47 and 48: SP-1231 Fig. I.4.3.1.1/1. Scheme of
Page 49 and 50: SP-1231 Fig. I.4.3.1.2/1. A: Compar
Page 51 and 52: SP-1231 54 I.4.3.2.1 Sedimentary Or
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Page 57 and 58: SP-1231 60 References regard to non
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Page 61: SP-1231 64 ities in</strong
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Page 67 and 68: SP-1231 Fig. I.5.2.2/2. Modelled Ti
Page 69 and 70: SP-1231 72 Galileo: images availabl
Page 71 and 72: SP-1231 74 TABLE I.6.2/1 EVIDENCE O
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Page 75 and 76: SP-1231 78 I.7.3 Organic Chemistry
Page 77 and 78: II.1 Introduction The</stro
Page 79 and 80: II.2 The Planet Ma
Page 81 and 82: cover the range 4.
Page 83 and 84: The depletion of c
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Page 87 and 88: plate tectonic evidence has been ob
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Page 91 and 92: II.3 The Martian M
Page 93 and 94: principal atmosphe
Page 95 and 96: Lines of evidence
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Page 101 and 102: Anders, E. (1989). Prebiotic organi
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SP-1231 118 orbital characteristics
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SP-1231 120 II.4.5 The</str
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SP-1231 122 II.4.6 Rovers and Drill
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SP-1231 124 Site 2: Apolin<
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SP-1231 126 Site 4: Chasma area, ea
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SP-1231 128 Sharp, M., Parkes, J.,
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SP-1231 Fig. II.5.1/1. The<
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SP-1231 132 II.5.3 Isotopic Analysi
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SP-1231 134 chemical composition de
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SP-1231 Fig. II.5.7.1/1. Th
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SP-1231 138 optical microscope. Bot
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SP-1231 140 protons of discrete ene
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SP-1231 142 atures should be per<st
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SP-1231 Fig. II.5.7.7/1. Example of
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SP-1231 146 soils. In this approach
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SP-1231 148 discrimin</stro
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SP-1231 150 determin</stron
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SP-1231 Fig. II.5.10.2/2. Enantiome
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SP-1231 154 References achiral colu
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II.6 Team III: The
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in width and heigh
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Searchin</
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minerals or oxidat
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A second group of rocks, however, m
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SCIENTIFIC METHOD AND REQUIREMENTS
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The sample materia
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surprising. To ach
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The main</
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Consideration of a diamond wire-saw
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Schoonen, M.A. & Barnes, H.L. (1991
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SP-1231 180 II.7.2 The</str
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SP-1231 Fig. II.7.4/1. Look
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SP-1231 184 II.7.5 A Possible <stro
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Team IV was asked to examin
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