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

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
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