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

I.6 Science and Experiment Strategy
‘Dans le champs de l’observation le hasard ne favorise que les esprits préparés.’ – Louis Pasteur, 1854.
(‘Where observation is concerned, chance favours only <strong>the</strong> prepared m<strong>in</strong>d.’)
Pasteur’s observation is particularly appropriate <strong>in</strong> <strong>the</strong> context of <strong>the</strong> search <strong>for</strong> life
beyond <strong>the</strong> Earth. <strong>The</strong>re is <strong>in</strong>evitably a large measure of chance <strong>in</strong>volved <strong>in</strong> f<strong>in</strong>d<strong>in</strong>g
evidence of ext<strong>in</strong>ct microbial life, and especially so when search<strong>in</strong>g <strong>for</strong> exist<strong>in</strong>g life.
But <strong>the</strong> chances of success can be improved by tak<strong>in</strong>g note of <strong>the</strong> lessons to be learned
from <strong>the</strong> Vik<strong>in</strong>g mission, from <strong>the</strong> relevant studies on Earth, as discussed <strong>in</strong> Sections
I.3 and I.4, and from <strong>the</strong> <strong>in</strong><strong>for</strong>mation derived from studies of <strong>the</strong> Martian meteorites,
discussed <strong>in</strong> Section II.3.
To maximise <strong>the</strong> chances of clear and unambiguous conclusions on <strong>the</strong> existence
of ext<strong>in</strong>ct or extant life <strong>in</strong> any analysis, it is absolutely essential that <strong>the</strong> coverage of
search <strong>in</strong>strumentation be broad enough to reduce <strong>the</strong> opportunities <strong>for</strong> alternative
<strong>in</strong>terpretations. At <strong>the</strong> same time, each component <strong>in</strong>strument must be carefully
designed so that it does not <strong>for</strong>m a serious weak l<strong>in</strong>k <strong>in</strong> <strong>the</strong> cha<strong>in</strong> of observations
across <strong>the</strong> <strong>in</strong>strument array.
Consequently, <strong>the</strong> search science strategy is to provide, as far as possible, a selfconsistent
set of <strong>in</strong>struments that will give concrete evidence, <strong>for</strong> or aga<strong>in</strong>st, <strong>the</strong>
existence of a range of biosignatures and associated images at a given search site. In
order to do that, an <strong>in</strong>tegrated package of analysis <strong>in</strong>struments is required.
What must not be attempted, under any circumstances, is a search <strong>for</strong> biosignatures
with <strong>in</strong>adequate <strong>in</strong>strumentation. It would be foolhardy to attempt to establish <strong>the</strong> existence,
or o<strong>the</strong>rwise, of life signs on Mars <strong>for</strong> example, by <strong>the</strong> use of a small narrowlybased
measurement unit located at one <strong>in</strong>adequately characterised land<strong>in</strong>g site. <strong>The</strong>
<strong>in</strong>evitable result<strong>in</strong>g confusion would simply discredit <strong>the</strong> subject of exobiology.
Clearly, a strategy rely<strong>in</strong>g on <strong>the</strong> careful collection of samples from <strong>the</strong> planet and
<strong>the</strong>ir return to Earth is highly desirable. <strong>The</strong> experience from <strong>the</strong> lunar programme
confirms this. Indeed, this is <strong>the</strong> route preferred by NASA <strong>in</strong> <strong>the</strong> search <strong>for</strong> life on
Mars. <strong>The</strong> returned samples can <strong>the</strong>n be subjected to <strong>in</strong>vestigations us<strong>in</strong>g <strong>the</strong> full
spectrum of <strong>in</strong>strumentation available on Earth by a diverse range of experts.
<strong>The</strong> problem with such a strategy is that, quite apart from <strong>the</strong> cost and technical
difficulties, it would be leav<strong>in</strong>g too much to chance to mount such a project be<strong>for</strong>e
carry<strong>in</strong>g out a careful programme of exploration and <strong>in</strong> situ analysis at several
selected locations on Mars. We do not yet know, <strong>for</strong> example, from what depth
samples must be obta<strong>in</strong>ed <strong>in</strong> order to be entirely free from <strong>the</strong> effects of <strong>the</strong> surface
oxidant. Nor do we know which of <strong>the</strong> currently listed sites on Mars are actually <strong>the</strong>
most <strong>in</strong>terest<strong>in</strong>g. <strong>The</strong>re is much to be done, and done well, be<strong>for</strong>e a sample return
mission should be attempted.
In <strong>the</strong> meantime, a sophisticated automated analysis system of <strong>the</strong> type considered
here can pave <strong>the</strong> way with essential observations, as well as <strong>in</strong>clud<strong>in</strong>g a large amount
of <strong>in</strong><strong>for</strong>mation on geochemistry and m<strong>in</strong>eralogy. It can do this with a reasonable
chance of succeed<strong>in</strong>g <strong>in</strong> <strong>the</strong> first detection of <strong>the</strong> signatures of life.
In <strong>the</strong> preced<strong>in</strong>g chapters we have seen that life as we know it relies upon <strong>the</strong>
existence of water, above all else. Without that, life ceases or goes <strong>in</strong>to a quiescent
mode. Hence, a search <strong>for</strong> extant life translates <strong>in</strong> <strong>the</strong> first place <strong>in</strong>to a search <strong>for</strong>
liquid water. This may lie at some considerable depth below <strong>the</strong> sterile surface of a
planet, as could be <strong>the</strong> case <strong>in</strong> certa<strong>in</strong> areas of Mars. An orbital survey to def<strong>in</strong>e <strong>the</strong>
water-ice boundary across Mars is of paramount importance. In <strong>the</strong> case of Europa,
<strong>the</strong> water-ice boundary is likely to be many kilometres below <strong>the</strong> icy carapace, except
<strong>in</strong> localised regions subjected to cryovolcanic flows.
I.6.1 Where to <strong>Search</strong>?
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