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

SP-1231
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II.6.16 Chemical
Inspection of Subsurface
Material
7.26 µm (C-H bend), and 7.90 µm (O-H or C-H bend). This wavelength range could
<strong>the</strong>re<strong>for</strong>e provide a direct test <strong>for</strong> <strong>the</strong> presence of organics.
Although carbon-bear<strong>in</strong>g molecules are most <strong>in</strong>terest<strong>in</strong>g from an exobiology
viewpo<strong>in</strong>t, a <strong>the</strong>rmal-IR system can also provide significant <strong>in</strong><strong>for</strong>mation on salts,
evaporites (e.g. phosphates, sulphates, sulphides, nitrites, chlorides) and <strong>the</strong> m<strong>in</strong>erals
that might be <strong>for</strong>med at hydro<strong>the</strong>rmal spr<strong>in</strong>gs. Carbonates are easily detected <strong>in</strong>
<strong>the</strong>rmal-IR spectra, and variations <strong>in</strong> anion composition can also be determ<strong>in</strong>ed.
Hydroxide-bear<strong>in</strong>g m<strong>in</strong>erals (e.g. clays) also have characteristic <strong>the</strong>rmal-IR spectral
features result<strong>in</strong>g from <strong>the</strong> fundamental bend<strong>in</strong>g modes of OH. <strong>The</strong> Al-O-H bend<strong>in</strong>g
mode near 11 µm <strong>in</strong> kaol<strong>in</strong>ite is a good example. Clays show clear spectral variations
directly related to differences <strong>in</strong> degree of hydration and leach<strong>in</strong>g. Oxides, which may
be important <strong>in</strong> sedimentary rocks, may be identified us<strong>in</strong>g <strong>the</strong>ir broad absorption
features <strong>in</strong> <strong>the</strong> 7-8.5 µm range. Silicates can also be detected us<strong>in</strong>g <strong>the</strong> broad
absorption features around 10 µm and 20 µm. <strong>The</strong> exact wavelengths of <strong>the</strong> absorption
depends upon <strong>the</strong> structure of <strong>the</strong> m<strong>in</strong>eral matrix.
<strong>The</strong>rmal emission spectroscopy can also be used to determ<strong>in</strong>e whe<strong>the</strong>r geo<strong>the</strong>rmal
or hydro<strong>the</strong>rmal activity is occurr<strong>in</strong>g. <strong>The</strong> presence of warm regions on <strong>the</strong> surface
would be extremely important <strong>in</strong> <strong>the</strong> search <strong>for</strong> extant or ext<strong>in</strong>ct martian biota (Walter
& DesMarais, 1993). Is Mars geologically dead or is <strong>the</strong>re sufficient heat produced to
drive a subsurface hydro<strong>the</strong>rmal circulation system? If this circulation is on a small
scale, <strong>the</strong>rmal emission spectroscopy may be a requirement <strong>in</strong> order to look <strong>in</strong> <strong>the</strong>
optimum place <strong>for</strong> life.
II.6.15.2 Instrumentation
A <strong>the</strong>rmal emission spectrometer will fly with <strong>the</strong> A<strong>the</strong>na rover on Mars Surveyor
2001. It is a m<strong>in</strong>iature version of <strong>the</strong> <strong>in</strong>strument now fly<strong>in</strong>g on <strong>the</strong> Mars Global
Surveyor mission. This device is mounted on a mast with <strong>the</strong> panchromatic camera to
allow accurate boresight<strong>in</strong>g, and covers a wavelength range of 5-28 µm to look at
water-deposited m<strong>in</strong>erals. <strong>The</strong> highest angular resolution of <strong>the</strong> device is 7 mrad. In
addition to m<strong>in</strong>eralogy, <strong>the</strong> device can provide temperature profiles <strong>in</strong> <strong>the</strong>
atmospheric boundary layer, and <strong>the</strong>rmophysical properties of rocks and soils (see
http://astrosun.tn.cornell.edu/a<strong>the</strong>na/m<strong>in</strong>ites.html).
II.6.16.1 Objectives
<strong>The</strong> chemistry of carbon and sulphur-bear<strong>in</strong>g phases and particularly <strong>the</strong>ir isotopic
composition provides redox-sensitive <strong>in</strong><strong>for</strong>mation about <strong>the</strong> environment as well as
evidence <strong>for</strong> <strong>in</strong>organic- and biologically-controlled processes with<strong>in</strong> <strong>the</strong> geochemical
cycle of sulphur. Strongly oxidis<strong>in</strong>g conditions prevail on <strong>the</strong> present surface. In order
to obta<strong>in</strong> redox-sensitive <strong>in</strong><strong>for</strong>mation, <strong>in</strong> particular reduced organic or m<strong>in</strong>eral phases,
it is important to study subsurface material, which might be shielded aga<strong>in</strong>st <strong>the</strong>
hostile surface conditions. Depth-controlled analysis of drill-core material should
reveal a redox-gradient. <strong>The</strong> presence of oxidised and reduced phases characterises
environmental conditions and – as part of it – possible signs of extant or ext<strong>in</strong>ct
biological activities.
II.6.16.2 Requirements
Carbon can be present as organic matter or as carbonate m<strong>in</strong>erals. Sulphur-bear<strong>in</strong>g
phases are likely part of a m<strong>in</strong>eral matrix, a soil or a rock. Liberation <strong>for</strong> subsequent
determ<strong>in</strong>ation of chemical and isotopic properties can be per<strong>for</strong>med via combustion
at m<strong>in</strong>eral-specific temperatures. Fur<strong>the</strong>r details are discussed <strong>in</strong> <strong>the</strong> Team II report
(Section II.5).
Be<strong>for</strong>e any analytical detection, it seems necessary to break/crush/pulverise <strong>the</strong>
matrix <strong>in</strong> order to easily liberate carbon and/or sulphur-conta<strong>in</strong><strong>in</strong>g compounds. Small
rock chips or, better, rock powder would be suitable material to guarantee a complete
liberation/volatilisation.