Exobiology in the Solar System & The Search for Life on Mars - ESA
Exobiology in the Solar System & The Search for Life on Mars - ESA
Exobiology in the Solar System & The Search for Life on Mars - ESA
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
pr<str<strong>on</strong>g>in</str<strong>on</strong>g>cipal atmospheric comp<strong>on</strong>ent, CO 2, now quoted <str<strong>on</strong>g>for</str<strong>on</strong>g> <strong>Mars</strong> is, <str<strong>on</strong>g>in</str<strong>on</strong>g> fact, <str<strong>on</strong>g>the</str<strong>on</strong>g> meteorite<br />
measurement, not from <str<strong>on</strong>g>the</str<strong>on</strong>g> Vik<str<strong>on</strong>g>in</str<strong>on</strong>g>g data.<br />
Once <strong>on</strong>e sample from <strong>Mars</strong> was identified it was possible to c<strong>on</strong>firm that o<str<strong>on</strong>g>the</str<strong>on</strong>g>rs <str<strong>on</strong>g>in</str<strong>on</strong>g><br />
<str<strong>on</strong>g>the</str<strong>on</strong>g> collecti<strong>on</strong> and returned from Antarctica were also martian. Because martian gases<br />
are not readily apparent <str<strong>on</strong>g>in</str<strong>on</strong>g> all of <str<strong>on</strong>g>the</str<strong>on</strong>g> samples, <str<strong>on</strong>g>the</str<strong>on</strong>g>ir presence or absence could not be<br />
taken as diagnostic. Indeed, some of <str<strong>on</strong>g>the</str<strong>on</strong>g> meteorites do not have any residual martian<br />
atmosphere. A far more useful and generalised criteri<strong>on</strong> is a sample’s oxygen stable<br />
isotopic compositi<strong>on</strong> (a measurement requir<str<strong>on</strong>g>in</str<strong>on</strong>g>g <strong>on</strong>ly a few milligrammes of sample).<br />
Indeed, this is how A84 was c<strong>on</strong>firmed as a martian sample – <str<strong>on</strong>g>in</str<strong>on</strong>g> most o<str<strong>on</strong>g>the</str<strong>on</strong>g>r respects,<br />
it is different to <str<strong>on</strong>g>the</str<strong>on</strong>g> o<str<strong>on</strong>g>the</str<strong>on</strong>g>rs of <str<strong>on</strong>g>the</str<strong>on</strong>g> family. Most notably it is extremely old (4.5 Gyr) –<br />
as old as <str<strong>on</strong>g>the</str<strong>on</strong>g> planet. Until recently, old age would almost certa<str<strong>on</strong>g>in</str<strong>on</strong>g>ly have c<strong>on</strong>signed it<br />
to be<str<strong>on</strong>g>in</str<strong>on</strong>g>g of asteroidal orig<str<strong>on</strong>g>in</str<strong>on</strong>g>. Indeed, it is ir<strong>on</strong>ic that A84 languished <str<strong>on</strong>g>in</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> Antarctic<br />
collecti<strong>on</strong> at Houst<strong>on</strong>, unrecognised and believed to be diogenite (Mittlefehldt, 1994).<br />
As already menti<strong>on</strong>ed, oxygen isotope systematics are <str<strong>on</strong>g>the</str<strong>on</strong>g> vital criteri<strong>on</strong> <str<strong>on</strong>g>for</str<strong>on</strong>g><br />
recognis<str<strong>on</strong>g>in</str<strong>on</strong>g>g meteorite families. Oxygen is 45wt‰ of any silicate-rich meteorite, so<br />
any characteristic discovered am<strong>on</strong>g <str<strong>on</strong>g>the</str<strong>on</strong>g> oxygen isotopes can be c<strong>on</strong>sidered as<br />
diagnostic. Fortunately, oxygen has three isotopes: 16 O (most abundant), 17 O (1/200th<br />
of total) and 18 O (1/500th). <str<strong>on</strong>g>The</str<strong>on</strong>g> exact relative proporti<strong>on</strong>s of 17 O and 18 O can be<br />
measured very precisely and quoted accord<str<strong>on</strong>g>in</str<strong>on</strong>g>g to <str<strong>on</strong>g>the</str<strong>on</strong>g> general <str<strong>on</strong>g>for</str<strong>on</strong>g>mula <str<strong>on</strong>g>for</str<strong>on</strong>g> δ 17 O and<br />
δ 18 O (‰, per mil).<br />
As <str<strong>on</strong>g>the</str<strong>on</strong>g> difference between 16 O and 17 O is 1 mass unit and between 16 O and 18 O is 2<br />
mass units, any samples from a homogenised parent body affected <strong>on</strong>ly by geological<br />
events must plot <strong>on</strong> a l<str<strong>on</strong>g>in</str<strong>on</strong>g>e of slope 1/2 (actually 0.52). All <str<strong>on</strong>g>the</str<strong>on</strong>g> rocks <strong>on</strong> Earth, its water<br />
etc def<str<strong>on</strong>g>in</str<strong>on</strong>g>e a well-accepted reference l<str<strong>on</strong>g>in</str<strong>on</strong>g>e called <str<strong>on</strong>g>the</str<strong>on</strong>g> terrestrial fracti<strong>on</strong>ati<strong>on</strong> l<str<strong>on</strong>g>in</str<strong>on</strong>g>e.<br />
Robert Clayt<strong>on</strong> and his colleagues at <str<strong>on</strong>g>the</str<strong>on</strong>g> University of Chicago (e.g. Clayt<strong>on</strong> &<br />
Mayeda, 1983) found that dist<str<strong>on</strong>g>in</str<strong>on</strong>g>ct meteorite groups, or groups that were related, had<br />
separate l<str<strong>on</strong>g>in</str<strong>on</strong>g>es <strong>on</strong> an oxygen isotope diagram, because <str<strong>on</strong>g>the</str<strong>on</strong>g> solar nebula was not<br />
adequately mixed be<str<strong>on</strong>g>for</str<strong>on</strong>g>e solid silicate bodies separated out. On this basis, <strong>Mars</strong> might<br />
be expected to have a dist<str<strong>on</strong>g>in</str<strong>on</strong>g>guish<str<strong>on</strong>g>in</str<strong>on</strong>g>g l<str<strong>on</strong>g>in</str<strong>on</strong>g>e. Indeed it does, although it is predictable that<br />
<str<strong>on</strong>g>for</str<strong>on</strong>g> a terrestrial type-planet, <str<strong>on</strong>g>for</str<strong>on</strong>g>med <str<strong>on</strong>g>in</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> <str<strong>on</strong>g>in</str<strong>on</strong>g>ner <str<strong>on</strong>g>Solar</str<strong>on</strong>g> <str<strong>on</strong>g>System</str<strong>on</strong>g>, <str<strong>on</strong>g>the</str<strong>on</strong>g> difference between<br />
<strong>Mars</strong> and Earth is small. <str<strong>on</strong>g>The</str<strong>on</strong>g> important parameter <str<strong>on</strong>g>in</str<strong>on</strong>g> def<str<strong>on</strong>g>in</str<strong>on</strong>g><str<strong>on</strong>g>in</str<strong>on</strong>g>g <strong>Mars</strong> (from martian<br />
meteorites) is <str<strong>on</strong>g>the</str<strong>on</strong>g> difference between <str<strong>on</strong>g>the</str<strong>on</strong>g> oxygen isotope compositi<strong>on</strong> of SNC<br />
meteorites and Earth, designated ∆ 17 O <strong>Mars</strong>. <str<strong>on</strong>g>The</str<strong>on</strong>g> ∆ 17 O <strong>Mars</strong> has now been measured to<br />
<str<strong>on</strong>g>the</str<strong>on</strong>g> martian meteorites/II.3<br />
Fig. II.3.3/1. Carb<strong>on</strong>ate globules <str<strong>on</strong>g>in</str<strong>on</strong>g> ALH 84001.<br />
A th<str<strong>on</strong>g>in</str<strong>on</strong>g>-secti<strong>on</strong> view, cover<str<strong>on</strong>g>in</str<strong>on</strong>g>g about 0.5 mm. <str<strong>on</strong>g>The</str<strong>on</strong>g><br />
‘nanofossil’ structures are found <str<strong>on</strong>g>in</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> rounded<br />
brownish and clear globules. <str<strong>on</strong>g>The</str<strong>on</strong>g> globules are<br />
made up of <str<strong>on</strong>g>the</str<strong>on</strong>g> brownish ir<strong>on</strong> carb<strong>on</strong>ate<br />
(siderite) and clear magnesium carb<strong>on</strong>ate<br />
(magnesite). <str<strong>on</strong>g>The</str<strong>on</strong>g> dark rims c<strong>on</strong>ta<str<strong>on</strong>g>in</str<strong>on</strong>g> ir<strong>on</strong> oxide<br />
and sulphide materials. (A. Treiman/Lunar &<br />
Planetary Institute)<br />
II.3.3 <str<strong>on</strong>g>The</str<strong>on</strong>g> Martian<br />
Meteorites<br />
97