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
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SP-1231<br />
86<br />
Table II.2.3/1. Bulk Compositi<strong>on</strong> of<br />
<strong>Mars</strong> Derived from SNC Meteorites.<br />
(Dreibus & Wänke, 1987).<br />
Mantle/Crust<br />
% ppm<br />
MgO 30.2 K 305<br />
Al 2 O 3 3.02 Rb 1.06<br />
SiO 2 44.4 Cs 0.07<br />
CaO 2.45 F 32<br />
TiO 2 0.14 Cl 38<br />
FeO 17.9 Co 68<br />
Na 2 O 0.5 Ni 400<br />
P 2 O 5 0.16 Cu 5.5<br />
Cr 2 O 3 0.76 Zn 62<br />
MnO 0.46 Ga 6.6<br />
ppb<br />
Br 145<br />
I 32<br />
Mo 118<br />
In 14<br />
La 480<br />
Tl 3.6<br />
W 105<br />
Th 56<br />
U 16<br />
Core (21.7% of <strong>Mars</strong> mass)<br />
%<br />
Fe 77.8<br />
Ni 7.6<br />
Co 0.36<br />
S 14.24<br />
H 2O added dur<str<strong>on</strong>g>in</str<strong>on</strong>g>g accreti<strong>on</strong> = 3.4%; H 2O<br />
reta<str<strong>on</strong>g>in</str<strong>on</strong>g>ed <str<strong>on</strong>g>in</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> mantle = 36 ppm, equivalent<br />
to a surface layer 130 m deep (100%<br />
degass<str<strong>on</strong>g>in</str<strong>on</strong>g>g).<br />
Fig. II.2.3/2. Estimated elemental abundances <str<strong>on</strong>g>in</str<strong>on</strong>g><br />
<str<strong>on</strong>g>the</str<strong>on</strong>g> Martian mantle as derived from SNC<br />
meteorites. <str<strong>on</strong>g>The</str<strong>on</strong>g> higher abundances of<br />
moderately volatile (Na, Ga, P, K, F, Rb, Zn)<br />
and volatile elements (Cl, Br) compared to<br />
those <str<strong>on</strong>g>in</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> terrestrial mantle is obvious. Note<br />
<str<strong>on</strong>g>the</str<strong>on</strong>g> depleti<strong>on</strong> of all chalcophile elements (In, Co,<br />
Cu, Ni) <str<strong>on</strong>g>in</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> martian mantle. Note especially<br />
<str<strong>on</strong>g>the</str<strong>on</strong>g> high mantle abundance of P <strong>on</strong> <strong>Mars</strong> and its<br />
depleti<strong>on</strong> <str<strong>on</strong>g>in</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> Earth’s mantle.<br />
<str<strong>on</strong>g>The</str<strong>on</strong>g> similar abundances of several geochemically (W to Rb <str<strong>on</strong>g>in</str<strong>on</strong>g> Fig. II.2.3/2) very<br />
different elements <str<strong>on</strong>g>in</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> mantle of <strong>Mars</strong> is fur<str<strong>on</strong>g>the</str<strong>on</strong>g>r str<strong>on</strong>g evidence <str<strong>on</strong>g>for</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> <str<strong>on</strong>g>for</str<strong>on</strong>g>mati<strong>on</strong><br />
of <str<strong>on</strong>g>the</str<strong>on</strong>g> <str<strong>on</strong>g>in</str<strong>on</strong>g>ner planets from two compositi<strong>on</strong>ally different comp<strong>on</strong>ents (R<str<strong>on</strong>g>in</str<strong>on</strong>g>gwood,<br />
1977; 1979; Wänke, 1981; Dreibus & Wänke, 1984). <str<strong>on</strong>g>The</str<strong>on</strong>g>se comp<strong>on</strong>ents are:<br />
Comp<strong>on</strong>ent A: Highly reduced and free of all elements with equal or higher volatility<br />
than Na, but c<strong>on</strong>ta<str<strong>on</strong>g>in</str<strong>on</strong>g><str<strong>on</strong>g>in</str<strong>on</strong>g>g all o<str<strong>on</strong>g>the</str<strong>on</strong>g>r elements <str<strong>on</strong>g>in</str<strong>on</strong>g> C1 abundance ratios. Fe and all<br />
siderophile elements are metallic, and even Si might be partly present <str<strong>on</strong>g>in</str<strong>on</strong>g> metallic<br />
<str<strong>on</strong>g>for</str<strong>on</strong>g>m.<br />
Comp<strong>on</strong>ent B: Oxidised and c<strong>on</strong>ta<str<strong>on</strong>g>in</str<strong>on</strong>g><str<strong>on</strong>g>in</str<strong>on</strong>g>g all elements (<str<strong>on</strong>g>in</str<strong>on</strong>g>clud<str<strong>on</strong>g>in</str<strong>on</strong>g>g <str<strong>on</strong>g>the</str<strong>on</strong>g> volatiles) <str<strong>on</strong>g>in</str<strong>on</strong>g> C1<br />
abundances. Fe and all siderophile (Co, Ni, Cu, Ga, W, etc) and lithophile elements<br />
are present ma<str<strong>on</strong>g>in</str<strong>on</strong>g>ly as oxides.<br />
As seen from Table II.2.3/1 and Fig. II.2.3/2, <str<strong>on</strong>g>the</str<strong>on</strong>g> SNC meteorites <str<strong>on</strong>g>in</str<strong>on</strong>g>dicate<br />
abundances of moderately volatile (Na, K, Rb, Zn) and volatile elements (Cl, Br, I) <str<strong>on</strong>g>in</str<strong>on</strong>g><br />
<str<strong>on</strong>g>the</str<strong>on</strong>g> martian mantle <str<strong>on</strong>g>in</str<strong>on</strong>g> excess of those <str<strong>on</strong>g>in</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> terrestrial mantle. A mix<str<strong>on</strong>g>in</str<strong>on</strong>g>g ratio of<br />
comp<strong>on</strong>ents A:B of 60:40 is obta<str<strong>on</strong>g>in</str<strong>on</strong>g>ed <str<strong>on</strong>g>for</str<strong>on</strong>g> <strong>Mars</strong>, compared to 85:15 <str<strong>on</strong>g>for</str<strong>on</strong>g> Earth. <str<strong>on</strong>g>The</str<strong>on</strong>g>re<br />
are, however, a number of elements supposedly derived from comp<strong>on</strong>ent B, which <str<strong>on</strong>g>in</str<strong>on</strong>g><br />
<str<strong>on</strong>g>the</str<strong>on</strong>g> Earth’s mantle have abundances similar to those of Fe, Na, Ga, K, F and Rb, but<br />
<str<strong>on</strong>g>in</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> martian mantle have c<strong>on</strong>siderably lower abundances (Co, Ni, Cu, In). <str<strong>on</strong>g>The</str<strong>on</strong>g>se<br />
elements all have str<strong>on</strong>g chalcophile character. <str<strong>on</strong>g>The</str<strong>on</strong>g>ir low abundances are taken to<br />
<str<strong>on</strong>g>in</str<strong>on</strong>g>dicate a homogeneous accreti<strong>on</strong> of <strong>Mars</strong>. Hence, c<strong>on</strong>trary to an <str<strong>on</strong>g>in</str<strong>on</strong>g>homogeneous<br />
accreti<strong>on</strong> of <str<strong>on</strong>g>the</str<strong>on</strong>g> Earth, as frequently favoured, <strong>on</strong> <strong>Mars</strong> <str<strong>on</strong>g>the</str<strong>on</strong>g> two comp<strong>on</strong>ents had <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
chance to equilibrate and were probably supplied almost simultaneously to <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
grow<str<strong>on</strong>g>in</str<strong>on</strong>g>g planet. <str<strong>on</strong>g>The</str<strong>on</strong>g> high abundance of comp<strong>on</strong>ent B, which supplied large amounts<br />
of sulphur, was obviously resp<strong>on</strong>sible <str<strong>on</strong>g>for</str<strong>on</strong>g> FeS becom<str<strong>on</strong>g>in</str<strong>on</strong>g>g a major phase and at its<br />
segregati<strong>on</strong> extracted all chalcophile elements 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>ir sulphide-silicate<br />
partiti<strong>on</strong> coefficients.<br />
<str<strong>on</strong>g>The</str<strong>on</strong>g> sulphide-silicate equilibrium <str<strong>on</strong>g>in</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> martian mantle <str<strong>on</strong>g>in</str<strong>on</strong>g>dicates its saturati<strong>on</strong> with<br />
FeS. <str<strong>on</strong>g>The</str<strong>on</strong>g> mantle’s FeO c<strong>on</strong>tent is about a factor of 2 higher than that of <str<strong>on</strong>g>the</str<strong>on</strong>g> terrestrial<br />
mantle. As a c<strong>on</strong>sequence, <str<strong>on</strong>g>the</str<strong>on</strong>g> sulphur abundance <str<strong>on</strong>g>in</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> martian mantle is expected<br />
to be substantially above <str<strong>on</strong>g>the</str<strong>on</strong>g> S abundance <str<strong>on</strong>g>in</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> Earth’s mantle, because <str<strong>on</strong>g>the</str<strong>on</strong>g> solubility<br />
of FeS <str<strong>on</strong>g>in</str<strong>on</strong>g> silicates <str<strong>on</strong>g>in</str<strong>on</strong>g>creases with <str<strong>on</strong>g>the</str<strong>on</strong>g> FeO c<strong>on</strong>tent. Hence, <str<strong>on</strong>g>the</str<strong>on</strong>g> observed high<br />
c<strong>on</strong>centrati<strong>on</strong>s of sulphur <str<strong>on</strong>g>in</str<strong>on</strong>g> mantle-derived magmas as represented by <str<strong>on</strong>g>the</str<strong>on</strong>g><br />
shergottites (sulphur c<strong>on</strong>tent 600-2800 ppm) is not surpris<str<strong>on</strong>g>in</str<strong>on</strong>g>g.<br />
<str<strong>on</strong>g>The</str<strong>on</strong>g> water abundance given <str<strong>on</strong>g>in</str<strong>on</strong>g> Table II.2.3/1 is derived from element correlati<strong>on</strong>s<br />
observed <str<strong>on</strong>g>in</str<strong>on</strong>g> shergottites and cosmochemical c<strong>on</strong>stra<str<strong>on</strong>g>in</str<strong>on</strong>g>ts. Two quite different<br />
approaches yielded an almost identical value. It might, however, be an upper limit<br />
<strong>on</strong>ly <str<strong>on</strong>g>for</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> water <str<strong>on</strong>g>in</str<strong>on</strong>g> <str<strong>on</strong>g>the</str<strong>on</strong>g> martian mantle.