Geoscience Reference
In-Depth Information
1
CO
2
Log abundance of
gas concentrations
0
1
−
N
2
−
2
40
Ar
−
3
−
4
−
5
20
Ne
36
Ar
−
6
84
Kr
129
Xe
−
7
132
Xe
−
8
−
9
−
10
−
10
−
9
−
8
−
7
−
6
−
5
−
4
−
3
−
2
−
10 1
Meteorite EETA79001
Figure 12.18
Gas in SNC (Martian) meteorites. The similarity of their gas content to that of the Martian
atmosphere measured by the Viking probes (standardized to nitrogen) is a very forceful argument
in favor of SNCs originating from Mars.
sulfate deposits are very abundant, clay minerals unusual, carbonate absent. Basalts and
more differentiated lavas were also identified. In addition, the origin of more than 40 mete-
orites has been ascribed to Mars by powerful arguments. These meteorites are known as
the SNC meteorites (from the three group-leading meteorites Shergotty, Nakhla, and Chas-
signy). The arguments that they are samples from Mars are: (1) these meteorites form a
homogeneous group in the (
gases trapped in the Martian meteorite EETA79001 found in the Antarctic ice is identical
to that of the atmosphere analyzed by the Viking probes (
Fig. 12.18
)
; and (3) the relatively
young ages (160-600 million years) found for most SNCs require the existence of a planet
with recent volcanic activity, while the ages of exposure to cosmic radiation determined
from these meteorites indicate that they have not spent more than a few million years in
interplanetary space and that they probably came from not very far away. Mars and its
super-volcanoes, with their very well-preserved morphology, such as Olympus Mons and
its caldera with vertical cliffs several kilometers high, is the only planet in the inner Solar
System that could be a possible candidate.
The SNCs are cumulate rocks (gabbro, peridotite) affected by the impact that allowed
them to escape Martian gravity to the extent that the plagioclase has lost its crystalline
structure and become amorphous. Little is certain from this blind sampling except that
the SNCs must represent the Martian lithosphere. The isotopic compositions of Sr, Nd,
and Hf reveal a very clear contrast between rocks from a source enriched in incompatible
elements and others from a depleted source: we can contrast samples from the Martian
“mantle” and samples from a Martian “crust.” The existence of isotopic anomalies left by
17
O,
δ
δ
