Chemistry Reference
In-Depth Information
Fig. 2.50 On the left is a false color Pancam image of the 'Karatepe' section, a piece of the
layered S-rich outcrop at Meridiani Planum exposed in the wall of Endurance crater. The image
was obtained with Mars Exploration Rover Opportunity's Panoramic Camera (Pancam;
http://
can see the tracks of the rover wheels curving down and where fresh outcrop has been exposed by
the Rock Abrasion Tool (RAT) for analyses with the APXS and Mössbauer spectrometers. On the
right are typical Mössbauer spectra of the outcrop material a a spectrum obtained at a velocity
range of ±12 mm/s and b at a reduced velocity range of ±4 mm/s to increase the resolution of
the central features in the spectrum where the Jarosite peaks occur. Fe
3
D
3
stands for the as yet
unassigned ferric mineral phase, possibly super paramagnetic hematite or Fe(SO
4
)(OH) [
90
]
jarosites, the quadrupole splitting DE
Q
increases in the order K [ Na [ H
3
O and
with Al
III
substitution for Fe
III
. The DE
Q
value for Martian jarosite is most con-
sistent with (K,Na)(Fe,Al)
3
(SO
4
)
2
(OH)
6
that is Na
+
-orK
+
-rich jarosite with
possible Al substitution.
Jarosite is a mineralogical marker for aqueous processes because it contains the
equivalent of *10 wt. % H
2
O in its structure as the OH anion. The average S-rich
outcrop rock at Meridiani Planum has the equivalent of *2%H
2
O associated
with jarosite alone. An important aspect of the jarosite detection is that acidic
conditions (pH \ 4 at room temperature) are required for its formation. The
alteration of basaltic precursor material under oxidizing, acid-sulphate conditions
to form jarosite and other phases in the S-rich outcrop rocks at Meridiani Planum
could have occurred under conditions provided, for example, by interactions with
acid-sulphate, possibly hydrothermal waters and/or condensation of SO
2
-rich
volcanic emanations [
91
].
