Chemistry Reference
In-Depth Information
All the Mössbauer spectra recorded on Mars have been publically released so
that every research group could dispose of them freely. Together with each
sampled spectrum, a spectrum of a reference absorber was collected and also the
recorded error signal of the driver is provided. As expected, the spectra are not
linear in the velocity scale, which causes some calibration difficulties. Agresti et al.
[
284
] calibrated the spectra by considering the a-Fe lines in the reference target,
combined with the error signal of the drive.
The many Mössbauer investigations of the spectra taken by the MERs on soils and
rocks along their pathways showed olivines and pyroxenes as the main silicates
[
294
-
296
]. Most interesting was the obvious observation of a jarosite spectrum,
inferring the former presence of water on Mars [
297
]. This was also corroborated by
the identification of goethite in the spectra from some locations [
298
].
Concerning the magnetic spectra, hematite and magnetite were found as the
predominant iron oxides. Unfortunately, those spectra with relatively high
hyperfine fields were not so well defined due to significant deviations from line-
arity experienced in the highest velocities, which are not covered by the sextet
lines of metallic Fe in the calibration spectra. The direct use of the hematite and
magnetite in the reference target for calibration was initially not simple because
along with the strongly overlapping spectral lines, hematite showed the two
magnetic phases in the temperature windows of the measurements. By determining
the hyperfine parameters of the five sextets of Fe metal, hematite and magnetite in
a laboratory sample consisting of the same reference target material measured at
the appropriate temperature windows, the spectra of the reference target could be
calibrated [
299
]. With this calibration method, several magnetic spectra recorded
on Mars at different temperature windows could be accurately analyzed [
299
,
300
]
(Fig.
3.53
).
For instance, magnetite could be fitted without parameter constraints. From the
fit there was also a strong indication that goethite exhibits two maxima in the
hyperfine field distribution inferring two different goethite formations. It was
further demonstrated that the Mars hematite measured in different temperature
windows between 210 and 260 K showed both the AF and WF phase giving an
idea about its morphology. Some spectra are shown in Fig.
3.53
. Similar results
were recently obtained by Agresti et al. [
301
], using a simfit method consisting of
the simultaneous analysis of 60 spectra with multi-spectrum constraints. However,
the latter results were interpreted by the presence of two kinds of hematites, i.e.
one showing the Morin transition and another remaining in the WF phase.
Finally, it is worth to mention that the Opportunity came across several
meteorites on the surface of Mars [
302
]. The first one was found in Meridiani
Planum [
303
]. The recorded Mössbauer spectra showed this meteorite to consist
mainly of kamacite, having about 7 wt % Ni [
304
,
319
]. No taenite spectrum was
observed indicating that this iron-nickel phase probably occurs in the meteorite
only in a very small amount below the detection limit of the spectrometer.
Further extraterrestrial Mössbauer measurements are planned using an
improved version of the MIMOS [
319
]. Phobos, one of the moons of Mars, is the
next target for Mössbauer investigations during the Phobos-Grunt mission by the
