Chemistry Reference
In-Depth Information
brought in contact with the painting on the wall for measurement. The two
Mössbauer spectra in Fig.
2.45
, one of a darker paint and the other for a light-
colored paint, distinguish between different iron oxide pigments [
80
].
A similar application is shown in Fig.
2.46
. A Greek lekythos vase was
investigated non-destructively with the help of MIMOS II [
81
]. The unpainted
surface shows a broad spectrum that can be associated with poorly crystallized iron
oxides produced during the firing clay process. The painted surface shows, in
addition to the characteristic spectrum of the non-painted area, a well-defined
sextet corresponding to a well-crystallized hematite. The Mössbauer spectrum
taken over the red painted details shows no significant difference from the non-
painted surface. Therefore, the red details are presumably iron-free. However, the
room-temperature analysis shows that the vase itself has a poorly crystallized iron
oxide (hematite, a-Fe
2
O
3
, or maghemite, c-Fe
2
O
3
) and some small Fe
III
particles
(intense lines in the middle of the measured spectral range). The poorly crystal-
lized iron oxide would have been formed from heated clay minerals (e.g. Fe-rich
smectites).
2.3.6.2 In Situ Mössbauer Spectroscopy on Mars
The NASA twin Mars Exploration Rovers (MER), Spirit and Opportunity, carry a
MIMOS II instrument each [
77
] (Fig.
2.47
). They were launched in June 2003 and
landed successfully in January 2004 in Gusev Crater and at Meridiani Planum,
respectively. Their nominal mission was only planned to be three months long, but
both rovers exceeded expectations considerably and have been actively exploring
the martian surface in their eight Earth year at the time of writing this paragraph
(March 2011). Both Mössbauer instruments continue to work as well, although
their decaying
57
Co sources (half-life 270 days) have resulted in significantly
longer integration times necessary to obtain statistically good quality spectra than
at the beginning of the mission.
The primary MER objective is to explore two sites on the Martian surface
where water may once have been present, and to assess past environmental con-
ditions at those sites and their suitability for life [
82
]. The Red Planet owes its
color to Fe-oxides, and surface materials are enriched in Fe relative to Earth. The
distribution of Fe between Fe-bearing minerals and its oxidation states constrains
the primary rock type (e.g. olivine-bearing vs. non-olivine-bearing basalt), the
redox conditions under which primary minerals crystallized (e.g. presence or
absence of magnetite), the extent of alteration and weathering (e.g. value of Fe
III
/
Fe
Total
), the type of alteration and weathering products (e.g. oxides vs. sulphates
vs. phyllosilicates), and the processes and environmental conditions for alteration
and weathering (e.g. neutral vs. acid-chloride vs. acid-sulphate aqueous process
under ambient or hydrothermal conditions) [
83
], making
57
Fe Mössbauer spec-
troscopy an extremely useful tool for Mars exploration.
