We
are investigating the surface mineralogy of Mars in order to provide clues to
its geologic history including when and where the water was present. The Compact
Reconnaissance Imaging Spectrometer for Mars (CRISM) is currently flying on
the Mars Reconnaissance Orbiter
(MRO) and collecting multiple new images of Mars each day.
CRISM is
mapping Mars with 100-200 meters/pixel images with 72 channels across the visible
and near-infrared wavelength region. CRISM is also collecting a few more
detailed images of targeted spots each day at 18 m/pixel with 544 channels.
CRISM is expanding the mineral identifications made on Mars with the European
Mars Express/OMEGA (Observatoire pour la Mineralogie, L'Eau, les Glaces et
l'Activite) at 300-1500 m/pixel surface resolution. The High Resolution Imaging
Science Experiment (HiRISE) camera on MRO is taking pictures at submeter
resolution, which can be combined with the spectral data from CRISM and OMEGA
to gain information about the surface textures.
Our group
is working on identification of clay minerals in CRISM
images as these minerals tell us about water on Mars. Clay minerals
typically form in marine sediments. They also form as volcanic ash and tephra
are altered in the presence of water. Hydrothermal activity produces clay
minerals as well. We are finding clays in the most ancient terrains that formed
4 billion years ago on Mars. These indicate that there were widespread bodies
of neutral water on Mars at that time. Two regions on Mars that show high
abundances of these clay minerals are called Mawrth Valles and Nili Fossae. Both are
under consideration as landing sites for future missions, including the Mars Science Lab
(MSL).
The
Mawrth Vallis
region contains one of the largest and most diverse outcrops of clays. Detailed
analyses of the CRISM
spectra in this region indicate the presence of expansive deposits of clays
called smectites, as well as smaller outcrops of kaolinite, hydrated silica and
mica. Smectite clays are also common in California. They expand readily to
accept more water and contract making huge cracks when the ground dries. These
are likely the clays responsible for shifting our homes here in California during wet and dry seasons so that our doors don't close properly. They also
tend to make the ground very hard and are the reason why we need to add soil
amendments to our gardens to make most plants grow well.
Reflectance
spectra exhibit dips or "bands" due to absorption of energy at the
frequency of molecular vibrations for species of interest. For detection of
clay minerals, we are investigating absorptions due to water and OH in the
mineral structure. The frequencies of these mineral absorption bands depend on
the mineral structure and which metal cations (Fe, Mg, Al) are bound to the
molecules. We match spectra from CRISM to spectra of minerals in the lab in
order to identify the specific types of clay minerals present as shown in
Figure 1 A below. We plot certain combinations of channels from the CRISM image
to generate mineral indicator maps as shown in Figure 1 B. Here the
Fe/Mg-smectite is orange, the Al-phyllosilicate is blue, and the hydrated
silica/mica is green. We also use HiRISE images to look in more detail at
specific locations. An example is shown in Figure 1C where we see a transition
from the Fe/Mg smectite at the bottom left to the Al-phyllosilicate and
hydrated silica material at the upper right.
The
identification of clay minerals on Mars with CRISM and OMEGA implies liquid
water was present on Mars. These clay layers are very old and were buried long
ago by mantles of volcanic material. We see the clays under this layer in
places where the volcanic material has been eroded away. We continue to search
for more pockets of these clays that are visible on the surface in order to
gain an understanding of the extent and character of the clay deposits and the
aqueous events that created them. We're following the water on Mars in the
search for evidence of life.
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