(A) CRISM spectra from several images compared to lab spectra of clay minerals, (B) mineral indicator map of CRISM image 43EC (~10 km across) draped over MOLA terrain with 10X vertical exaggeration where several of the spectra were collected, and (C) a HiRISE image showing the transition from the smectite we see in many regions to hydrated silica we see surrounding this on the higher edges.
The Nili Fossae are valleys that have cut into the ancient crust of Mars,exposing clay minerals.
Credit: NASA/JPL/University of Arizona
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.
By Janice L. Bishop
Carl Sagan Center, SETI Institute
