Mars Bedrock Mineralogy

Over the course of Mars’ geologic history, the actions of wind, ice, meteor impacts, gravity and liquid water have produced a thin but extensive layer of sand and dust that is mixed with rocks on the martian surface. Because sand is pervasive in the dark regions of Mars, it dominates most spectral measurements that are taken from orbit. In the past, true bedrock exposures were rarely resolved by orbiting spectrometers. However, the THEMIS instrument aboard the 2001 Mars Odyssey spacecraft has provided a new view (Figure 1) of the martian surface with 100 m/pixel daytime and nighttime multispectral infrared imaging. Numerous exposures of bedrock have been identified using THEMIS data. These exposures are found in a variety of southern highlands terrains, including crater floors and intercrater plains.

Figure 1. A plains bedrock exposure discovered in THEMIS infrared images. This feature cannot be distinguished in the MOC wide angle or MOLA shaded relief. Light tones in THEMIS IR mosaic indicate warmer areas. Rocky surfaces are warm at night, relative to the surroundings.

We are characterizing the composition, thermophysical properties, and morphology of these rocky, largely sediment-free exposures in the intercrater plains of Mars [Rogers et al., 2005a]. Targeting bedrock sites is important because: 1) primary/secondary mineral identification may be placed in a stratigraphic and geologic context, which is a valuable asset in understanding the processes that affected that surface, and 2) they offer a method of locating small-scale compositional heterogeneities and mineral enrichments that are important for understanding aqueous, igneous, and weathering processes (example, Fig. 2).

Figure 2. Based on geologic mapping utilizing spectral, thermophysical, morphologic and topographic information, Rogers et al. [2005b] derived a geologic map and stratigraphic section for a region in upper Ares Vallis that contains bedrock exposures. The mapping revealed the presence of 2 olivine-rich basaltic layers that were ~100 to >250 m thick, separated by ~1.4 km of olivine-deficient rocks. Colors in the decorrelation stretch mosaic highlight spectral variability within the channel.

To characterize the mineralogy and surface textures and morphology of each exposure (example, Figure 3), we are using several datasets (TES, OMEGA, MOC, MOLA, and THEMIS). Once the bedrock exposures are characterized, we will classify the exposures based on similar mineralogic and morphologic properties, and then examine each class in the context of local and global stratigraphy, morphology, age, elevation and latitude to understand their origin.

Figure 3. Left: Bedrock location in Hesperia Planum (indicated with dotted line). The background image is a THEMIS daytime radiance mosaic and the color information is TES albedo (linear color scale, purple = 0.10, red=0.14). The white box designates the location where TES spectra were extracted for analysis. Right: Measured and modeled TES surface emissivity from Hesperia Planum. The dominant mineral groups used in the model are plagioclase and pyroxene.

The early geologic history of Mars is of considerable interest to the scientific community because of the strong evidence for water-related activity and perhaps a warmer and wetter climate. However, the question of the duration and intensity of the wetter periods has not been resolved [e.g., Jakosky and Phillips, 2001; Solomon et al., 2005]. By characterizing these interesting and unique sites, we are providing new information about the processes that formed and modified the ancient terrains of Mars.

References

Jakosky, B. M. and R. J. Phillips, Mars' volatile and climate history, Nature, 412 (6843), 237-244, 2001.

Solomon, S.C., O. Aharonson, J.M. Aurnou, W.B. Banerdt, M.H. Carr, A.J. Dombard, H.V. Frey, M.P. Golombek, S.A. Hauck, J.W. Head, B.M. Jakosky, C.L. Johnson, P.J. McGovern, G.A. Neumann, R.J. Phillips, D.E. Smith, and M.T. Zuber, New perspectives on ancient Mars, Science, 307 (5713), 1214-1220, 2005.

Rogers, A. D., O. Aharonson, J. L. Bandfield, and P. R. Christensen, Spectroscopic analysis of bedrock exposures in the Martian highlands, Abstract, Fall 2005 AGU Meeting, 2005a.

Rogers, A.D., P.R. Christensen, and J.L. Bandfield, Compositional heterogeneity of the ancient martian crust: Surface analysis of Ares Vallis bedrock with THEMIS and TES data, Journal of Geophysical Research-Planets, 110 (E05010), doi:10.1029/2005JE002399, 2005b.

More information about using infrared observations to locate bedrock and determine mineralogy is available here.