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Developing Tools to Highlight the Presence of Carbonates in CRISM Images of Mars

Introduction: CRISM hyperspectral images of Mars have recently been used to identify what are interpreted as magnesium carbonate deposits composed of magnesite mixed with one or more hydrated phases [1]. Using existing approaches to parameterize spectral data from CRISM [2], thousands of high-resolution targeted observations have been examined for occurrences of this material. Results to date indicate that it is found primarily in association with ultramafic materials located circumferentially to the Isidis basin [1]. Given the large number of targeted observations (>9500), the weakness of the carbonate absorptions, and the presence of instrumental noise and artifacts in the data, it is unclear if this result reflects the true extent of the deposits or is limited by methods used to process the data. We are attempting to develop new techniques to identify these carbonates, leveraging the high spectral and spatial resolution of CRISM, to better assess their spatial distribution and geological context. Approach: Martian carbonates have been observed in telescopic data [3,4], meteorites [5], and possibly dust deposits [6]. They are an expected weathering product of water and basalt in an atmosphere having CO2 [7] and the conditions appropriate for their formation have been predicted for Mars [8]. However, until recently [1], no carbonate-bearing rock outcrops had been identified. The fact that these deposits have been observed in CRISM targeted observations (~18 m/pixel spatial resolution) indicates that high spatial resolution is essential to detecting them. In addition, improved approaches may be needed to accentuate these materials at these greater spatial resolutions. A common approach already employed by the CRISM team is the use of spectral parameter maps that highlight the presence of specific mineralogies within a spectral data set. These have been employed with great success on many planetary bodies [e.g., 9, 10] and can effectively identify absorption features associated with a given mineral in a given scene. The existing set of parameters for CRISM [2] was developed based on occurrences of olivine, pyroxene, sulfates, and phyllosilicates detected by OMEGA, and augmented with parameters designed to detect minerals not yet observed, such as carbonates. In the case of carbonates, a calcitic spectrum was assumed. Fig. 1 illustrates three presently employed parameters that are sensitive to magnesite. The D2300 and BD3400 parameters are part of the standard multispectral parameter suite [2] and BD2500 has been recently added for hyperspectral images, as formulated in [1]. These parameters measure the strengths of overtone absorptions near 2.30, 2.51, and 3.4 μm. The presence of all three features can be used to map out the location of carbonates on the surface of Mars, as a "carbonate browse product" (a parameter at 3.9-μm which takes advantage of a strong overtone absorption is also under development).