Endmember spectra and classified mosaics derived from CRISM mapping data at the south pole of Mars

Overview

Multispectral mapping data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) provide a unique opportunity to characterize south polar ice deposits at higher spectral sampling, spatial resolution, or spatiotemporal coverage than previous work. This new perspective can help to constrain the nature and distribution of different mixtures of CO₂ ice, H₂O ice, and dust that influence the formation, evolution, and preservation of Mars climate records. We processed 1103 CRISM observations spanning southern summer of six Mars Years through a combination of k-means clustering and random forest classification. Using a set of 12 spectral endmembers directly tied to previous work with high-resolution CRISM targeted data, we made a series of temporally restricted mosaics showing surface spectral variation over time. The endmember set and classified mosaics produced in this work can provide critical context for future studies of the dynamic processes that shape south polar ice deposits.

This is follow-on work to a previous study that produced a series of classified maps and a spectral library from CRISM targeted data. That work can be accessed with the following links:

• Publication: https://doi.org/10.1029/2022JE007372
• Repository: https://doi.org/10.5281/zenodo.6960943

Contents

The repository contains two .zip files, which can be expanded to access the files described below:

• classified_mosaics.zip
  • lookup_files
    • SP_CRISM_RF_ColorMap.clr : An ESRI-formatted color map file that can be used to apply the endmember color scheme to random forest-classified maps in ArcGIS (see Symbology settings). Note that for continuity with Cartwright (2022), there are 21 colors in this file, though only endmembers associated with 12 of those colors are present in the classified mosaics.
    • SP_CRISM_RF_ColorMap.txt : A text file that can be loaded into Python as a numpy array and used to generate a matplotlib color ramp. Row indices correspond to endmember numbers (see below) and columns are red, green, and blue values scaled from 0 to 1. Note that for continuity with Cartwright (2022), there are 21 colors in this file, though only endmembers associated with 12 of those colors are present in the classified mosaics.
    • SP_CRISM_RF_Mapping_Endmember_Lookup.csv : A lookup table that can be used to cross-reference between endmember numbers (as stored in GeoTiffs or the indices of the colormaps above) and corresponding endmember names (C1m, Dc3m, etc.). Note that for continuity with Cartwright (2022), the endmember numbers are not sequential and instead correspond to the number of the original reference spectrum (e.g., C1m in this work has the same endmember number as C1 in Cartwright (2022)).
  • random_forest_classification
    • Contains GeoTiff mosaics of random forest classification results. Mosaics compile all observations falling in 10º bins of solar longitude (Ls) for a given year, provided that data was acquired in that range. Classified observations are stacked in ascending order of Ls. Mosaics of MSW data have a resolution of ~90 m/pixel while mosaics of MSP or combined MSP and MSW data have a resolution of ~180 m/pixel. Filenames indicate the observation type ("MSP", "MSW", or combined "MSP-MSW"), Mars Year ("MY28", "MY29", etc.) and Ls range ("Ls300-310" indicates all observations acquired between Ls 300º and Ls 310º). These images are not rendered to display colors consistent with the publication figures, but instead store the endmember classification for each pixel as a number from 0 to 21; use the contents of lookup_files to find the corresponding endmember name or render the image with the color scheme from the publication. To view rendered summary plots of each observation, see the contents of summary_plots below.
  • summary_plots
    • Contains summary plots of the endmember-classified mosaics provided in random_forest_classification above. Each plot presents the classified mosaic over a grey outline approximating the extent of high-albedo CO₂ ice in the south polar residual cap (SPRC). Note that the summary view focuses on the area in and around the SPRC, but the full mosaics may extend beyond these bounds. 
• spectral_library.zip
  • Contains spectral libraries with the median spectrum of each endmember as presented in Figure 3 in the publication. A basic text file listing the wavelength (WVL) and normalized reflectance values for each endmember is included (SP_CRISM_MappingEndmemberMedians.txt) as well as an ENVI-formatted spectral library (SP_CRISM_MappingEndmemberMedians_ENVI.sli). Note that a subset of the 55 wavelengths sampled in the source CRISM data were removed to avoid error-prone bands, leaving these spectral libraries with 51 bands. The spectra are normalized by the value at 1.330 µm.