EXCAVATION OF SUBSURFACE ICE ON MARS BY NEW IMPACT CRATERS
Here we report on natural probes of the martian subsurface which have ‘detected’ ice in this critical mid-latitude zone. New impact craters discovered in the Context Camera (CTX) dataset, and not present in
previous imagery, occur in this latitudinal zone. In five such cases (with latitudes spanning 43.3° to 55.6° N), these impacts have excavated bright material which in High Resolution Imaging Science Experiment (HiRISE) data have a brightness and color consistent with water ice (see Figure 1). Real-time collaboration between instrument teams allowed for a coordinated investigation of these features.
Bright material composition: This bright material appears in both the ejecta (Figure 1) and floors (Figure 2) of these new craters. These craters are typically a few meters in diameter, excavate to several decimeters
and have associated bright material a few meters across. We examined hyperspectral data acquired by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) for spectral evidence of water ice.
Four of the five sites have bright material which occupies less than 10% of a CRISM pixel (~18m in size) and showed no spectral evidence of water ice. However, one site (depicted in Figure 1) had a large enough
bright deposit to occupy a significant fraction of a CRISM pixel. Spectra from this site show clear waterice absorption features at 1.25, 1.5 and 2 μm.
Exposed surface ice is not stable at the latitudes of these sites (43.3°-55.6° N) and would be expected to sublimate. We are monitoring these sites and have seen this ice both shrinking in area and fading. Figure
2 shows an example of icy material in craters of an impact cluster. Bracketing CTX images confirm this particular cluster formed between June and August 2008 (Ls 81-111). The two craters containing the
bright blue material are 5-6m across and shadow measurements indicate depths of ~70cm. Figure 2 shows these ice patches fading to background color and albedo over the course of several months and they are essentially indistinguishable from background terrain in recent images (at Ls 180 and 187, not shown here).
