A.M. High Albedo Events in Martian northern polar craters: An Investigation Using THEMIS Infrared Data

Following the work done on surveying High Albedo Events (HAEs) in Mars" northern polar craters (Armstrong et al., 2007), we use data from THEMIS (Thermal Emission Imaging System) on board the Mars Odyssey spacecraft to correlate the findings of the TES (Thermal Emission Spectroscopy) survey. In the TES survey, there were 9 craters (65°N to 71°N) with a diameter larger than 10km which displayed HAE's in a.m. hours (0140-0330 local time) during late spring to mid-summer, at temperatures well above the sublimation point of carbon dioxide frost. According to Armstrong et al., this implies that water ice forms overnight, sublimates during the day, and re-deposites in the following night. Using the THEMIS image database at http://themis.asu.edu, we found a total of 40 infrared images and several visible images which fell into our search criteria for the a.m. crater HAE's. These images were captured at a local time ranging from 0419-0511 during Ls 47° to 180°, with solar incidence angles ranging from 95.4° to 72.1°. An investigation of these images showed mixed results. Some craters (65.43°N/128.33°E, 66.35°N/163.44°E, 66.38°N/144.02°E, 70.04°N/352.05°E) exhibit what could be interpreted as signs of interior crater frost but none of them is definitive, while others (64.82°N/209.40°E, 64.90°N/350.77°E, 65.49°N/283.67°E, 69.87°N/285.01°E, 70.88°N/193.55°E) display no sign of interior crater frost at all. Additionally, there are a number of craters (64.90°N/350.77°E, 65.43°N/128.33°E, 66.35°N/163.44°E, 66.38°N/144.02°E, 70.04°N/352.05°E, 70.88°N/193.55°E) which show definite signs of water frost along the crater rim. These observations suggest one of the followings: (1) the interior crater frost sublimates before the THEMIS images were taken; (2) the THEMIS data cannot capture the diffuse, scattered, and thin frost deposits well enough to be used in any conclusive manner; or (3) there was simply no interior crater frost to begin with. Further investigations are in progress.