Thermally distinct ejecta blankets from Martian craters

Abstract

Utilizing the Termoskan data set of the Phobos '88 mission we have recognized a new feature on Mars: ejecta blanket distinct in the thermal infrared (EDITH). Virtually all of the more than 100 features discovered in the Termoskan data are located on the plains near Valles Marineris. EDITHs have a startlingly clear dependence upon terrains of Hesperian age, implying a spatial or temporal dependence on Hesperian terrains. Almost no thermally distinct ejecta blankets are associated with any of the thousands of craters within the data set that occur on the older Noachian units. EDITHs also do not appear on the portions of the younger Tharsis Amazonian units seen in the data. The Hesperian terrain dependence cannot be explained by either atmospheric or impactor variations; Noachian and Hesperian terrains must have experienced identical atmospheric and impactor conditions during Hesperian times. Thermally distinct ejecta blankets therefore reflect target material differences and/or secondary modification processes. Not all lobate ejecta blankets are thermally distinct, but all EDITHs correlated with visibly discernible ejecta blankets are associated with lobate ejecta blankets. The boundaries of the thermally distinct areas usually follow closely the termini of the fluidized lobate ejecta blankets, even when the ejecta blankets show a high degree of sinuosity. Thus, the thermally distinct nature of EDITHs must be due to the primary ejecta formation process. The coupling of these thermal anomalies to morphology is unlike most sharp Martian inertia variations which are decoupled from observed surface morphology. Some thermally distinct ejecta blankets occur near otherwise similar craters that do not have thermally distinct ejecta blankets. Thus, wind patterns or locally available aeolian material cannot provide a single overall explanation for the observed variations. We compiled a data base of 110 EDITH and non-EDITH craters ranging in diameter from 4.2 km to 90.6 km. There are almost no correlations within the data base other than occurrence on Hesperian terrains. We postulate that most of the observed EDITHs are due to excavation of thermally distinctive Noachian age material from beneath a relatively thin layer of younger, more consolidated Hesperian volcanic material. The plausibility of this theory is supported by much geological evidence for relatively thin near-surface Hesperian deposits overlying massive Noachian megabreccias on the EDITH-rich plains units. We suggest that absence of thermally distinct ejecta blankets on Noachian and Amazonian terrains is due to absences of distinctive near-surface layering. Thermally distinct ejecta blankets are excellent locations for future landers and remote sensing •cause of relatively dust free surface exposures of material excavated from depth. . Laboratory experiments that vary atmospheric pressure and particle size have also reproduced some lobate crater morphologies Gault, 1979, 1984]. One of our motivations in studying Martian thermally distinct ejecta blankets in d...

An Alternative Approach to Mapping Thermophysical Units from Martian Thermal Inertia and Albedo Data Using a Combination of Unsupervised Classification Techniques

“…These craters can be identified by concentric circular structures of thermophysical units that contrast with the units dominating the surrounding terrain. This is consistent with observations of distinct high thermal inertia rims and impact ejecta surrounding many Martian craters [55,170]. Three interesting impact craters are shown in Figure 19.…”

High-Resolution Thermal Inertia Mapping from the Mars Global Surveyor Thermal Emission Spectrometer

“…2). This observation was also evident in Phobos-2 thermal inertia and thermal data (Selivanov et al 1998, Betts andMurray 1993). Betts and Murray (1993) reported thermally distinct crater ejecta deposits for numerous small craters.…”

“…This observation was also evident in Phobos-2 thermal inertia and thermal data (Selivanov et al 1998, Betts andMurray 1993). Betts and Murray (1993) reported thermally distinct crater ejecta deposits for numerous small craters. Dark "splotch" deposits on craters floors were observed in Viking IRTM data to have thermal inertias higher than the surrounding terrains (Zimbelman and Kieffer 1979, Christensen 1983, Zimbelman 1986, Edgett and Christensen 1994.…”

“…However, not all craters exhibit a thermal inertia signature at all, suggesting that these surfaces are mantled, masking the interior, rim, and ejecta deposits. We have examined several craters reported by Betts and Murray (1993) and have not been able to confirm the presence of thermally distinct ejecta deposits in high-resolution maps; nondetection may be due to a combination of limited spatial coverage and potential miscorrelation for small craters between the MGS inertial and USGS cartographic frames. Zimbelman (1986) examined the visible and thermal properties of the Pettit wind streak.…”

Combinations of processes responsible for Martian impact crater “layered ejecta structures” emplacement

“…The inner LES also has a broad low terminal rampart as shown in the MOLA profile (Figure 10c), resulting in a moat between the crater's rim crest and the rampart. Thermal properties of LESs could offer information regarding the state of LES surface conditions [ Betts and Murray , 1993; Baratoux et al , 2005]. The THEMIS nighttime infrared (IR) image of the same impact crater shows that the inner LES has a brighter surface compared with the surface of the outer LES (Figure 11).…”