Lunar and Planetary Science XXXIII (2002) 1765.pdf
WEATHERING OF THE CONTINUOUS EJECTA BLANKET ASSOCIATED WITH CASSINI IMPACT BASIN, MARS. J. D. King1 and E. F. Albin2, 1School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332 ([email protected]), 2Department of Space Sciences, Fernbank Science Center, Atlanta, GA 30307 ([email protected]).
Introduction: Cassini basin is a 400-km diameter bedforms, and intrusive volcanic features beneath the basin in the Arabia Terra region of Mars. The struc- ejecta blanket. The volcanic features, associated with ture’s continuous ejecta unit extends roughly 1.5 to 2.5 tectonic fractures caused by the initial shock of the basin radii from the rim. Basin ejecta is significant in impact that created the Cassini basin, may include in- that it is representative of upper crustal material ex- trusive dikes, sills, and batholiths. posed by colossal impact events, and it can serve as a Fluvial Features: Many channels are found relatively coherent stratigraphic marker for relative age throughout the impact ejecta unit. The friable or non- dating purposes. However, the thickness and state of indurated nature of impact ejecta deposits make them preservation of martian basin deposits is unclear. Pre- susceptible to erosion by water or other fluids that may vious work [e.g., 1, 2] studied the nature of Cassini have flowed across the surface. Channels are often ejecta blanket and found many features indicative of characterized as small valley networks. intense weathering – including considerable evidence Mass Wasting Features: Dark slope features are that aeolian, fluvial, and mass wasting processes have ubiquitous across the ejecta blanket and are inferred to been at work. This investigation is concerned with the be mass wasting features. Dark slope streaks (Figure degree or extent of removal of the continuous ejecta 2) are thought to result from the removal of dust from blanket through the study of weathering features using relatively steep slopes [3]. MOLA and MOC data. Features Associated with Weathering: High resolution Mars Orbiter Camera (MOC) images show an array of aeolian, fluvial, and mass wasting features found on and within the ejecta blanket of Cassini.
Figure 2: Dark slope streak (MOC Image M0704514)
Discussion: Based on the presence of the weather- ing features described above, it is clear that the ejecta Figure 1: Pedestal crater (MOC Image M0701675) blanket of the Cassini basin has been continuously evolving since the formation of the impact structure in Aeolian Features: Features characteristic of ae- the late Noachian epoch. The present day topographic olian erosion are found across much of the Cassini expression of the basin ejecta deposit can be glimpsed ejecta blanket. For instance, pedestal craters occur on from a regional MOLA map of the Cassini structure the ejecta blanket and are indicative of aeolian removal (Figure 3). An estimated thickness profile of the basin of surface deposits (Figure 1). Yardangs also represent deposit, at the time shortly after impact, is shown in significant erosion of material by wind. Wind erosion Figure 4. This profile was modeled based on an ejecta has been extensive in places and has exhumed craters, thickness function described elsewhere [4, 5, and 6].
Lunar and Planetary Science XXXIII (2002) 1765.pdf
WEATHERING OF BASIN EJECTA: J. D. King and E. F. Albin
Our hypothetical profile of the Cassini ejecta deposit, References: [1] Albin E. F. (1997) LPSC XXVIII, 1151– when compared to recent MOLA data for the area, 1154. [2] King J. D. and Albin E. F. (2001) DPS 33, 48.09. suggest that considerable basin ejecta has been re- [3] Albin E. F. and King J. D. (2001) LPSC XXXII, 1380. [4] moved by weathering. In fact, we estimate that weath- McGetchin T. R. et al. (1973) Earth Planet. Sci. Lett. 20, 226-236. ering processes have stripped perhaps in excess of sev- [5] Melosh H. J. (1989) Impact Cratering, 245 p. [6] Garvin J. B. eral hundred meters of material. Any work that utilizes and Frawley J. J. (1998) GRL 25, 4405-4408. basin deposits must take into account the removal of a significant proportion of the original basin’s ejecta deposit.
Figure 3: Compiled MOLA Map of the Cassini Impact Basin.
Figure 4: Theoretical Profile of Ejecta Depth for a 400-km Diameter Basin.