Noachian Megabreccia on Mars

Details Noachian Megabreccia on Mars Noachian Megabreccia on Mars

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.p43d..03m&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #P43D-03

Other

5400 Planetary Sciences: Solid Surface Planets, 5420 Impact Phenomena, Cratering (6022, 8136), 5460 Physical Properties Of Materials, 5464 Remote Sensing, 6225 Mars

Scientific paper

Megabreccia consists of randomly oriented angular blocks, many larger than 1 m diameter, which forms suddenly in energetic environments such as impact events. Megaregolith is a related term for unconsolidated material resulting from heavy cratering, but megabreccia may be indurated. We have, to date, identified megabreccia in more than 50 locations on Mars from HiRISE images, generally in the form of indurated bedrock. It is commonly found in the central uplifts of large craters in or near Noachian (>3.8 Ga) terrains, near the rims of large basins such as Isidis, and in deep exposures such as the floor of Uzboi Valles and in parts of Valles Marineris. Well-exposed rock outcrops are required to identify megabreccia, in particular from the diversity of colors and textures indicating diverse lithologies. CRISM has identified hydrated minerals, especially clays, in many of these locations and perhaps alteration will be found in nearly all of the deposits once the data are acquired and analyzed, but they also appear to contain unaltered clasts. These may be among the very oldest rocks exposed on the surface of Mars, dating back to the time of heavy bombardment. In some cases megabreccia with relatively small (1-5 m) clasts probably formed by post Noachian cratering, particularly when found in the pitted and ponded material filling the crater floors, which may be analogous to suevite. The indurated megabreccia with large (>10 m) blocks is only found in locations consistent with deep bedrock, such as the central uplifts, although further brecciation and alteration may have occurred in the crater that exposed the rocks. It has long been assumed that Mars has a ~2 km thick layer of porous and permeable megaregolith, but we suggest that it may be largely cemented by melt and hydrothermal alteration. Heavy bombardment of the ice-rich crust of Mars could have produced a very different surface layer than on the dry Moon. Life on Earth may have begun during the period of heavy bombardment, but the most ancient rocks are heavily metamorphosed. Mars preserves a much better record of the environmental effects of heavy bombardment into a water-rich crust. Another reason to study the oldest outcrops in that the younger Noachian may have been relatively dry. At least 3 of the 7 remaining candidate Mars Science Laboratory (MSL) landing sites (Nili Fossae, Holden, and Eberswalde) would provide access to Noachian megabreccia; we may already have a sample of this type of deposit in ALH84001.

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