Thirteen Iron Meteorites Found at Gale Crater, Meridiani Planum, and Gusev Crater — Exogenic Witnesses to Weathering Processes Near the Martian Equator

Friday, 19 December 2014
James Ashley, NASA Jet Propulsion Laboratory, Pasadena, CA, United States
At least 20 meteorites and meteorite candidates have now been found by science teams at three Mars rover landing sites, all within 15 degrees of the martian equator. Thirteen of these are iron meteorites, comprising 65% of the population — an order of magnitude greater abundance than found among witnessed iron falls in Earth-based collections (~6%). Chondritic meteorites, which comprise some 86% of Earth-based falls, are conspicuously absent from the Mars inventory. The reasons for this disproportion may involve a) post-fall environmental resistance differences favoring iron survivability; b) fragmentation from impact shock (and possibly internal weathering stresses associated with oxide production in desert environments [1]); combined with c) selection biases arising from residual chondritic fragments appearing less conspicuous. Impact features along rover traverses often show evidence of dark materials likely to be impactor fragments [e.g., 2], which could represent the missing chondritic fraction.

The reactivity of reduced (metallic) iron to aqueous alteration, combined with the near equatorial and widely distributed locations of these rocks, makes them particularly useful to the assessment of climate models arguing for geologically recent ice at the martian equator. Exposure histories involving alternating wind/water cycles are imprinted on several Meridiani irons, for example [3]. Evidence for oxide coating removal demonstrates the current epoch to be one of coating destruction, not production, showing that atmospheric exposure alone is insufficient to produce the coating. Cavernous weathering is likely associated with acidic corrosion, while evidence of aeolian scouring is found in Widmanstätten patterns, sharp-crested scallops, regmaglypt enlargement, and abundant pitting. Further study of these features could help constrain wind direction and velocity during epochs of sculpting [e.g., 4], and assist in exposure age estimation.

References: [1] Ashley J. W. and M. A. Velbel (2000), MAPS, v. 35 no. 5 Supplement, p. A22. [2] Golombek et al., (2010) J. Geophys. Res., 115, E00F08, doi:10.1029/2010JE003628. [3] Ashley J. W. et al., (2011) J. Geophys. Res., 116, E00F20, doi:10.1029/2010JE003672. [4] Greeley R. et al., (2002) J. Geophys. Res., 107, E1, 5005, doi : 10.1029/2000JE001481.