Martian meteorites have been launched from some 4 to 8 sites on Mars within the last 20 My. Some 75% to 88% of the sites ejected igneous rocks < 1.3 Gy old. Thus 75% to 88% of the rock-launching sites represent only 29% of Martian time. We hypothesize this imbalance arises not merely from poor statistics, but because much of the older Martian surface is inefficient in launching rocks during impacts. There are three lines of evidence. First, intense Noachian cratering must have produced surface layers with > 100 m of regolith, which reduces launch efficiency due to dominance of fines and possible effects of ice in the regolith. Second, both Mars Exploration Rovers in 2004, found that some older coherent strata are weak sediments, 1-2 orders of magnitude weaker than Martian igneous rocks. Low strength favors low launch efficiency, and even if launched, such rocks may produce recognizable meteorites on Earth. Third, the smaller fresh impact craters in Martian upland sites are rarely surrounded by secondary impact crater fields (cf. Barlow and Block, 2004). In a survey of 200 craters, the smallest Noachian, Hesperian, and Amazonian craters with fields of secondaries are ∼ 45 km, ∼ 24 km, and ∼ 10 km, respectively. With 40% of Mars being Noachian, and 74% being either Noachian or Hesperian, these effects could play an important role in the statistics of recognized Martian meteorites and production rates of secondary crater populations. Reference: Barlow N.G., Block, K.M. (2004), DPS abstract 47.04.
AAS/Division for Planetary Sciences Meeting Abstracts #37
- Pub Date:
- December 2005