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W. K. Hartmann, J. A. Grier, D. C. Berman, G. A. Esquerdo (Planetary Science Institute)
Mars displays lava flows formed within the last 180 Ma, and possibly within the last 20 Ma, and has also experienced liquid water movement on or near the surface within the same time frame. This constrains geophysical models.
Mars meteorites divide into 3 groups. One specimen is 4500 Ma old, and may represent a primordial crust underlying much of Mars' surface. The nakhlite group crystallized 1300 Ma ago. The shergottite group crystallized 180-300 Ma ago, possibly at a single impact site. Several nakhlites have been exposed to liquid water; Shih et al. (1998) and Swindle et al. (2000) dated this exposure at 670 Ma, possibly with younger episodes. Mars meteorites give us data from three to five random sites on Mars; they establish geothermal activity within the last 4% of Martian time.
Crater counts give us more selectivity in geologic interpretation. We calibrate ages by using lunar counts adjusted by estimates of the relative Mars/Moon impact ratio. Ongoing independent studies by W. Bottke (USA) and B. Ivanov (Russia) both suggest that for a fixed bolide size, Mars gets 2.0 ± 0.4 times as many hits as the moon. Adjusting for impact velocity and gravity, we predict Martian isochrons, or crater densities for surfaces of varied ages. Mars Global Surveyor images show that the youngest lava flows in both Elysium Planitia and Amazonis Planitia have less than 1% as many craters (11 m < D<~100 m) as the lunar maria, and ages of the order 20 Ma or less (Hartmann and Berman 2000; Hartmann et al., in preparation). This suggests volcanism within the last 0.4% of Martian time.
For every geothermal heating episode that produces lava flows on Mars, many lesser ones melt permafrost ice and produce underground aquifers. The apparent water-erosion gullies found by Malin and Edgett (2000) at high latitudes may be produced in this way on time scales less than those cited above.
References: Hartmann and Berman 2000, J. Geophys. Res. 105, 15011-10526; Malin and Edgett 2000, Science 288, 2330-2335; Shih et al. 1998, LPSC XXIX, #11435; Swindle et al. 2000, Meteor. Planet. Sci. 35,107-116.
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