DPS Pasadena Meeting 2000, 23-27 October 2000
Session 58. Mars Surface and Satellites I
Oral, Chairs: J. Connerney, N. Barlow, Friday, 2000/10/27, 1:30-3:00pm, C106

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[58.03] Constraining the Age of Martian Polar Strata by Crater Counts

J. A. Grier, W. K. Hartmann, D. C. Berman, E. B. Goldman, G. A. Esquerdo (Planetary Science Institute)

Mars Global Surveyor images are capable of giving good counts on craters down to about D~11 m. We studied 70 north polar images covering 2513 km2, mostly at latitudes 79-86 degrees, detecting a few probable impact craters and placing upper limits from non-detections in other frames. From these data we conclude that impact craters in the diameter range 11 m < D < 88 m indicate a survival lifetime of craters and crater-like topography in the north polar regions of < a few hundred Ka.

The crater counts suggest a much flatter slope in the diameter distribution of the young polar laminae than found in the production function on young, low-latitude lava surfaces, confirming the rapid obliteration of smaller craters even in recent geologic time (Plaut et al. 1988). To obliterate small craters, if vertical relief on the order of 30 m is completely blanketed and removed in < 500,000 yrs, then an inferred upper limit on the sediment deposition rate is 6 x 10-5 meters/year or 60 \mu/y.

These results are consistent with models which call for enhanced dust deposition at the poles due to a process whereby dust particles act as condensation nuclei for winter ice and are preferentially dropped out of the polar atmosphere. Pollack et al. (1979) calculated polar deposition at 300 \mu/y. Our age results are also consistent with Herkenhoff and Plaut (2000) who sought craters of D > 300 m on Viking images of the north cap and derived the same age, < 100,000 years. They used the same logic to infer a higher deposition limit of 1200 \mu/y. The measured north polar deposition rates are one to three orders of magnitude above the 1 to 4 \mu/y suggested at lower latitudes (Hartmann 1966, 1971; Matijevic et al. 1997).

References: Hartmann 1966, Icarus 5:406; Hartmann 1971, Icarus 15: 410; Herkenhoff and Plaut 2000, Icarus 144: 243; Matijevic et al. 1997, Science 278:1765; Pollack et al. 1977, J. Geophys. Res. 84: 2929; Plaut et al. 1988 Icarus 75:357.

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