Frequency of Steep Slopes on Mars: Evidence for Active Permafrost Layer at High Latitudes During High Obliquity Periods

Details Frequency of Steep Slopes on Mars: Evidence for Active Permafrost Layer at High Latitudes During High Obliquity Periods Frequency of Steep Slopes on Mars: Evidence for Active Permafrost Layer at High Latitudes During High Obliquity Periods

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.p32c..05k&link_type=abstract

American Geophysical Union, Fall Meeting 2003, abstract #P32C-05

Mathematics

Logic

1863 Snow And Ice (1827), 5416 Glaciation, 5455 Origin And Evolution, 6225 Mars

Scientific paper

Mapping of the frequency of the steepest (above 20 deg) slopes on Mars showed that the steep slopes are much less frequent at high latitudes (above 50 deg in both hemispheres, excluding the polar caps) than in the equatorial zone (below 40 deg latitude). In the narrow transitional belts around 45 deg latitude in both hemispheres, we have found a strong asymmetry of steep slopes (Kreslavsky and Head, GRL, 30, 1815, 2003GL017795, 2003): steep equator-facing slopes are much more frequent than pole-facing ones. Costard et al. (Science, 295, 110-113, 2002) calculated that the day-average summer temperatures above the water freezing point can be reached only at high obliquity, and only at high latitudes or at pole-facing slopes at mid latitudes. They suggested that the summer-time melting of a tens-cm-thick permafrost layer could be responsible for gullies formation. We suggest that the repeating formation of active layer during summer in these areas at high obliquity is also responsible for a number of other features, including the absence of steep slopes: erosion (including gullies formation) at the presence of meltwater could preferentially eliminate the steepest slopes. Our study of morphology and topography of individual craters confirms this suggestion. The trend in the steep slope occurrence indicates the presence of ground ice or thin snow packs at least down to 35 deg latitude at high obliquity. The summer-time insolation and hence the day-average temperature strongly depend not only on the latitude, obliquity and slope orientation, but also on the distance of Mars from the sun during summer, that is on Mars orbit eccentricity and the areocentric longitude of the Sun at perihelion. The perfect inter-hemisphere symmetry of the steep slope occurrence boundary indicates that this boundary reflects a cumulative result of a great number of geologically short episodes of high summer temperature maxima. Preservation of steep slopes at low latitudes indicates that Mars has never had a globally "warm" climate for a geologically appreciable duration at least since the Late Hesperian. This also restricts the former rotation pole location within 5-10 deg from the present one for the same time span.

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