Evidence for Amazonian mid-latitude glaciation on Mars from impact crater asymmetry

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We find that crater slopes in the mid-latitudes of Mars have a marked
north-south asymmetry, with the pole-facing slopes being shallower. We
mapped impact craters in two southern hemisphere sites (Terra Cimmeria
and Noachis Terra) and one northern hemisphere site (Acidalia Planitia)
and used elevation data from the High Resolution Stereo Camera (HRSC)
onboard Mars Express to find the maximum slope of impact crater walls in
the four cardinal directions. Kreslavsky and Head (Kreslavsky, M.A.,
Head, J.W. {[}2003]. Geophys. Res. Lett. 30), using Mars Orbiter Laser
Altimeter (MOLA) track data, also found that, in general, conjugate
slopes are shallower in the pole-facing direction, but over a narrower
(similar to 10 degrees) and more constrained latitude band. They linked
the asymmetry to active-layer formation (thaw) at high obliquity.
However, Parsons and Nimmo (Parsons, R.A., Nimmo, F. {[}2009]. J.
Geophys. Res. 114) studied crater asymmetry using MOLA gridded data and
found no evidence of a relationship between crater asymmetry and
latitude. Our work supports the observations of Kreslavsky and Head
(Kreslavsky, M.A., Head, J.W. {[}2003]. Geophys. Res. Lett. 30), and
shows that asymmetry is also found on conjugate crater slopes below the
resolution of MOLA, over a wider latitude band than found in their work.
We do not systematically find a sudden transition to asymmetric craters
with latitude as expected for thaw-related processes, such as
solifluction, gelifluction, or gully formation. The formation of gullies
should produce the opposite sense of asymmetry to our observations, so
cannot explain them despite the mid-latitude location and pole-facing
preferences of gullies. We instead link this asymmetry to the deposition
of ice-rich crater deposits, where the base of pole-facing slopes
receive ten to hundreds of meters of additional net deposition, compared
to equator-facing ones over the mid-latitudes. In support of this
hypothesis we found that craters in Terra Cimmeria that have deposits on
both their floor and pole-facing walls, occur preferentially at the
mid-latitudes and have marked positive asymmetry. These deposits were
likely laid down during high obliquity excursions (>45 degrees) at least
5 My ago and potentially over the whole Amazonian epoch. Their
preservation to the present-day relies on the presence of a surface lag
of debris, which inhibits sublimation. (C) 2013 Elsevier Inc. All rights