Comprehensive analysis of glaciated martian crater Greg

Research areas:
  • Structure et évolution comparée des planètes
JAN 15
The 66-km diameter martian crater, Greg, east of Hellas, hosts various
distinctive features, including dendritic valleys filled with
chevron-textured masses (south wall), and lobate tongues a few
kilometers long (north wall). We analyze these features by various
quantitative techniques to illuminate martian geologic and climatic
history. Crater retention model ages indicate that Greg is at least 1-3
Gy old, but surface layers of mantles and glacial features are orders of
magnitude younger. Properties of the dendritic valleys, combined with
climate models, suggest that fluvial activity began under a thicker,
warmer atmosphere, soon after the crater's formation. The oldest exposed
fluvial systems have surface crater retention ages of a few hundred My,
indicating runoff in recent geologic time. Much of Greg is covered by
ice-rich mantle deposits, for which we infer gradual accumulation and
depths of order 30-85 m; they mask pre-existing landforms. The lobate
tongues are interpreted as glaciers with mean slope of 10.2 +/- 2.3
degrees and average thickness of 33 19 m. Our calculations and data
suggest that these glaciers were originally ice-rich and that their
surface layers have been depleted by volatile loss. The glaciers
probably formed when ice-rich mantle deposits reached critical thickness
and flowed downhill. The top 5-10 m of the mantle and glaciers show
crater survival times of order a few My to similar to 15 My, which,
remarkably, is the time since the last 1-4 episodes of obliquity >45
degrees. Global climate models affirm that Greg lies in one of two
nonpolar areas with extremes of ice deposition during high-obliquity
epochs. This match with observations supports the use of such models in
studies of planetary climate change. (C) 2013 Elsevier Inc. All rights