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S. Byrne, B.C. Murray (Division of Geological and Planetary Sciences, California Institute of Technology)
We report on the division of the north polar layered deposits into two distinct geologic units based on Mars Global Surveyor high resolution imaging and topographic data. An accurate self-consistent way of co-registering these two datasets, was developed and used to begin a stratigraphic analysis of the northern polar region.
A distinct change in the layering style exists at a definite stratigraphic horizon near the base of the north polar layered terrains. Occurrences of the contact between two distinct layered units can be mapped hundreds of kilometers apart at nearly the same MOLA elevation. The lower layered unit has a consistent association with sand dunes leading to the conclusion that it is an eroding sand rich deposit that predates most of the overlying north polar layered terrain, which exhibits the expected features of a dust-ice mixture. These results suggest that an areally extensive erg was in existence before the present ice-cap and that the present circumpolar erg is likely composed of material reworked from this older deposit. The volume of this lower unit is estimated to be on the order of 105 km3. The presence of this deposit implies that there existed a period in Mars’ history when there was no icy polar cap. A dramatic climatic change leading to the deposition of the upper layered (icy) unit in the present day polar layered terrain represents a major event in Mars’ history which may be related to a chaotic obliquity change, disappearance of a northern ocean or the end of large scale volcanism on the planet. However, due to uncertainties in the mechanics of layered terrain formation, such an event cannot be dated at this time.
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