The essential Atmospheric Paradox: If the atmosphere on Mars was self-sustaining in the past, why is it no longer present?

Added to this we have the observation that many Noachian features on Mars are relatively undegraded, implying that Mars has not had a significant atmosphere over most of geologic time.

The conventional answer to the Paradox is that the early atmosphere was eroded by impacts blasting it away into space and by various phochemical and solar wind processes stripping away ions and atoms from the atmosphere. In this way Mars could have lost its early atmosphere and collapsed to its present dry state.

Unfortunately, for this to occur, Mars needs to have been depleted of most of its volatile inventory, leaving a very dry barren planet indeed. There is plenty of dissenting evidence that large amounts of volatiles remain in the regolith, especially in high lattitudes. The current volatile inventory of Mars seems adequate to recreate the early greenhouse, yet it does not.

An alternative explanation is that Mars' spin axis is unstable, due to the lack of a large Moon. Modelling of the obliquity of Mars' spin axis shows a large range of variation either side of the present 25.2 degrees. In fact this can range from ~12 to ~36 degrees or more (depending on numerical models). When the obliquity is low, both poles remain cold and the atmosphere is thin (perhaps as low as 1 millibar?). When obliquity is high, the summer pole is pointed more directly at the Sun and the polar icecaps are dispersed each summer (but only to a depth of a few metres). The winter pole becomes acutely cold and thick ice deposits form, but it takes time for the atmosphere to move from one pole to the other, so a moderate atmospheric pressure may result (a few tens of millibars?). The Polar layering and Polar Layered Terrain appear to be signatures of this variation. Invoking even more variation in the past attempts to explain how a thick atmosphere might have temporarily been possible at various times in Mars' history, but this is not a very satisfactory solution, since the outburst "floods" are so localised. they are not the result of general atmospheric conditions.

The White Mars answer to the Paradox is that the early Noachian atmosphere was not self-sustaining and was only ephemerally stable in the immediate aftermath of a major impact when significant surface heating had occurred. After that, on a timescale of hours to days, or perhaps at most a few years, the atmosphere collapsed again to an icy layer. In this model Mars still has a large amount of its original volatile inventory and relatively minor amounts have been lost from a near-surface active reservoir. The remainder is stored safely at depth in the regolith.
 

      Created: May 2002
      Last modified: May 2002
      Authorised by:  Head, Earth Sciences

      Maintained by: Nick Hoffman
      Email: nhoffman@unimelb.edu.au