White Mars

Previously, we looked at the dominant model for the history of Mars based on early greenhouse atmospheres and fluvial erosion. It successfully explains many of the observations, but there are a few issues it fails to resolve. In particular, the Faint Young Sun Paradox and the Carbonate Paradox are problematic for this model.

As an alternative, Nick Hoffman at La Trobe University, offers a CO₂ based model of Martian History. According to this model, there is not now, nor has there ever been any liquid water on Mars, not even a little bit. Immediately, this point resolves the two paradoxes mentioned above. The Faint Young Sun is ok, because there is no longer a need to warm the surface of Mars, because liquid water is not required. Since liquid water is not required, carbonates would not be expected, explaining why we don't see them.

The White Mars model may resolve the paradoxes introduced by the aqueous model, but can it explain the observations as well? The valleys and gullies we see certainly appear to have been carved by water, but they need not have been. There is plenty of water on White Mars, but much of it is bound up with CO2 in the form of a clathrate which coexists with liquid CO2 under pressure. As you can see from the accompanying phase diagram, liquid CO2 is only stable at pressures above 5.1 bars, corresponding to a burial depth of about 50 meters. Also notice that the liquid/solid boundary for water is negative. Any liquid water buried in the subsurface and migrating upwards would freeze before vaporizing. On the other hand, liquid CO2 will boil when depressurized and may erupt phreatically.

Phase diagrams of water and CO2 from Hoffmann [2000].

How then, do you depressurize the CO₂ and have it erupt at the surface? This requires the removal of 50 meters of rock, not something that happens regularly. However, near steep slopes, such as at the dichotomy boundary or crater rims, high shear stresses can develop, inducing a landslide. Newly uncovered terrain will have clathrates and liquid CO₂ buried in it which will no longer be stable. The CO₂ will flash to the gas phase, blasting the upper layers apart. When the upper layers are blown off, deeper layers will be uncovered and the decompression front will migrate downwards. This high-pressure gas will support a cloud of debris which will rush downslope like a terrestrian pyroclastic flow. These "cryoclastic flows" will also carry along any clathrates in the regolith which vaporize much more slowly. These clathrates will gradually provide a fresh supply of gas to the debris flow extending the duration. These cryoclastic flows take on the basic geomorphic shapes of a pyroclastic flow, but are much larger, up to 1000 km long. See the diagram below for the initiation of the flow.

Martian Cryoclastic Outbursts on Mars. Taken from Hoffmann [2000].

The cryoclastic flows can also easily climb obstacles up to 1 km high, wheras water would first have to fill the basin before it could spill over. The water would also leave tide marks around the basin at that height and these are not observed.

Although this model resolves the paradoxes left by the aqueous model and explains the observations, it is not perfect. No phreatic cones are observed which might be the source of the CO₂ eruptions. Also, these flows are dependent on landslides to get them started which can only happen in certain unstable regions. Furthermore, the resemblence of these features to those carved by water on Earth are striking, as pointed out byVictor Baker of the University of Arizona. Additional objections are made by Bruce Jakosky of the University of Colorado who would prefer to see all the features explained by one coherent model, rather than one at a time [O'Hanlon, 2001]. Clearly, further exploration is necessary to determine which (if either) of these models are correct. Whatever the history of the Martian atmosphere, it is clear that there is a vast pool of volatiles available, a critical factor in terraforming.

Most of the material on this page was taken from Hoffmann [2000].

These pages designed by James Roberts
Last updated:  05 May 2002

http://anquetil.colorado.edu/~jhr/terraform/whitemars-nf.html