Nick any comments on the peer reviewed paper below from JGR Planets?

http://www.gps.caltech.edu/~sstewart/StewartNimmo_CO2MarsJGR2001Preprint.pdf

Surface runoff features on Mars: Testing the
carbon dioxide formation hypothesis

Sarah T. Stewart
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA
Francis Nimmo1
Space Sciences Division, 245-3, NASA/Ames Research Center, Moffett Field, CA

Abstract. Fresh gully-like features on Mars strongly suggest that fluid flowed on
the surface in the recent past. Here we consider the possibility that CO2 vapor supported flows formed the gullies. We find that neither condensed CO2 nor CO2 clathrate hydrate are likely to accumulate in significant quantities in the Martian
crust. In addition, if condensed CO2 were present under lithostatic pressures,
exposure to the atmosphere would produce features analogous to terrestrial
pyroclastic flows, not surface runoff. Finally, the source volume of CO2 required
to support a flow excludes clustered or episodic gully formation. Therefore, we
conclude that CO2 cannot have formed the gullies. In light of these results, liquid
water flow remains the preferred formation mechanism for the recent surface runoff
features.

4. Summary

After examining the geologic processes that may emplace
CO2 in the Martian crust and its stability in the
present climate, we find that neither solid or liquid CO2
nor CO2 clathrate can be accumulated in bulk quantities.
Subsurface solid or liquid CO2 formed in a hypothesized
early, more massive atmosphere would not persist
to the present day because diffusion rates through the
crust are much shorter than the billion-year timescales
for climate change. CO2 condensation in the current
climate is limited to ¡Ü 1000 kg m−2 in the polar regions,
and outgassed CO2 from magmas would escape
to the atmosphere at geologically fast rates. Therefore,
except for seasonal polar caps, it is extremely unlikely
that CO2 has driven surface modification processes in
the present climate.

We find that the mass of CO2 required to support
a single debris flow to be larger than plausible CO2
content in the present day crust. Furthermore, sudden
exposure of condensed CO2 to the surface probably produces
volcanic-like jets of CO2 vapor that are unlikely
to form the observed tapering V-shaped channels and
depositional aprons. We conclude that the gullies were
not formed by CO2-based flows.
In the absence of other possibilities, liquid water is
the most likely formation fluid for the surface runoff
features. Since liquid water is not stable near the surface,
the formation mechanism of these features probably
involves rapid melting or transport of liquid water
from depth. Even though we do not know the absolute
amount of water available in the regolith, H2O vapor
or liquid migration can transport H2O near to the
surface [Clifford, 1993; Mellon et al., 1997]. If liquid
water could be brought to the surface, it would have
no diffculty in carving the observed gullies, as watersupported
debris flows on Earth easily reproduce the
morphology of the observed gullies. Unlike CO2-based
formation, the issues related to melting or transport of
water to form gullies are tractable problems with geologically
plausible solutions [c.f. Knauth et al., 2000;
Saunders and Zurek, 2000; Doran and Forman, 2000;
Gaidos, 2001; Mellon and Phillips, 2001].
Acknowledgments. The authors thank A. Albee, S.