CO2 Sublimation and Jet-Like Activity
The seasonal CO2 cycle on Mars is, to first approximation, the exchange of CO2 between the atmosphere and the seasonal polar caps in response to changes in insolation. Insolation changes arise from Mars’s orbit combined with the planet’s obliquity (=25.19°). A significant perturbation is produced by Mars’s relatively large eccentricity (e =0.0934) which increases the duration of winter in the southern hemisphere. The seasonal insolation variation is especially large at the poles which see no insolation for more than 300 earth days. The absence of a thick atmosphere acting as a blanket results in energy loss from the poles through thermal radiation. As the temperature of the cap drops, condensation of CO2 onto the unilluminated polar cap occurs, thereby releasing latent heat and stabilizing the temperature, T.
The measured temperature (148 K) leads to a deposition rate of 45 µg/m2/s which, assuming a frost bulk density of around 1 g/cm3, leads to a build-up of a CO2 frost layer at a rate of around 4 mm per day or approximately a 1.3 m layer over the entire Martian winter season. This then sublimes in spring producing a number of remarkable effects.
The Mars Orbiting Camera [MOC; Malin et al., 1998] showed the presence of fan-like dark patches on the surface of the subliming southern polar caps. Kieffer  suggested that the unique properties of CO2 ice are responsible for this phenomenon which probably results from a type of solid-state greenhouse effect [Matson and Brown, 1989; Kaufmann et al., 2006; Piqueux et al., 2003; Kieffer, 2007; Piqueux and Christensen, 2008] involving CO2 slab ice. By being translucent at optical wavelengths, but relatively opaque at infrared wavelengths, a thin CO2 ice sheet (the result of condensation in winter onto a dusty or rocky substrate) can result in solar energy being deposited at the boundary between the ice sheet and the substrate without a ready means of escape. This leads to sublimation at the ice-substrate interface with subsequent pressure build-up. When the pressure becomes sufficient to exploit a weakness in the ice sheet, it is released by venting, effectively producing a jet-like activity. We have been addressing this “paradigm” in several ways.
Firstly, the pressure under the CO2 slab ice can crack the slab and produce fissure-like activity [Portyankina et al., 2010]. There is no obvious reason why these fissures should appear in the same position at each Martian year. Hence, we have started investigating the second and third Mars year of observations of the southern polar region to assess reproducibility. Secondly, we have been studying sources of activity. For example, on slopes there appears to be a correlation between the local topography and the rate of initiation of activity which we are seeking to quantify. Work on this subject has already been started in the framework of a bachelor thesis [Gfeller, 2009]. Finally, the deposition pattern observed is frequently correlated to the local topography indicating near-surface flow rather than “fountain-like” behaviour [Thomas et al., 2010]. We also observe bright CO2 deposits surrounding the dark deposits. We are currently using fluid dynamics tools in an initial attempt to model these observations.
Our colleague, Candy Hansen, had the idea of mapping fan structures over the much of the southern polar region of Mars with the help of the general public. This was taken up and implemented in the Zooniverse project, PlanetFour. The project is supported by former Bern post-docs Anya Portyankina and Michael Aye.
Hansen, C.J., S. Byrne, G. Portyankina, M. Bourke, C. Dundas, A. McEwen, M. Mellon, A. Pommerol, and N. Thomas, (2013), Observations of the northern seasonal polar cap on Mars: I. Spring sublimation activity and processes. Icarus, 225, 881-897, doi:10.1016/j.icarus.2012.09.024
Portyankina, G., A. Pommerol, K.-M. Aye, C.J. Hansen, and N. Thomas, (2013), Observations of the Northern seasonal polar cap on Mars II: HiRISE photometric analysis of evolution of northern polar dunes in spring, Icarus, 225, 898-910, doi: 10.1016/j.icarus.2012.10.017.
Pommerol, A., T. Appéré, G. Portyankina, K.?M. Aye, N. Thomas, and C.J. Hansen (2013), Observations of the Northern seasonal polar cap on Mars III: CRISM / HiRISE observations of spring sublimation, Icarus, 225, 911-922, doi:10.1016/j.icarus.2012.08.039
Portyankina, G., A. Pommerol, K.-M. Aye, C.J. Hansen, and N. Thomas, (2012) Polygonal cracks in the seasonal semi-translucent CO2 ice layer in Martian polar areas, J. Geophys. Res. (Planets), 117, Issue E2, CiteID E02006, doi: 10.1029/2011JE003917.
Thomas, N., G. Portyankina, C. J. Hansen, and A. Pommerol (2011), Sub-surface CO2 gas flow in Mars' polar regions: Gas transport under constant production rate conditions, Geophys. Res. Lett., 38, L08203, doi:10.1029/2011GL046797.
Pommerol, A., G. Portyankina, N. Thomas, K.-M. Aye, C.J. Hansen, M. Vincendon, and Y. Langevin, (2011) Evolution of south seasonal cap during Martian spring: insights from highresolution observations by HiRISE and CRISM/MRO, J. Geophys. Res. (Planets), 116, 8007, doi: 10.1029/2010JE003790.
Hansen, C.J., M. Bourke, N.T. Bridges, C. Colon, S. Diniega, C. Dundas, K. Herkenhoff, A. McEwen, M. Mellon, G. Portyankina, and N. Thomas, (2011), Seasonal Erosion and Restoration of Mars’ Northern Polar Dunes, Science, 331, 575-578, doi:10.1126/science.1197636.
Thomas, N., G. Portyankina, C.J. Hansen, and A. Pommerol, (2011), HiRISE observations of gas sublimation-driven activity in Mars' southern polar regions: IV. Fluid dynamics models of CO2 jets, Icarus, 212, 66-85, dx.doi.org/10.1016/j.icarus.2010.12.016