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Spiders on Mars

Participants: F. Schmidt MCF, F. Costard DR, J. Gargani, MCF, Ch. Marmo IR., external collaboration : S. Douté, B. Schmitt (LPG, Grenoble), Y. Langevin, M. Vincendon (IAS, Orsay)

Strange but true, there is “spiders” in the South Polar Region of Mars. We refer to the geomorphologic feature discovered one decade ago with the high resolution imaging. Spiders are dendritic network, with a shape that looks like an arthropod (see fig. 1). Spiders are often associated with a dark fan that usually begins at the center of the feature.

Figure 1 : Spiders and jets as seen from HiRise/Mars Reconnaissance Orbiter (NASA) with a spatial resolution lower than 1 meter.

Every year, during the polar winter, the surface temperature is low enough to reach 140K so that atmospheric CO2 condensates. In the springtime, a layer of condensed CO2 drapes the whole landscape with a thickness up to 1 meter. The spiders’ formation is most probably related to the CO2 sublimation during the spring (see fig. 2). According to the current understandings, sublimation occurs at the bottom of a translucent CO2 ice layer creating a trapped gas that goes into pressure. When pressure is large enough to crack the surface CO2 layer, the gas escapes, carrying a large amount of dust and creating the vents and fans. The spiders are the cavities built trough the year in the regolith by this venting process.

Spiders and jets formation model sketch (from Kieffer et al, 2006.). Figure 1 is probably taken in the step c) or d).

The “cryptic region” is a dark region of the South Polar Region where most of the spiders are located (see fig. 3). Using the OMEGA/Mars Express (ESA) near infrared spectra, it is now possible to test this exotic process. The first results shows that there is no slab ice in the cryptic region but rather in the South Permanent cap, where no spiders are present (see fig. 3). This new view challenges the current knowledge. Mars and especially the cryptic region haven’t revealed their mysteries yet...

Figure 3 : Translucent slab ice in the South Polar Region for early spring (from Schmidt et al, 2009.). The cryptic region, where most of the spiders occur, is indicated by the black line. The slab index is very low, showing that the CO2 ice is not in a translucent state. The smaller area in black, near the geographical south polar cap, is the permanent polar cap, that is most likely covered by a translucent slab ice.

Reference :

Kieffer, H. H.; Christensen, P. R. & Titus, T. N., CO2 jets formed by sublimation beneath translucent slab ice in Mars’ seasonal south polar ice cap, Nature, 2006, 442, 793-796

Piqueux, S.; Byrne, S. & Richardson, M., Sublimation of Mars’s southern seasonal CO2 ice cap and the formation of spiders, Journal of Geophysical Research (Planets), 2003, 108, 3-1

Schmidt F., Douté S., Schmitt B., Langevin, Y., Bibring J.-P. and the OMEGA Team, Slab ice in the seasonal south polar cap of Mars; Proceedings of EUROPLANET Conferences, 2009

IDES Université Paris-Sud 11 CNRS Faculté des Sciences
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