CO 2 jets formed by sublimation beneath translucent slab ice in Mars’ seasonal south polar ice cap Hugh H. Kieffer 1,2 , Philip R. Christensen 3 & Timothy N. Titus 2 The martian polar caps are among the most dynamic regions on Mars, growing substantially in winter as a significant fraction of the atmosphere freezes out in the form of CO 2 ice. Unusual dark spots, fans and blotches form as the south-polar seasonal CO 2 ice cap retreats during spring and summer. Small radial channel networks are often associated with the location of spots once the ice disappears. The spots have been proposed to be simply bare, defrosted ground 1–3 ; the formation of the channels has remained uncertain. Here we report infrared and visible observations that show that the spots and fans remain at CO 2 ice temperatures well into summer, and must be granular materials that have been brought up to the surface of the ice, requiring a complex suite of processes to get them there. We propose that the seasonal ice cap forms an impermeable, translucent slab of CO 2 ice that sublimates from the base, building up high-pressure gas beneath the slab. This gas levitates the ice, which eventually ruptures, producing high-velocity CO 2 vents that erupt sand-sized grains in jets to form the spots and erode the channels. These processes are unlike any observed on Earth. The vast majority of spots and fans occur in the south polar region of Mars 1,2 , and are common in ‘cryptic’ terrain identified in Mars Global Surveyor Thermal Emission Spectrometer (TES) data as having CO 2 slab ice and anomalously low albedos 4 . Spots and fans (Fig. 1) are often associated with dunes but are common on mesas and individual layers of the polar layered materials 1–3 . Spots are remarkably repeated from year to year 2,5 , and often correspond to the centres of radially branching ‘spiders’ eroded into the substrate (Fig. 2) 6,7 . Blotches are larger than spots, hundreds of metres to tens of kilometres in size, with less distinct boundaries, and, unlike most spots, have albedo patterns that are similar whether CO 2 ice is present or not. Previous studies have suggested that these dark features are areas of early ice defrosting and exposing dark soil 3,5,8,9 . Mars Odyssey Thermal Emission Imaging System (THEMIS) 10–13 infrared images of the temperature of spots and fans that show that they are within 3–5 8C of the CO 2 ice temperature (,145 K), and remain at these temperatures for more than 120 days after sunrise. These tempera- tures are far too cold to be bare soil, which warms to .225 K within days of CO 2 ice removal 4,14 . Thus, the spots must be on, under, or within a layer of CO 2 ice, and an alternative to defrosting is necessary to explain their formation. We selected a representative mesa and trough system (nicknamed ‘Manhattan Island’) for an intense monitoring campaign (Fig. 3). In this region some spots were present at sunrise (Fig. 3a) and increased in number significantly over the next week (Fig. 3b). Some regions remained spot-free for up to 100 days then developed a large number of spots within one week (Fig. 3c, d). Fans were rarely present when spots first form, typically forming days to weeks later. Some fans lengthened by up to 1 km in multiple events, suggesting surface transport at rates well within reasonable wind velocities. Two major brightenings occurred at Ls 2018 and 2418 (Ls is the aerocentric longitude of the Sun and Ls 1808 is the southern spring equinox) with a contrast reversal between ‘Manhattan Island’ and the surroundings (Fig. 3c, e). TES albedos increased to ,0.29 and ,0.32 in dark and bright regions respectively; temperatures in all regions remained near 150 K. Many spots and fans became indistinct, suggesting the deposition of a thin veneer of bright material. Over the next several weeks the spots and fans reappeared in their previous positions, suggesting the removal of the overlying bright material or, which is less likely, the formation of new fans in the same positions. Blotches, unlike spots, have similar albedo patterns in winter with CO 2 ice present and in summer when the ice is gone. This suggests that the ice is translucent, allowing the underlying surface albedo to be seen in winter. LETTERS Figure 1 | Examples of spots and fans in the south polar region of Mars. a, MOC image E09-00231 at Ls 2488 is ,3 km £ 5 km in size, centred near 85.28 S and 181.48 E at ,3 m per pixel. b, MOC image E07-00159 at Ls 2078 is ,3 km £ 6 km in size centred near 86.38 S and 94.48 E at ,11 m per pixel. The dark fans are typically tens to hundreds of metres in length, 10–308 in angular size, originate from a dark spot, and point in a similar direction within a given area, sometimes with curvilinear trends influenced by topographic slopes. Occasionally fans form in multiple directions from a single spot. Some multiple fans have one diffuse edge, suggesting subsequent transport at an angle to the initial orientation. Fans are typically dark relative to the surrounding surface, although relatively bright fans also occur. 1 Celestial Reasonings, Carson City, Nevada 89703, USA. 2 US Geological Survey, Flagstaff, Arizona 86001, USA. 3 Department of Geological Sciences, Arizona State University, Tempe, Arizona 85287, USA. Vol 442|17 August 2006|doi:10.1038/nature04945 793 © 2006 Nature Publishing Group