In the 1970s, orbiter missions around Mars revealed that during southern spring, large areas near Mars's south pole became much darker than the rest of the seasonal ice cap. How could this area be in the polar region and not be covered in bright ice? Intrigued, planetary scientists called the area the 'cryptic region' of the south seasonal cap.
Art © Mark Schultz and available now in his new book “Various Drawings Vol. 2” from Flesk Publications HERE.
The mystery deepened in the late 1990s when new observations showed that the temperature of the cryptic region was close to -135º C. At that temperature, carbon dioxide ice had to be present. So, scientists developed the idea that a one-metre-thick slab of clear carbon dioxide ice covered the cryptic region, allowing the dark surface underneath to be seen.
However, the new observations from Mars Express's OMEGA instrument show that this interpretation cannot be correct. The only way to reconcile the apparently conflicting observations is that there is indeed a thick slab of dry ice in this area, but its surface is so heavily covered by dust that few of the Sun’s rays make it to the deeper layers and back again.
How does the dust get on top of the slab? The answer could be provided by the mysterious markings that dot the cryptic region. Known as spots, 'spiders' and 'fans' depending upon their shapes, they were discovered in 1998–1999 by NASA's Mars Global Surveyor. Planetary scientists that sunlight heating the soil causes carbon dioxide bubbles below the ice to erupt as geysers erupts throwing dust onto the surface creating the fans. No signature of clear CO2 ice from the 'cryptic' regions in Mars' south seasonal polar cap. 2006. Y.Langevin et al. Nature 442: 790-792.
Mars is an Icehouse (no frozen zombies though...):