But THEMIS' photos and temperature maps of the additional meter-thick "seasonal" cap of more frozen CO2 that forms around the south pole in midwinter and vanishes during summer has also revealed something intriguing. One big patch in the middle of it which stays dark (the "Cryptic" region) turns out to be so cold that it too seems to be frozen CO2 rather than an isolated patch of bare ground.

The way in which CO2 freezes out on the surface to form the temporary winter caps at Mars' poles isn't well understood. In most areas it seems to be deposited as light-colored fine-grained frost. But in the Cryptic region it may freeze out as solid "slab" ice, forming a layer which is transparent and still shows the darker ground underneath despite being a meter thick.

And solid CO2 has the same greenhouse effect as CO2 gas: sunlight piercing it will warm the underlying surface, whose resultant infrared warmth will be unable to radiate back up through the ice. This warms the underlying soil surface considerably -- to the point that high-pressure pockets of CO2 gas will thaw out at the bottom of the "dry ice" layer.

This seems to explain the round, dark-colored "Dalmatian spots" that form all over the seasonal CO2 caps when they're thawing in spring: they're growing circular patches of dust sprayed out onto the surface of the remaining frost around such a vent. (This is much more plausible than Hungarian scientist Andras Horvath's theory that they're actually short-lived patches of Martian algae growing in briefly thawed surface water ice.)

It also explains the strange, horizontal "spider" patterns of dark streaks that form under the ice of the Cryptic region in spring: the thawed CO2 gas can force open fewer vents in the solid ice there, so it streams horizontally toward each such uncommon vent from the surface for kilometers around, eroding individual gas-blown channels in the soil under the transparent "dry ice", running along horizontally toward the vent.

THEMIS' photos of the "spider" patterns show their branches to look dark in visible light, but warmer and brighter in IR light, as would be expected. Its photos of the patterns in different IR bands also indicate that their central vents do have a cloud of dust and water ice particles over them.

On another subject, it had been hoped that THEMIS' high-resolution photos of Mars' surface in five different visible colors would show local differences providing more clues to the planet's mineral distribution -- but, in another DPS talk, Cornell's James F. Bell revealed that the spectral color differences found so far are extremely subtle and small-scale, whether in Mars' orange dust-covered regions or its smaller areas of gray exposed rocks. The far more detailed color photos to be taken by the 2005 "MRO" orbiter may reveal more. But a few interesting things are showing up.

For one thing, there are areas on the floors of some craters that are faintly more "bluish", although too subtly to be seen with the naked eye. These seem to be actual mineral differences rather than just an effect from different-sized soil particles, but their meaning isn't known.

And Odyssey's detailed color photos of the famous patch of "White Rock" in Pollack crater -- which some scientists once hoped might be surface carbonate minerals -- have confirmed the indications from earlier Viking and Global Surveyor photos and spectra that it's actually just the same light-colored orange as Mars' other regions of especially fine windblown dust. It only looks "white" by comparison with its darker surroundings when you crank up the contrast level in photos to make details stand out better, and it seems to be just an especially concentrated patch of "duricrust" made out of soil glued together by salts that were left behind when a small concentration of surface soil water evaporated at some earlier point in Mars' complex climate history.

Click for Part Five

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