Geysers on Mars

One of the most interesting features spotted by the high resolution cameras in orbit around Mars are the dark "spiders" that appear on sand dunes in the spring, especially around the South Pole. Most of these measure about 150 to 300 meters across.

They are accompanied by "dark spots."

Dark spots can also appear without spiders,  as in this image.

After much argument, opinion among Mars scientists has coalesced around the notion that both spiders and dark spots are related to carbon dioxide frozen beneath the surface of the dunes. When the dunes war in the spring, some of the CO₂ erupts upward, creating impressive geysers; these are the dark spots with the trailing "shadows." (Artist's reconstruction above.) In other circumstances the sublimating CO₂ travels through channels toward surface vents, creating the spider patterns.

It would be nice to get a close-up look, but so far NASA has deemed it too dangerous to risk one of their precious rovers in an explosive geyser field.