Gullies coming down the south-facing wall of a trough in the Sirenum Fossae/Gorgonum Chaos region of the martian southern hemisphere. Each channel and its associated fan--or apron--of debris appears to have started just below the same hard, resistant layer of bedrock located approximately 100 meters (about 325 feet) below the top of the trough wall. The layer beneath this hard, resistant bedrock is interpreted to be permeable, which allows ground water to percolate through it and--at the location of this trough--seep out onto the martian surface. The channels and aprons only occur on the south-facing slope of this valley created by faults on each side of the trough. The depression is approximately 1.4 km (0.9 mi) across. Image: NASA/JPL/Malin Space Science Systems. Caption: MSSS
These theories are very different -- one involves eruptions of liquid water from underground, the other the thawing of buildups of surface ice -- but they have one major thing in common: they both do agree that the gullies are indeed water-produced.
But there are at least one, and maybe two, serious alternate theories proposing that the gullies are not water-produced -- and one of them states that the "fluid" that created them isn't even liquid!
The first was described in some detail by Dr. Michael Carr of the U.S. Geological Survey at the press conference in which the discovery was first announced.
Carr is arguably the world's leading authority on the subject of water on Mars, but he is seriously skeptical about these gullies being water-produced for the two reasons given above.
He thinks there is a serious chance that the substance responsible is not subsurface water, but subsurface carbon dioxide.
It's been recognized for a long time that Mars' subsurface -- because the planet's temperatures are so much lower than Earth's -- almost certainly contains large amounts of stored CO2, in several different possible forms: as solid frozen "dry ice"; a "clathrate" mixture of CO2 incorporated into water ice; liquid CO2 (which can exist at pressures above 5 bars); and/or gaseous CO2 "adsorbed" by Mars' cold soil (that is, chemically stuck to its grains).
There is still a lot of debate about how much there is, but Jeffrey Kargel and Kenneth Tanaka have calculated that the total amount of CO2 outgassed by Mars' geothermal vents over its history "could form a global mean layer thickness of CO2 clathrate over 200 meters -- and possibly over 1300 meters -- thick, if all the CO2 is present in hydrate form."
In reality, a great deal of that amount must either have escaped from Mars completely over the eons due to its low gravity, or been converted into carbonate minerals. But that's still a lot of underground CO2.
Since there's also a good deal of subsurface water ice on Mars, most CO2 beneath the surface is probably mixed with it as a solid frozen "clathrate".
This substance can only exist under a fair amount of pressure -- equivalent to that found under about several meters of Martian soil and/or rock -- and it must be sealed off from even any small vents to the near-vacuum of Mars' surface. However, a layer of ordinary water-ice permafrost surrounding a body of clathrate can seal it off in this way.
According to Carr's theory, if such a body of clathrate is suddenly exposed to Mars' surface by a landslide on a slope, the CO2 will explode violently out of the clathrate as gas -- and the resultant powerful blast of gas could sweep a cloud of ice and rock fragments and soil down the slope, plowing a gully which could bear a surprisingly close visual resemblance to the narrow one produced by an eruption of fluid, rather than the broader trail left by an ordinary avalanche.
This kind of traveling cloud of "gas-suspended" debris occurs fairly often in volcanic eruptions, where it's known as a "nuee ardente" (burning cloud). One such cloud from Mount Pelee killed all but one of the 30,000 inhabitants of the downslope city of St. Pierre in 1902.
Carr's Martian version, needless to say, wouldn't be glowing ash and pebbles; it might be more accurately called a "refrigerated cloud". According to this theory, these eruptions would occur only on Mars' colder slopes simply because only they would be cold enough for permafrost, and an underlying layer of CO2 clathrate, to be close to the surface and thus likely to be exposed by landslides. (It should be kept in mind that these runoff gullies are quite rare on Mars.)
However, Carr told SpaceDaily that he still has considerable doubts about the feasibility of his tentative theory on the grounds of "kinetics". The key question being whether a Martian gas blast would be powerful enough to avoid immediately dissipating into the near-vacuum atmosphere of Mars to carry debris the distances being seen in the new images.
For this reason, a variant on this theory has been suggested by Dr. Kenneth Tanaka.