That is true, acknowledges Ken Tanaka, a planetary geologist at the United States Geological Survey, but he warns that it isn't productive to always grab at the first available earthly explanation to explain extraterrestrial geology.

"We shouldn't forget that this is another planet." Tanaka said. "We haven't set foot on it. And even though we have robots that have been there, their observations have been very limited. The very fact that these [features] were such a surprise demonstrates how naive we are."

A different possibility, which Tanaka suggests, is that the features are not caused by water, but by some other liquid, say, carbon dioxide, or a substance closely-related to water called clathrate. Clathrate is a special crystal in which water forms a lattice structure that can hold larger molecules, such as methane or carbon dioxide, in its center.

On Mars the water may be locked up in the form of a carbon dioxide clathrate, Tanaka said. This molecule has certain characteristics that cause it to behave somewhat differently from water. For instance, water can be melted by a sudden exertion of pressure. Squeeze an ice cube with enough force, and it will suddenly melt. Clathrate has the opposite property. With a sudden drop in pressure, clathrate ice could quickly turn to water.

If clathrate were buried underground, and a landslide suddenly uncovered some of this subsurface clathrate ice, the pressure release might be enough to cause a violent chain-reaction of melting. This could dump out a fizzing and bubbling wave of liquid that would pour down the hillside, carrying debris and boiling away as it gushes downward.

It is also possible that the now-famous gully features are created not by liquid at all, but by explosive flows of hot gas or water vapor that might be related to volcanism. Outbursts of gas might create fluid-like flows, which could rush downhill, carrying dust and debris and creating the newfound features.

These might be small-scale versions of the kinds of hot gas and ash flows witnessed in the 1980 eruption of Mount Saint Helens, Tanaka said.

Whatever the real mechanism that created these features is, Tanaka said they are exciting because they appear to be so young.

But other scientists said it is important to remain cautious about the ages. Tim Parker is a planetary geologist at NASA's Jet Propulsion Laboratory. He studies Martian geological processes, and has been involved in analyzing landing sites for past and future NASA Mars missions.

Scientists arrive at the rough ages for planetary bodies by counting the craters left in their surfaces. The older a surface is, the longer it will have been exposed to the destructive impacts from meteorites, and the more cratered it will be.

Counting the number of craters on a surface is a relatively reliable method for judging the surface's age, Parker said, but it is not foolproof. It is best when used across large areas, and less reliable at small scales. Also, it is not always clear whether a surface looks devoid of craters because it is fresh, or because it has been freshly uncovered.

For example, a region of rock could be protected from cratering processes because it is covered by sand dunes. If the dunes were to retreat, or be cleared by wind, an old surface could be exposed and appear new.

It is possible that some of the so-called water-influenced features were actually caused by landslides, Parker said. "As much as I would like to believe that water is involved, some of them look like they could be dry-debris-flow features."

Parker and others said these features need to be studied in more detail by scientists throughout the planetary science community before the water hypothesis is confirmed.