There's pretty widespread acceptance now that many of the features of Mars are best explained by the presence of liquid water, but that water seems to have frozen in the distant past. We can spot what appear to be glacial features on the planet now, but the local conditions are such that, instead of melting to liquid, this water will go directly into the vapor phase. The main question that has been up for debate is how long water lasted on the red planet; a paper in the latest issue of Geology suggests that the answer is much longer than we'd generally thought.

The paper focuses on features in an area of the planet called Ares Vallis, a drainage channel cut in the early history of Mars when an area called Iani Chaos (can I just say I love that name?) melted. Some flat, rimless features in the Ares Vallis formed long after this channel was cut, and have been the topic of much debate. Some have argued that they formed as subsurface ice underwent sublimation into vapor, causing the surface to collapse; others argue the basins were dry basins that had been occupied by ancient lakes.

The new paper takes advantage of images obtained by the Mars Reconnaissance Orbiter, which carries some of the most sensitive cameras ever sent away from Earth. The new images reveal details down to six meters/pixel, and clearly show channels that travel among the depressions, generally linking one basin with another one that's lower in elevation. The authors are confident that these once carried water between what they now feel must represent ancient lakes.

Based on a model that incorporates the rates at which Martian features are obliterated by impact craters, the authors estimate that these features date from about 3.3 billion years ago—well into Mars' Hesperian period, and long after the Martian atmosphere should have been able to support liquid water. The authors argue that a climate fluctuation, possibly linked to the period's intense volcanism, may have produced a temporary warming. They highlight the similarity of nearby features with those found in areas where permafrost has melted.

Why is this of interest? The results push the era in which Mars supported (at least sporadically) liquid water to over a billion years. "The extension of warm and wet surface conditions on Mars from the early Noachian (4.5 Ga) into the late Hesperian (3.0 Ga) may have facilitated long-term evolution of microbial life in isolated pockets of stable near-surface and subsurface water," the authors conclude.

Geology, 2010. DOI: 10.1130/G30579.1