The find suggests that substantial amounts of water pooled and persisted on the martian surface hundreds of millions of years later than many planetary scientists currently estimate, the team says.

A 3.4-billion-year-old Lake Champlain on Mars? That's what scientists say they have uncovered as they analyzed images from the Mars Reconnaissance Orbiter. An artist's rendering of the lake is on the left. On the right, a segment of the MRO image the researchers scrutinized. Click on "Enlarge" and the arrow points to faint parallel dune ridges indicating an old shoreline.

Moreover, the images the steam studies bear witness to a vast delta that would have formed when water gushed into the valley to form the lake. This would have been prime real estate for microbes, based on what biologists have learned about microbial ecosystems buried within Earth’s river deltas.

Images from NASA’s Mars Reconnaissance Orbiter’s HiRISE camera and topographical data from the Mars Global Surveyor orbiter’s laser altimeter sealed the deal, according to team leader Gaetano Di Achille. He’s a planetary scientist at the University of Colorado at Boulder. The data, he says, provided “the first unambiguous evidence of shorelines on the surface of Mars.”

The formal report of the results has been accepted for publication in the journal Geophysical Research Letters.

By the way, it’s hard to get an idea of what the team saw from the images we can post here. For a closer look — a much closer look — download and explore the full images from the HiRISE web site.

The team estimates that in its heyday, the lake covered 80 square miles and reached a depth of some 1,500 feet.

Planetary scientists have been hunting for such so-called paleolakes of this decade as NASA Mars missions have continued with their theme of “follow the water.” But until now, no one has been able to provide anything resembling smoking-gun evidence for ancient shorelines in other possible lakes — including two sites NASA is considering as landing spots for the Mars Science Laboratory, now scheduled for launch in 2011.

Shalbatana Vallis is one impressive valley. It’s some 800 miles long and about 12 miles across at its widest point. To the north, it drains into a vast basin. And, a bit like Arizona’s Grand Canyon, it has a handful of smaller valleys that branch off and lead upward to the surrounding highlands.

One of these, an 11-mile long side valley, is thought to have been the funnel through which at least one, and perhaps several, extreme floods flowed to form the lake. The telltale delta — formed as flood-borne sediment fanned out as it reached the valley floor — took at least 1,000 years to build, Di Achille’s team estimates.

With only one clearly identifiable shoreline, the team posits that the lake met its end either by draining fairly quickly or by freezing over, with the ice subsequently going from frozen to water vapor without passing through a liquid stage. This process also would have prevented new shorelines from forming as well.

Achille explains that most researchers hold that any warm, wet climate the planet enjoyed would have ended about 3.7 billion years ago. After that, conditions on the planet’s surface prevented water from gathering and persisting for significant periods of time. The new results suggest that significant amounts of water could have pooled on the surface far longer based on regional conditions, even if the overall climate no longer supported the presence of large bodies of standing water.