WASHINGTON (Reuters) - A long, deep canyon and the remains of beaches are perhaps the clearest evidence yet of a standing lake on the surface of Mars -- one that apparently contained water when the planet was supposed to have already dried up, scientists said on Wednesday.

This is reconstructed landscape showing the Shalbatana lake on Mars as it may have looked roughly 3.4 billion years ago. Data used in reconstruction are from NASA and the European Space Agency. REUTERS/University of Colorado/G. Di Achille

Images from a camera called the High Resolution Imaging Science Experiment aboard the Mars Reconnaissance Orbiter indicate water carved a 30-mile-(50-km-)long canyon, a team at the University of Colorado at Boulder reported.

It would have covered 80 square miles (200 sq km) and been up to 1,500 feet deep, the researchers wrote in the journal Geophysical Research Letters.

There is now no dispute that water exists on the surface or Mars -- robot explorers have found ice. There is also evidence that water may still seep to the surface from underground, although it quickly disappears in the cold, thin atmosphere of the red planet.

Planetary scientists have also seen what could be the shores of giant rivers and seas -- but some of the formations could also arguably have been made by dry landslides.

“This is the first unambiguous evidence of shorelines on the surface of Mars,” said Gaetano Di Achille, who led the study.

“The identification of the shorelines and accompanying geological evidence allows us to calculate the size and volume of the lake, which appears to have formed about 3.4 billion years ago,” Di Achille said in a statement.

Water is key to life and scientists are looking desperately for evidence of life, past or present, on Mars. Having water on the planet could also be useful to future human explorers.

“On Earth, deltas and lakes are excellent collectors and preservers of signs of past life,” said Di Achille. “If life ever arose on Mars, deltas may be the key to unlocking Mars’ biological past,” Di Achille said.

“Not only does this research prove there was a long-lived lake system on Mars, but we can see that the lake formed after the warm, wet period is thought to have dissipated,” assistant professor Brian Hynek said.

The lake probably either evaporated or froze over after abrupt climate change, the researchers said. Its waters would have turned into vapor. No one knows what turned Mars from a warm, wet planet into the frozen, airless desert it is now.