At least some of the channels on Mars were created by deluges of rain somewhere from 3.5 billion to 3.7 billion years ago, a new paper published in Science Advances argued on Wednesday, based on comparison with gullys in the American southwest.
"Contrary to previous NASA observations which focused on small gullies on south facing slopes where they think, or thought, water or brine may still flow today in tiny quantities during 'warm' Mars summer days, our paper analyzes the relics of ancient larger-scale flows," Hansjoerg Seybold of the ETH Zurich science university told Haaretz.
Large indeed: the channels that Seybold and colleagues studied are mostly several hundreds of meters wide and kilometers long, he says. These are valleys that we can observe and that survived 3.5 billion to 3.7 billion years, he points out.
How exactly do we know their age? "Usually in planetary geology, ages of geologic units are reconstructed by counting how many meteorites hit a certain area," Seybold explains. "The more craters you find, the older the unit. How to convert this into real age numbers is a whole field in planetary geology."
So we shall let that go, but do note meanwhile that the team isn't arguing that rainstorms pounding the land created all of Mars' gullys. There may well be places on Mars where, as on Earth, groundwater sapping was the major driver for the erosion that caused the valley network.
"Some of the largest-scale channels on Mars were probably formed by huge mega-floods, not rainfall," Seybold told Haaretz.
Also, other processes such as glaciers and lava flows could have incised at least some of the scars on Mars' face. "But more and more evidence from recent observations and space missions points to surface water," says Seybold, by which he doesn't mean today, but rather in Martian prehistory.
"This would mean that the climate on Mars 3.7 billion years ago was much different than today," he observes.
Who dunnit and who couldn't have
So-called "canals" on Mars have been intriguing Earthlings since the invention of the telescope. Early observers wondered if they had been artificially constructed by little green men. Although the jury's still out on whether life does or ever did exist on Mars, it did not take long to realize that the channels were natural.
The new rainfall hypothesis arose from noticing similarities between certain narrow Martian channels, and features of the Upper Colorado "Dirty Devil" basin, say the researchers.
A study on gully networks in the American southwest desert showed a relationship between aridity and valley-branching angles, the team explains. Despite differences in gravity, atmospheric pressure and a host of other things, basic physics would apply to the red planet, too.
On Earth, the more arid the climate, the narrower the branching angles of the valley channels, explain the researchers. The more humid the climate, the wider these branching angles are. The scientists found that valley networks on Mars tend to branch at narrow angles similar to those found in Dirty Devil.
The Dirty Devil is a hell of a name: it's an 130-kilometer-long tributary of the Colorado that meanders southward through some quite deep canyons.
Meanwhile on Mars, the scientific conclusions now presented were based on two independently obtained data sets. Around 3.7 billion years ago, Mars had an arid climate with the occasional rainstorm, resulting in runoff, resulting in gully formation as happened at the Dirty Devil. Not re-emerging groundwater: real rain.
So is there life on Mars? Was there once?
First off, since the gouges in the Martian surface formed billions of years ago, going by the impact record, the failure to detect rain or flowing water today is irrelevant. But it does indicate that once, billions of years ago, the planet may have been more hospitable to life than today. So it supports the thesis that at least once, Mars may have hosted some form of life.
On Tuesday the world press reported yet another Martian mystery: the reported discovery of a massive blue sand dune inside a crater, dwarfing the "normal" Martian dunes around it. A photograph of the cnidarian-shaped dune was taken by the Mars Renaissance Orbiter.
Scientists think the dune may be made of different stuff from the dunes around it.
But there is no reason to think it's a sign of life, no more than the organic chemicals and methane spikes reported on the planet last month were.
Recent discoveries on Mars, including the possibility that it didn't lose its water after all, but the water was absorbed by the crustal rocks, are incredibly intriguing. But based on the principle of Occam's razor, according to which the simplest explanation should do, as long as the phenomena on Mars can be explained by geology rather than wishful thinking, they should remain explained by geology.