Mars Had Big Rivers For Billions of Years, Study Suggests (space.com)

A new study suggests that Mars once had giant rivers larger than anything on Earth after the planet lost most of its atmosphere to space. "That great thinning, which was driven by air-stripping solar particles, was mostly complete by 3.7 billion years ago, leaving Mars with an atmosphere far wispier than Earth's," reports Space.com. "But Martian rivers likely didn't totally dry out until less than 1 billion years ago, the new study found." From the report: "We can start to see that Mars didn't just have one wet period early in its history and then dried out," study lead author Edwin Kite, an assistant professor of geophysical sciences at the University of Chicago, told Space.com. "It's more complicated than that; there were multiple wet periods." The team's work suggests that Martian rivers flowed intermittently but intensely over much of the planet's 4.5-billion-year history, driven by precipitation-fed runoff. The rivers' impressive width -- in many cases, more than twice that of comparable Earth catchments -- is a testament to that intensity.

It's unclear how much water Martian rivers carried, because their depth is hard to estimate. Determining depth generally requires up-close analysis of riverbed rocks and pebbles, Kite said, and such work has only been done in a few locations on Mars, such as Gale Crater, which NASA's Curiosity rover has been exploring since 2012. The ancient Mars rivers didn't flow in just a few favored spots; rather, they were distributed widely around the planet, Kite and his colleagues found.

Comments

Re:No rain?

[TheSync]

Meteorite data show that Mars had a much denser atmosphere billions of years ago, perhaps as dense as half that of the Earth (now it is only 0.06 times as dense as the Earth's). Back then it had a magnetic field to avoid atmosphere stripping by the solar wind.

Re:No rain?

[Anonymous Coward]

That seems implausible. If the solar wind was strong enough to strip away CO2 and CH4, it would have also been strong enough to strip away water vapor.

The stripping wasn't exactly instant, it would take millions of years for the solar winds to get rid of the atmosphere.
Water vapor would probably one be in the atmosphere for a couple of weeks until it fell down again. Nowhere close to the time needed for solar winds to carry it away.

Also, with "most of the atmosphere" I assume we are meaning just above 50% of it because without atmosphere the water vapor will be the gas with the highest density around and will stay at the bottom.

Re:No rain?

[Rei]

Could have been large impact-driven vaporization events that temporarily create a denser, water-rich atmosphere, perhaps? I haven't read the full study, so I'm not sure what they're positing. Water does need a certain minimum pressure to be able exist as a liquid at all. Hygroscopic salts at high concentrations can let it exist as a liquid at much lower pressures, though I'm not sure how you'd sustain huge brine-filled rivers for billions of years; you'd expect the source of said salts to be quickly exhausted by such flows.

Personally I'm more curious about Venus's rivers, like Baltis Vallis, the longest riverbed in the solar system. We don't even know what fluid carved them, let alone where it came from or where it went. Theories cover everything from liquid sulfur to supercritical CO2, but most likely is that it's thermal erosion by rare (by Earth standards) types of low-temperature lavas, such as carbonatites or similar.

(Love carbonatites... look like crude oil during the day, glow maroon in the dark, flow like water, and rapidly oxidize to bright white after cooling. Also tend to be very rich in valuable minerals)