Liquid water could have carved out Mars’ famous gullies

US researchers believe that the mysterious gullies on Mars could have been carved by liquid water that formed when the planet was tilted more on its axis. During these periods, increased solar radiation vaporized carbon dioxide at the planet’s poles, raising atmospheric pressure and allowing liquid water to form.1

Mars has ice at each pole and subsurface, but is thought to be free of liquid water because its atmospheric pressure is below the triple point of water (612 kPa). Under such conditions, ice sublimates rather than melts when it warms. But in 1999, observations by NASA’s Mars Global Surveyor mission revealed river gullies on steep slopes.


Some researchers have postulated that they are actually formed by compressed gas. “Scientists have made phenomenal observations of changes in these ravines that are well-timed when carbon dioxide from the ice turns into a gas,” says Jay Dixon of UCLA and Brown University in Rhode Island. “I’ve looked at these observations and it’s pretty clear to me that it’s loose sand and dust, and that’s not enough to completely cut these channels. The hypothesis also fails to explain why there are no channels above 4,500 m from the surface.

The obliquity, or axial tilt, of Mars is more variable than that of Earth: it is currently 25°, but has been as high as 35° over the past million years. In 2011, Roger Phillips at the Colorado School of Mines and colleagues proposed that in this case, increased solar radiation on the south polar ice cap evaporates more carbon dioxide, increasing atmospheric pressure and allowing liquid water.2

Dixon and colleagues modeled the solar orbit of Mars at a 35° inclination to the Sun: “In the middle of the day, Mars rises above freezing quite often,” Dixon says; “We found this correlation between where the pressure is high enough and where these signs appear, which we think is an unlikely coincidence.” The pressure drop with height naturally explains the disappearance of channels above 4500 m. Therefore, Dixon believes, “650,000 years ago, Mars had enough liquid to carve these channels, which is good news if you’re looking for small amounts of life on the Martian surface in the relatively recent past.”

Jack Sinnard, also at Brown University but not involved in the study, sees it as an important contribution to a long-standing problem that has divided scientists into several camps. “The idea that it’s liquid water has always been troubling because of the instability of liquid water at the surface,” he says. He believes the current study puts a lot of previous speculation on a foundation of quantitative modeling: “Putting all these pieces together, they can say in a declarative sense, ‘Yes, this is more than just speculation—we can make this statement. “‘

Norbert Schorghofer of the Arizona Planetary Science Institute is more cautious: “It’s an exciting correlation, but it remains to be seen what it actually means,” he says. He points out that the stability of liquid water would require the partial pressure of water vapor in the Martian atmosphere to rise above the triple point, and currently less than 0.1% of the total atmospheric pressure on Mars is water vapor: “That’s why it’s so hard to be in liquid water on Mars, and the authors don’t really explain it,” he says; “It takes a lot more than total pressures above the triple point and temperatures above 273 K to form liquid water.”

Dickson agrees that further detailed research will be needed to help “scale up”. [this] global investigation”. “Our study looks at whether Mars was able to achieve the absolute minimum conditions for liquid water at sinkhole sites, and we hope it inspires researchers to look further into this topic,” he adds.

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