Planets

No Water Required: Mars Gullies Caused by 'Dry Ice'?

More than a decade ago, NASA’s Mars Global Surveyor returned stunning images of gullies shaped like water-carved streams on Earth, raising the prospect that Mars may be a friendly haven for life. But appearances can be deceiving.

More than a decade ago, NASA's Mars Global Surveyor returned stunning images of gullies shaped like water-carved streams on Earth, raising the prospect that Mars may be a friendly haven for life.

But appearances can be deceiving.

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New research shows that the Martian gullies were much more likely to be sculpted by seasonal outbursts of dry ice -- frozen carbon dioxide -- than water.

"When dealing with other worlds, we must take care to remember that unfamiliar processes are possible and even likely in alien environments," planetary scientist Colin Dundas with the U.S. Geological Survey in Flagstaff, Ariz., wrote in a commentary in this week's Nature Geoscience.

The gullies, which appear in slopes around the planet's mid-latitudes, have been a puzzle because modern-day Mars is too cold to support the amount of surface water needed to carve the features.

The gullies are relatively young, just a few million years old. Even more intriguing is that repeat imagery by a series of Mars orbiters shows some gullies are forming today.

PHOTO: New Gully on Mars Found

Two French researchers believe they have the answer: seasonal pressure changes in pockets of dry ice trapped beneath the planet's surface are periodically erupting, which disrupts the regolith and triggers debris flows.

The theory is not completely new, but Cedric Pilorget and François Forget, with the University of Paris-Sud, and Paris' Pierre and Marie Curie University, respectively, flesh out the idea with some hard numbers.

Their new computer model calculates seasonal changes and impacts of an underlying layer of regolith, a carbon dioxide ice layer and the carbon dioxide-dominated gas atmosphere above. The simulation can take into account a variety of latitudes, slopes and other parameters.

The scientists found that most of the gullies could be created in a process that does not require any liquid water.

PHOTOS: When Liquid Water Gushes on Mars

"Dry-ice-related processes seem to have played a more important role in the evolution of the martian landforms than previously thought," the researchers conclude in a study published in this week's Nature Geoscience.

"The role of liquid water in gully formation should, therefore, be reconsidered, raising the question of the importance of its occurrence in Mars' recent past," they said.

These NASA Mars Global Surveyor images show fresh gully deposits inside an unnamed crater in the Centauri Montes region of the Red Planet.

Once again the world is abuzz about water on Mars. Sure, we already

know

that there's a plentiful supply of water ice at the Red Planet's poles; we

know

that approximately 2 percent of the Martian regolith (at Mars rover Curiosity's location in any case) is composed of water; we also

know

that ancient Mars was a wet world, possessing rivers, lakes and even seas -- according to the sedimentary rock and minerals that could have only been formed in an abundance of liquid water. But now NASA has found pretty solid evidence that the seasonal short-lived, dark channels seen on steep slopes are formed by salty liquid water gushing over the apparently barren Mars surface

today

.

MORE: Mystery Solved: Water DOES Flow on Mars

Shown here is a 3-D map of the slopes of Hale Crater which is based on observations by the High-Resolution Imaging Science Experiment (HiRISE) camera on board NASA's Mars Reconnaissance Orbiter. The dark channels running down the slopes are known as "recurring slope lineae," and they could be one of the biggest hints yet that some form of basic, yet extreme, microbe may use that water as a means to eke out an existence just below the surface.

After arriving in Mars orbit in 2006, the Mars Reconnaissance Orbiter began an epic campaign of observing the surface in high resolution. In 2011,

planetary scientists reported

the formation of recurring slope lineae, or RSLs, as the southern hemisphere passed from winter to summer. Like drips running down a melting ice cream cone, these dark features resembled flowing liquid water on the Martian surface. However, the finding was not conclusive.

MORE: NEWSFLASH: Mars is Toxic

Surface missions to the Red Planet have gradually been forming a picture of the chemicals in the Mars soil, or regolith. Of particular note was the 2008 NASA Phoenix Mars lander that uncovered evidence for droplets of liquid water on the surface. However, liquid water should be impossible on current-day Mars; the atmospheric pressure and temperature is too low for liquid water to persist, water either exists in a solid (ice) or gaseous (vapor) state. Ice doesn't melt on the Martian surface, it sublimates, like carbon dioxide ice (a.k.a. "dry ice") does at room temperature on Earth. The possibility of liquid water on the surface as seen by Phoenix added credence to the possibility that the surface must be rich in a highly toxic salts, known as perchlorates. When mixed with water, these salts lower water's freezing point, potentially allowing liquid water to persist in pools just under the surface, despite the chilly conditions. Since then, NASA's Curiosity has also uncovered evidence for perchlorates laced in the soil.

MORE: Briny Water May Pool in Mars' Equatorial Soil

Image: More evidence for RSLs in Garni Crater, as dramatically shown in this 3-D map constructed from HiRISE data.

Perchlorates form the key behind Monday's grand announcement. Through the use of another instrument on board the Mars Reconnaissance Orbiter, a spectrometer called CRISM (Compact Reconnaissance Imaging Spectrometer for Mars), and using a new analysis technique, scientists have been able to study the chemical composition of these dark RSL channels. What they found was the chemical signatures of hydrated salts -- basically the residue of perchlorates left behind after surges of salty water, known as "brines."

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Although exciting, the source of these brines apparently supplying the RSL channels is something of a mystery. Scientists point to the possibility of the seasonal melting sub-surface ice, or perhaps moisture from the atmosphere somehow condensing to supply the flow of water, but neither explanation is satisfactory.

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Image: An oblique view from the Mars Reconnaissance Orbiter of slopes in Newton Crater, highlighting warm season flows in Newton Crater.

This may be strong evidence for liquid water flows on the surface of Mars, potentially revealing sub-surface aquifers under an otherwise barren landscape, but scientists are quick to point out that these brines are not particularly conducive to life, at least

life as we know it.

The extreme salinity of the water causing the RSLs would likely be too dense for the survival, let alone evolution, of microbes. But on Earth, despite its toxicity, some microbes in desert locations use perchlorate salts as an energy source. Regardless, the location of these possible outflows would make it very difficult for us to send robotic explorers to check them out. Although researchers are hopeful that perhaps Curiosity may find some RSLs of its own on the slopes of Mount Sharp in Gale Crater, the best way to study them is, currently, from orbit. But it would be nice if we could send a human expedition to the Red Planet to check out these outflows first-hand.

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