Interstellar Clouds Eroded Martian Atmosphere

It turns out that solar wind may not be the only culprit behind Mars' thinned atmosphere.

A long time ago, Mars had an atmosphere thick enough to allow running water on its surface. But today these vast gullies -- and features that some scientists interpret as ocean shorelines -- are bone-dry.

Something thinned the Red Planet's atmosphere over time, and there's a mission in orbit to find out how, when it happened and how quickly.

New results based on observations from MAVEN (Mars Atmosphere and Volatile EvolutioN Mission) have found a possible new source for losing the atmosphere. For years, scientists have known that the solar wind -- that stream of charged particles coming from the sun -- can strip away hydrogen molecules on Mars. The new study suggests that interstellar clouds are also partially responsible.

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"MAVEN made it clear that ionizing radiation from the sun is the main driver as we it see now. I agree with those results 100%, and am looking at geological timescales where encounters with interstellar clouds also becomes important," said lead author Dimitra Atri, of the Blue Marble Space Institute of Science, in an email interview with Discovery News.

The changes come as our solar system moves around the galaxy, Atri explained. From time to time, we move through interstellar clouds -- vast, dense clouds of gas and dust -- in events that last for about a million years. This has happened at least 135 times since the solar system was formed 4.5 billion years ago, Earth's geological records suggest. Each time, the gas and dust in the cloud strike the solar wind and create a bow shock. This shock accelerates charged particles (protons), which Atri suggests will make Mars lose 0.5% of its atmosphere each time.

Because each of these events is so long -- roughly a million years apiece -- Atri's calculations suggest that over time, they contributed to losing half of the Martian atmosphere. Other events attributed to Martian atmosphere loss, such as solar flares and supernovae, have a smaller contribution; Atri says even the strongest solar flare has eight times less magnitude on the escape of the Martian atmosphere than one pass through an interstellar cloud.

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"This study can be extended to other planetary atmospheres as well," he added. "The most important thing I found was the planetary magnetic field. If a planet, like the Earth, has a strong magnetic field, these particles will not be able to penetrate its atmosphere and it will be able to preserve its atmosphere."

Atri hopes to follow up with the research by looking at historical solar events and how they affect the Martian atmosphere. His current study was published in the Monthly Notices of the Royal Astronomical Society.

SEE PHOTOS: The Beauty of Mars Dunes

Mars plays host to a huge number of dune fields -- regions where fine wind-blown material gets deposited to form arguably some of the most beautiful dunes that can be found on any planetary body in the solar system. Using the powerful High-Resolution Imaging Science Experiment (HiRISE) camera on board NASA's Mars Reconnaissance Orbiter, planetary scientists have an orbital view on these features that aid our understanding of aeolian (wind-formed) processes and Martian geology. Here are some of our favorite Mars dunes as seen by HiRISE. Pictured here are shell-like "barchan dunes" in the ancient Noachis Terra region of Mars.

Special thanks to Ari Espinoza of the HiRISE team at the University of Arizona for helping to compile this list.

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Dunes of many shapes, sizes and formation processes can be found on the Red Planet. Shown here are elegant "linear dunes" with deposits of larger rocks and possibly ices in their troughs.

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These slug-like dark dunes are striking examples of "dome dunes" -- elliptical accumulations of fine material with no-slip surfaces. These domes contrast greatly with the often jagged appearance of barchan dunes. Found at the bottom of Proctor Crater, they are darker than the surrounding crater floor as they are composed of dark basaltic sand that was transported by the wind.

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Looking like a wind-blown silk sheet, this field of "star dunes" overlays a plain of small ripples, another aeolian feature. The ripples move more slowly across the bottom of Proctor Crater, so the large dune field will travel over the smaller ripples. Dunes are continuously evolving and moving with the wind, ensuring that the Martian surface is never static.

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These "transverse dunes" are undergoing seasonal changes. Likely entering Mars summer, this region of dunes is stained with pockets of subliming ices -- likely carbon dioxide. As the ices turn from solid to vapor, dune material slumps, revealing dark, sandy material underneath.

Resembling the mouths of a shoal of feeding fish, this is a group of barchan dunes in Mars' North Polar region. Barchan dunes betray the prevailing wind direction. In this case, the prevailing wind is traveling from bottom right to top left; the steep slope of material (plus dune "horns") point to the downwind direction. The HiRISE camera monitors barchans to see if they move between observing opportunities, thereby revealing their speed of motion across the Martian plains.

This is the same barchan dune field, zoomed out, a "swarm" of dunes covering the plains.

Not all barchan dunes "behave" and form neat "horny" shapes. They can become muddled and overlapping, creating "barchanoid dunes," as shown here.

This very fluid-looking collection of barchans is accompanied by a wind-blown ridge in the Hellespontus region of Mars but...

...only when zoomed out does the true nature of this fascinating region become clear. The prevailing wind is eroding the mesas (small hills) to the right of the image, carrying fine material downwind (from right to left), creating a startling pattern of barchans and a viscous-looking trail of sandy ridges across the plains.

The band Train sang about the "Drops of Jupiter" -- what about the "Drops of Mars"? Sure, they're not made of any kind of fluid, but they do make for incredibly-shaped dunes. These raindrop-shaped dunes are found in Copernicus Crater and are known to be rich in the mineral olivine, a mineral that formed during the wet history of Mars' evolution.

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These craggy-looking dunes are old barchanoids eroding away through seasonal processes (sublimation of sub-surface ices) and the persistent Martian wind.

These linking barchan dunes are at the leading edge of a dune field -- grains of dust have been blown across a plain, deposited and left to accumulate in elongated arrow shapes.

Dome-shaped dunes and barchans seem to "reach out" and touch their downwind partners with slumped material.

Barchan dunes inside Arkhangelsky Crater in the southern hemisphere of Mars reveal a wind direction from top left to bottom right. Note the tracks of Martian dust devils over the dune slopes.

For more on the HiRISE camera, see the HiRISE website and regularly updated Tumblr page. For more on Mars dune definitions, check out the USGS Mars Dunes site.