Gullies on Mars once thought to have been carved by the action of running water, have now been shown to be the result of frozen carbon dioxide, or dry ice.
The Mars Reconnaissance Orbiter (MRO), launched into orbit around Mars on 12 August 2005, was used to photograph gullies for examination. Astronomers were interested in studying how the features change during different seasons on the Red Planet.
Gullies on the surface of Mars were first spotted in 2000, generating a great excitement among scientists and the general public. At first, many researchers believed the structures were formed by the action of liquid water running across the Martian landscape.
Water vapor has been found in the atmosphere of Mars, and water ice has been discovered just beneath the surface of the planet. Liquid water still remains elusive, and if the gullies had been cut by running water, it would have greatly increased chances of finding microbial life there, past or present.
"As recently as five years ago, I thought the gullies on Mars indicated activity of liquid water. We were able to get many more observations, and as we started to see more activity and pin down the timing of gully formation and change, we saw that the activity occurs in winter," Colin Dundas from the Astrogeology Science Center, said.
Gullies at 356 sites on Mars were carefully examined, using the High Resolution Imaging Science Experiment (HiRISE) camera aboard the orbiting observatory. Observations for the study began in 2006. Of the hundreds of sites examined, 38 showed evidence of active gully formation, including the deposit of material at the bottom of slopes. Researchers found the timing of this process coincided with carbon dioxide thrust, and temperatures there would not have been high enough to allow liquid water to flow.
Dry ice does not exist naturally anywhere on the Earth, but is commonly found on Mars, particularly in the polar caps. When frozen carbon dioxide is exposed to warmer temperatures, it sublimates - converting directly from solid to gaseous state, without ever forming a liquid. Astronomers believe this sublimation may be responsible for the gullies seen on Mars. The idea states that as gas pressures build up within the structures, it could provide room for additional dry ice to move through the formation. Another possibility is that frozen carbon dioxide continues to build up in gullies until the weight becomes great enough to cause a small landslide.
"I like that Mars can still surprise us. Martian gullies are fascinating features that allow us to investigate a process we just don't see on Earth," Dundas stated in a NASA press release about the discovery.
Study of the gullies on Mars and the seasonal processes that could have created the features from dry ice was published in the Journal Icarus.
