Scientists may be one step closer to unraveling the mystery of Mars' current atmosphere thanks to a dry-ice discovery.
Scientist Roger Phillips of the Southwest Research Institute in Boulder, Colo., and his colleagues have found a significant amount of dry ice on Mars' south pole - a find that may show the planet's atmosphere was once thick enough to have had large amounts of liquid water on its surface.
While past studies have suggested that Mars' south pole is covered in water with a thin top layer of dry ice, the team's study - which used radar measurements from the Mars Reconnaissance Orbiter - proposes there is significantly more dry ice than previously suspected. Not only that, but the dry ice is atmospheric carbon dioxide deposited on the surface, which Phillips described as possibly being "the last gasp of an ancient, thicker atmosphere."
The radar used by the team works by bouncing radio waves off reflectors at various depths on the planet, Phillips said, allowing the group to determine the velocity of the waves as they pass through material. For a particular deposit, the group found a wave velocity corresponding to dry ice, which is very different from the wave velocity of liquid water, he said.
But how did dry ice end up on the poles? Phillips said it has to do with the tilt of Mars' rotational axis.
Like Earth's, Mars' tilt oscillates, he said, moving from a state of high tilt to low tilt and back again in roughly 100,000 years. Mars at its highest tilt would create an atmosphere with twice as much carbon dioxide by turning the dry ice from a solid immediately to a gas in a process called sublimation. At low tilt, all the carbon dioxide would collapse onto the planet's poles.
"The present climate is very dynamic," Phillips said. "Four billion years ago, there was a lot more atmosphere. That was a time when Mars was warmer and wetter, and river systems formed."
Most of the carbon dioxide was lost to space or was tied up in rocks deep in the crust, he said, explaining that dry ice formed at the poles when Mars' atmosphere began to cool and atmosphere was lost.
"What we're likely seeing is the last vestiges of that, and it's moving into and out of the atmosphere as the tilt slowly changes," he said.
Shane Byrne, an assistant professor of planetary sciences at the University of Arizona, worked on the project and said the findings are exciting.
The thought of how much dry ice is on Mars has waxed and waned, he said.
Scientists initially believed there was a lot of dry ice, enough to produce a massive atmosphere and a warm planet, but then went to the opposite extreme to say there was very little - a tiny fraction of what's in the current atmosphere.
"We're kind of getting our way toward the truth slowly, oscillating back and forth, closer to the right answer," he said.
The discovery is exciting because if the carbon dioxide were to enter the atmosphere again, it could drive climate change, Byrne said.
Byrne isn't the project's only connection to UA.
Phillips said he and his colleagues also looked at high resolution images from HiRISE - the High Resolution Imaging Science Experiment - that was built under the direction of the UA Lunar and Planetary Laboratory. Sublimation features seen in the images provide independent evidence that the radar-discovered deposit is dry ice, Phillips said.
"It really is a buried treasure," said Jeffrey Plaut of the NASA Jet Propulsion Laboratory who was part of the discovery team reporting online Thursday in the journal Science. "We found something underground that no one else realized was there."
Though the newfound store sounds like a lot, it's only enough carbon dioxide to double the mass of the feeble Martian atmosphere if released - not enough to warm up the planet substantially or allow water to pool.
"The atmosphere would still be quite thin and would not have the density necessary to warm things up enough to have liquid water stable on the surface," said Peter Thomas of Cornell University, who had no role in the mission.
The mystery of what happened to Mars' atmosphere has long intrigued scientists. NASA plans to explore the upper atmosphere and study how gases are lost to space with a new spacecraft in 2013.
Victoria Blute is a NASA Space Grant intern. E-mail her at email@example.com. The Associated Press contributed to this story.