The Dunes of Mars
A dune is a hill of sand built by the wind. Dunes occur in different forms and sizes. Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter slip face in the lee of the wind. Dunes can be found in any environment where there is a substantial atmosphere, winds, and dust to be blown. Dunes are common on Mars, and they have also been observed in the equatorial regions of Titan. Sand dunes in a vast area of northern Mars long thought to be frozen in time are changing with both sudden and gradual motions, according to research using images from a NASA orbiter. These dune fields cover an area the size of Texas in a band around the planet at the edge of Mars' north polar cap. The new findings suggest they are among the most active landscapes on Mars. However, few changes in these dark-toned dunes had been previously detected before a campaign of repeated imaging by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter.
Scientists had considered the dunes to be fairly static, shaped long ago (perhaps 30,000 years) when winds on the planet's surface were much stronger than those seen today. Several sets of before-and-after images from HiRISE over a period covering two Martian years or four Earth years -- tell a different story.
Thought to be encased in a frozen, static crust, the Martian north pole is actually a dynamic place, with sand dunes skidding and sliding in spring.
A report by Hansen and co-authors in this week's edition of the journal Science identifies the seasonal coming and going of carbon-dioxide ice as one agent of change, and stronger-than-expected wind gusts as another.
A seasonal layer of frozen carbon dioxide, or dry ice, blankets the region in winter and changes directly back to gaseous form in the spring. This layer of frozen carbon dioxide ice may temporarily stabilize a dune's surface. When it melts or sublimates directly to vapor, the dune surface is suddenly unstable and prone to collapse.
"This gas flow destabilizes the sand on Mars' sand dunes, causing sand avalanches and creating new alcoves, gullies and sand aprons on Martian dunes," Hansen said. "The level of erosion in just one Mars year was really astonishing. In some places, hundreds of cubic yards of sand have avalanched down the face of the dunes."
Wind drives other changes. Especially surprising was the discovery that scars of past sand avalanches could be partially erased by wind in just one Mars year. Models of Mars' atmosphere do not predict wind speeds adequate to lift sand grains, and data from Mars landers show high winds are rare. This suggests that polar winds are stronger.
In all, dune changes or modifications were seen in about 40 percent of these far-northern dune sites over the two-Mars-year period of the study.
Related HiRISE research previously identified gully-cutting activity in smaller fields of sand dunes covered by seasonal carbon-dioxide ice in Mars' southern hemisphere. A report four months ago showed that those changes coincided with the time of year when ice builds up.
Researchers have tracked changes in gullies on the faces of sand dunes in seven locations on southern Mars. The periods when changes occurred, as determined by comparisons of before-and-after images, overlapped in all cases with the known winter build-up of carbon-dioxide frost on the dunes. Before-and-after pairs that covered periods only in spring, summer and autumn showed no new activity in those seasons.
The findings were published in the article, "Seasonality of present-day martian dune-gully activity," in the November 2010 issue of the journal Geology.
"The role of the carbon-dioxide ice is getting clearer," said Serina Diniega of NASA's Jet Propulsion Laboratory, Pasadena, Calif., lead author of the earlier report and a co-author of the new report. "In the south, we saw before-and-after changes and connected the timing with the carbon-dioxide ice. In the north, we're seeing more of the process of the seasonal changes and adding more evidence linking the changes with the carbon dioxide."
The new findings contribute to efforts to understand what features and landscapes on Mars can be explained by current processes, and which require different environmental conditions.
"Understanding how Mars is changing today is a key first step to understanding basic planetary processes and how Mars changed over time," said HiRISE Principal Investigator Alfred McEwen of the University of Arizona, Tucson, a co-author of both reports. "There's lots of current activity in areas covered by seasonal carbon-dioxide frost, a process we don't see on Earth. It's important to understand the current effects of this unfamiliar process so we don't falsely associate them with different conditions in the past."
For further information: http://www.jpl.nasa.gov/news/news.cfm?release=2011-039&rn=news.xml&rst=2900