Balme, a research scientist with the Tucson-based Planetary Science Institute, says TARs have no exact analog on Earth. These fields of rippled sand, found over large swaths of Mars, are smaller than the planet's gigantic dunes but larger than the sand ripple fields found on Earth.

Wind-driven particles form TARs. They're called "Aeolian Ridges" because planetary scientists refer to phenomena involving air movement as "Aeolian processes."

The ridges assume many shapes, such as simple ripples, forked ripples, snake-like sinuous waves, barchan-like (crescent-shaped) forms or complex, overlapping networks.

TARs are worth studying because they are an intriguing scientific mystery, hold clues to past and present climate processes, and because they're death to rovers, says Balme, who also is a research fellow at Great Britain's Open University and based in Milton Keynes, England.

In 2005, NASA's Opportunity rover bogged down for six weeks with its wheels firmly mired in what Balme believes was a small TAR. So it's important to know where TARs are located to avoid landing among them on future rover missions, he said.

Balme and his colleagues have conducted a pole-to-pole planet survey of more than 10,000 images taken by the Mars Orbiter Camera, which is flying aboard the Mars Global Surveyor spacecraft.

They found that TARs:

+ Are more common in the southern hemisphere than in the northern.

+ Are found in an equatorial belt between 30 degrees north and 30 degrees south latitude.

+ Exist in two distinct environments: near layered terrain or adjacent to Large Dark Dunes (LLDs). Those adjacent to dunes have formed recently, while those near layered terrain are millions of years old.

+ Are abundant in the Meridiani Planum region and in southern-latitude craters.

The Opportunity rover's TAR encounter provided additional data, showing that at least that TAR was composed of an outer layer of granule-sized material ranging from about 2mm to 5 mm in diameter, Balme said. Beneath this was a mixed mass of fine and coarse particles.

TARs need two things to form, Balme explained: a supply of sediment and strong winds. The sediment requirement helps explain why they're found near dunes and layered terrain and why they're confined to a central belt around the planet, Balme said.

Both dunes and layered terrain (which could have been formed by ancient sand dunes, ocean or lake deposits, or layers of volcanic ash) provide the raw material.

Steep slopes also can provide additional particles for TAR formation as they erode. The lack of steep slopes and other sources of coarse and fine material in the middle to far north and south latitudes may explain the absence of TARs in those areas, he said.

The TARs near layered terrain are generally several million years old and inactive, while the ones near LLDs are young and may still be actively forming and moving.

"My theory is that the very young TARs are found near the Large Dark Dunes, which are also very young, because the sand blowing off the dunes provides the energy needed to form TARs," Balme said.

"Meanwhile you have areas near layered landforms that used to have active sediment transport, but no longer. This shows a dynamic environment that has changed, and we might be able to use TARs as paleo markers to help decipher ancient climates."

Current Martian circulation models don't provide much evidence that wind patterns and atmospheric densities on Mars were significantly different in the past than from what they are today. "But I think the geology we are seeing suggests that there might have been different patterns and densities," Balme said.

"The observations we're getting now from Mars Global Surveyor and the HiRISE camera (flying aboard the Mars Reconnaissance Orbiter) are giving us really good data to drive the models."

Although Blame and his team have discovered much about TARs, they still don't know what materials compose the various TAR fields or why they're seeing these large features on Mars but not on Earth.

"Over the next couple of years we should be seeing many more images from HiRISE that can give us more information, for example, about the heights versus spacing and whether TARs have more in common with dunes or the ripple fields found on Earth," Balme said.

"And they could provide insights into present and past climate patterns as we learn more about them and use that data to help drive general circulation models."

HiRISE images of the same areas of the planet taken over long time intervals also might show small movements within some of the TAR fields, indicating which ones are still active and possibly demonstrating how they form, Blame said.

Other researchers working with Balme include: Daniel Berman and Mary Bourke, of the Planetary Science Institute; Scot Rafkin, of Southwest Research Institute; and James Zimbelman of the Smithsonian Institution.

Balme's TARs research is supported by NASA's Mars Data Analysis Program.

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