The latest images from NASA's Mars Global Surveyor spacecraft show giant plates of solidified volcanic lava and evidence for active dunes near the planet's north pole, with sands that have hopped or rolled across the surface in recent months.

The images will be presented on Thursday, October 29, by members of the mission science team at the annual meeting of the Geological Society of America in Toronto, Canada.

The close-up views of Mars' Elysium Basin reveal the first evidence of huge plates of solidified lava, rather than lakebed sediments, that appear to have been broken up and transported across the Martian surface millions of years ago as they floated on top of molten lava. This implies that the area in the planet's northern lowlands was once the site of giant ponds of lava flows hundreds of kilometers across, according to Dr. Alfred S. McEwen of the University of Arizona, Tucson, a member of the Global Surveyor science team.

"NASA Viking mission images of the same region showed a surface of dark plates with intervening bright surfaces that did not quite make sense," McEwen said. "Some scientists thought they could somehow be volcanic, while others thought they might be related to differences in the way that wind had eroded a dried lakebed. With these new images in hand, it is now quite easy to understand the older, lower-resolution Viking images."

McEwen and his co-authors believe that lava erupted near this area and the upper surface became crusted, then cooled and cracked. Some cracks widened and portions of the surface crust became rafts of solid rock that moved in the direction that the molten lava was flowing underneath. Other Viking and Global Surveyor images have shown similar plate-like lava textures in nearby Marte Vallis, implying that some of the lava from Elysium Basin spilled into this valley and flowed thousands of kilometers to the northeast.

"The sparse occurrence of impact craters on these plate-like lava surfaces suggests that the eruptions happened relatively recently in Mars' history," McEwen explained. "These eruptions could be much younger than the youngest of the large Martian volcanoes like Ascraeus Mons and Olympus Mons in the Tharsis region, but they would still have occurred many, many millions of years ago. So these images should not be treated as evidence that Mars is volcanically active today."

Additional close-up views of Martian sand dunes in the north polar region are showing scientists detailed patterns of ongoing movement of sand across the planet for the first time. Drs. Kenneth S. Edgett, staff scientist at Malin Space Science Systems, San Diego, CA, and Michael Malin, Mars Global Surveyor camera principal investigator, report the presence of many fresh dunes that have been active as recently as July or August.

"The north polar cap of Mars is surrounded by a zone of dark dunes," Edgett said. "These were first seen by Mariner 9 as a rippled texture, and by the Viking orbiters as definitive sand dunes. Between late July and mid-September 1998, Mars Global Surveyor's closest passage over the planet took us right over the north polar dune fields four times a day. This provided us with many opportunities to take high-resolution pictures of these mounds."

Martian dunes typically contain granular fragments of rocks and minerals ranging from 0.06 to 2 millimeters (0.002 to 0.08 inches) in size, which puts them in the geologic classification of "sand." The sand appears to have been transported by wind in one of two ways: either by hopping over the ground, a geological process called "saltation," or by rolling along the ground, a process known as "traction."

Some of the dunes appear to be coated with thin, bright frost that was left over from the northern winter season that ended in mid-July, according to Edgett and Malin. This frost is covered with dark streaks emanating from small dark spots that dot the bases of many of the dunes. "The simplest explanation is that gusts of wind have blown the dark sand out across the frost- covered dunes, creating a streak of deposited sand over the frost," Malin said. "Some spots seen in the close-ups have multiple streaks, each one indicating that a different wind gust has moved in a different direction."

The images are available on the Internet at the following locations:,, and .

Mars Global Surveyor is part of a sustained program of Mars exploration known as the Mars Surveyor Program. The mission is managed by the Jet Propulsion Laboratory, a division of the California Institute of Technology, Pasadena, CA, for NASA's Office of Space Science, Washington, DC. JPL's industrial partner is Lockheed Martin Astronautics, Denver, CO, which developed and operates the spacecraft.

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