Douglas Isbell
Headquarters, Washington, DC           October 28, 1998
(Phone: 202/358-1547)

Diane Ainsworth
Jet Propulsion Laboratory, Pasadena, CA
(Phone: 818/354-5011)

RELEASE:  98-200

NEW MARS IMAGES SHOW LAVA FLOW PLATES AND ACTIVE DUNES 

     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 0.002 to 0.08 inches (0.06 to 2 
millimeters) 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: http://www.jpl.nasa.gov , 
http://photojournal.jpl.nasa.gov , http://mars.jpl.nasa.gov and 
http://www.msss.com  .

     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.

                              -end-