Douglas Isbell
Headquarters, Washington, DC                  December 7, 1998
(Phone:  202/358-1547)

Cynthia M. O'Carroll
Goddard Space Flight Center, Greenbelt, MD
(Phone:  301/614-5563)

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

RELEASE:  98-219

LASER PROVIDES FIRST 3-D VIEW OF MARS' NORTH POLE

     Measurements by a laser altimeter instrument orbiting aboard 
NASA's Mars Global Surveyor spacecraft are providing striking new 
views of the north pole of the red planet and the processes that 
have shaped it.  

     This first three-dimensional picture of Mars' north pole 
enables scientists to estimate the volume of its water ice cap 
with unprecedented precision, and to study its surface variations 
and the heights of clouds in the region for the first time.

     The elevation measurements were collected by the Mars Orbiter 
Laser Altimeter (MOLA) aboard Global Surveyor during the spring 
and summer of 1998, as the spacecraft orbited Mars in an interim 
elliptical orbit.  MOLA sends laser pulses toward the planet and 
measures the precise amount of time before the reflected signals 
are received back at the instrument.  From this data, scientists 
can infer surface and cloud heights.

     Approximately 2.6 million of these laser pulse measurements 
were assembled into a topographic grid of the north pole with a 
spatial resolution of .6 miles (one kilometer) and a vertical 
accuracy of 15-90 feet (5-30 meters).  A peer-reviewed paper based 
on the measurements will be published in the Dec. 11 issue of 
Science magazine.

     The topographic map reveals that the ice cap is about 750 
miles (1,200 kilometers) across, with a maximum thickness of 1.8 
miles (3 kilometers).  The cap is cut by canyons and troughs that 
plunge to as deep as 0.6 miles (1 kilometer) beneath the surface.  
"Similar features do not occur on any glacial or polar terrain on 
Earth," said Dr. Maria Zuber of the Massachusetts Institute of 
Technology and NASA's Goddard Space Flight Center, Greenbelt, MD.  
"They appear to be carved by wind and evaporation of ice."

     The MOLA data also reveal that large areas of the ice cap are 
extremely smooth, with elevations that vary by only a few feet 
over many miles.  In some areas the ice cap is surrounded by large 
mounds of ice, tens of miles across and up to half a mile in 
height.  "These structures appear to be remnants of the cap from a 
time when it was larger than at present," Zuber said.  Impact 
craters surrounding the cap appear to be filled with ice and dust 
that was either deposited by wind or condensation, or perhaps 
remains from an earlier period when the ice cap was larger.

     The shape of the polar cap indicates that it is composed 
primarily of water ice, with a volume of 300,000 cubic miles (1.2 
million cubic kilometers).  The cap has an average thickness of 
0.64 miles (1.03 kilometers) and covers an area 1.5 times the size 
of Texas.  For comparison, the volume of the Martian north polar 
cap is less than half that of the Greenland ice cap, and about 
four percent of the Antarctic ice sheet.  

     The estimated volume of the north ice cap is about 10 times 
less than the minimum volume of an ancient ocean that some 
scientists believe once existed on Mars.  If a large body of water 
once existed on the red planet, the remainder of the water must 
presently be stored below the surface and in the much smaller 
south polar cap, or have been lost to space.  But such a large 
amount of unaccounted-for water is not easily explained by current 
models of Martian evolution.

     During its mapping of the north polar cap, the MOLA 
instrument also made the first direct measurement of cloud heights 
on the red planet.  Reflections from the atmosphere were obtained 
at altitudes from just above the surface to more than nine miles 
(approximately 15 kilometers) on about 80 percent of the laser 
profiles.  Most clouds were observed at high latitudes, at the 
boundary of the ice cap and surrounding terrain.  

     Clouds observed over the polar cap are likely composed of 
carbon dioxide that condenses out of the atmosphere during 
northern hemisphere winter.  Many clouds exhibit dynamic structure 
probably caused by winds interacting with surface topography, much 
as occurs on Earth when winds collide with mountains to produce 
turbulence. 

     The principal investigator for MOLA is Dr. David E. Smith of 
Goddard.  The MOLA instrument was designed and built by the Laser 
Remote Sensing Branch of Laboratory for Terrestrial Physics at 
Goddard.  The Mars Global Surveyor Mission is managed by NASA's 
Jet Propulsion Laboratory, Pasadena, CA, for the NASA Office of 
Space Science.

     Further information about MOLA and images created from its 
measurements of the north pole are available on the Internet at 
the following address: 
http://ltpwww.gsfc.nasa.gov/tharsis/mola.html  
A color image of the north pole from the camera aboard Mars Global 
Surveyor is available at: 
http://www.msss.com/mars/global_surveyor/camera/images/MENUS/poles_list.html