Douglas Isbell
Headquarters, Washington, DC                   May 27, 1999
(Phone:  202/358-1753)

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

Mary Hardin
Jet Propulsion Laboratory, Pasadena, CA
(Phone:  818/354-0344)

RELEASE:  99-66

FIRST GLOBAL 3-D VIEW OF MARS REVEALS 
DEEP BASIN AND PATHWAYS FOR WATER FLOW

     An impact basin deep enough to swallow Mount Everest and 
surprising slopes in Valles Marineris highlight a global map 
of Mars that will influence scientific understanding of the 
red planet for years.

     Generated by the Mars Orbiter Laser Altimeter (MOLA), an 
instrument aboard NASA's Mars Global Surveyor, the high-
resolution map represents 27 million elevation measurements 
gathered in 1998 and 1999.  The data were assembled into a 
global grid with each point spaced 37 miles (60 kilometers) 
apart at the equator, and less elsewhere.  Each elevation 
point is known with an accuracy of 42 feet (13 meters) in 
general, with large areas of the flat northern hemisphere 
known to better than six feet (two meters).  

     "This incredible database means that we now know the 
topography of Mars better than many continental regions on 
Earth," said Dr. Carl Pilcher, Science Director for Solar 
System Exploration at NASA Headquarters, Washington, DC.  
"The data will serve as a basic reference book for Mars 
scientists for many years, and should inspire a variety of 
new insights about the planet's geologic history and the ways 
that water has flowed across its surface during the past four 
billion years."

     "The full range of topography on Mars is about 19 miles 
(30 kilometers), one and a half times the range of elevations 
found on Earth," noted Dr. David Smith of NASA's Goddard 
Space Flight Center, Greenbelt, MD, the principal 
investigator for MOLA and lead author of a study to be 
published in the May 28, 1999, issue of Science. 

    "The most curious aspect of the topographic map is the 
striking difference between the planet's low, smooth Northern 
Hemisphere and the heavily cratered Southern Hemisphere," 
which sits, on average, about three miles (five kilometers) 
higher than the north, Smith added.  The MOLA data show that 
the Northern Hemisphere depression is distinctly not 
circular, and suggest that it was shaped by internal geologic 
processes during the earliest stages of martian evolution.

     The massive Hellas impact basin in the Southern 
Hemisphere is another striking feature of the map.  Nearly 
six miles (nine kilometers) deep and 1,300 miles (2,100 
kilometers) across, the basin is surrounded by a ring of 
material that rises 1.25 miles (about two kilometers) above 
the surroundings and stretches out to 2,500 miles (4,000 
kilometers) from the basin center.

     This ring of material, likely thrown out of the basin 
during the impact of an asteroid, has a volume equivalent to 
a two-mile (3.5-kilometer) thick layer spread over the 
continental United States, and it contributes significantly 
to the high topography in the Southern Hemisphere.  

     The difference in elevation between the hemispheres 
results in a slope from the South Pole to North Pole that was 
the major influence on the global-scale flow of water early 
in martian history.  Scientific models of watersheds using 
the new elevation map show that the Northern Hemisphere 
lowlands would have drained three-quarters of the martian 
surface.

     On a more regional scale, the new data show that the 
eastern part of the vast Valles Marineris canyon slopes away 
from nearby outflow channels, with part of it lying a half-
mile (about one kilometer) below the level of the outflow 
channels.

     "While water flowed south to north in general, the data 
clearly reveal the localized areas where water may have once 
formed ponds, " explained Dr. Maria Zuber of the 
Massachusetts Institute of Technology, Cambridge, MA, and 
Goddard.

     The amount of water on Mars can be estimated using the 
new data about the south polar cap and information about the 
North Pole released last year.  While the poles appear very 
different from each other visually, they show a striking 
similarity in elevation profiles.  Based on recent 
understanding of the North Pole, this suggests that the South 
Pole has a significant water ice component, in addition to 
carbon dioxide ice.

     The upper limit on the present amount of water on the 
martian surface is 800,000 to 1.2 million cubic miles (3.2 to 
4.7 million cubic kilometers), or about 1.5 times the amount 
of ice covering Greenland.  If both caps are composed 
completely of water, the combined volumes are equivalent to a 
global layer 66 to 100 feet (22 to 33 meters) deep, about 
one-third the minimum volume of a proposed ancient ocean on 
Mars. 

     During the ongoing Mars Global Surveyor mission, the 
MOLA instrument is collecting about 900,000 measurements of 
elevation every day.  These data will further improve the 
global model, help engineers assess the area where NASA's 
Mars Polar Lander mission will set down on Dec. 3, and aid 
the selection of future landing sites.  MOLA was designed and 
built by the Laser Remote Sensing Branch of the Laboratory 
for Terrestrial Physics at Goddard.  The Mars Global Surveyor 
mission is managed for NASA's Office of Space Science, 
Washington, DC, by the Jet Propulsion Laboratory, Pasadena, 
CA, a division of the California Institute of Technology.

     MOLA topographic images may be viewed at the following 
web address:  

http://pao.gsfc.nasa.gov/gsfc/spacesci/pictures/mola/mars3d.h
tm

     More details about the MOLA instrument and science 
investigation can be found at:

      http://ltpwww.gsfc.nasa.gov/tharsis/mola.html

                            -end-

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