Douglas Isbell 
Headquarters, Washington, DC                    August 10, 1999
(Phone: 202/358-1753)

Franklin O'Donnell
Jet Propulsion Laboratory, Pasadena, CA
(Phone: 818/354-5011)

RELEASE:  99-91

SHARPEST-EVER MARS IMAGES REVEAL ACTIVE RED PLANET

     Newly released images from NASA's Mars Global Surveyor show 
that the red planet is a different place today than it was two 
years ago when the spacecraft arrived -- a world constantly 
reshaped by forces of nature including shifting sand dunes, 
monster dust devils, wind storms, frosts and polar ice caps that 
grow and retreat with the seasons.

     "Mars is a cold, dry desert, but our camera has shown it is 
far from being a stagnant place," said Dr. Michael Malin, 
principal investigator for the Mars Global Surveyor camera at 
Malin Space Science Systems, San Diego, CA.  "Over the past few 
months, we have captured a unique record of seasonal and 
meteorological events, which demonstrates that Mars is active and 
dynamic today."

     The spacecraft's camera monitors the planet's weather on a 
daily basis from orbit, just like weather satellites on Earth.  
Today, Mars is a much more dynamic place than the planet the 
Viking spacecraft saw in the late 1970s.  The weather has been 
particularly active during the past two months, as spring arrived 
in the southern hemisphere and autumn approached in the north.

     "Storm clouds have been brewing over the north polar ice cap 
all through the month of July, and soon, ever-increasing portions 
of the north polar cap will be plunged into wintertime darkness," 
Malin said.  "As the season changes rapidly, clouds will cover 
much of the northern plains and it might begin to snow as the 
polar cap expands."

     In other regions of Mars, dust devils are the prevailing 
weather story.  Dust devils result from spinning vortices of air 
that arise when the ground is heated and general wind flow is 
light.  On Earth they are relatively small features, but on Mars, 
dust devils are thought by some to be a major transporter of the 
fine, pinkish dust that gives the sky its unearthly brownish 
color, as seen by the Mars Pathfinder and Viking landers.  Dust 
devils may also help initiate the seasonal raising of dust over 
wide areas of Mars.

     In mid-May, swirling columns of dust as high as five miles 
(eight kilometers) were observed in northern Amazonis Planitia.  
Dust devils in this area, northwest of the large Tharsis 
volcanoes, appear to be common; they were also seen by the Viking 
missions of the 1970s.  The average dust devil is slow-moving and 
may carry several tons of dust within its height of 1.2 miles (two 
kilometers).  

     Each lasts for a few hours at most during the hottest part of 
the Martian day in the late afternoon, Malin said.  Although the 
winds in these vortices are sufficient to raise dust, they have 
much less power than tornadoes on Earth, which develop under very 
different meteorological circumstances.

     Global Surveyor's camera has also returned tantalizing 
evidence of recent shifting sands in dune fields first seen in 
Mariner 9 pictures of Mars from the early 1970s.  Scientists are 
interested in dune fields isolated within large impact craters 
because their dark color suggests that the dust which covers much 
of the rest of the planet does not accumulate on their sandy 
surfaces.  

     "This indicates that the dunes must be moving and that over 
time we may be able to see changes that will allow us to measure 
the rates of wind erosion on Mars," Malin explained.

     Sand dunes also are giving Mars scientists some new insights 
as to how Mars' seasonal polar ice caps retreat at the end of each 
winter as seasonal warming occurs.  The most dramatic views show 
patches of dark sand poking through fields of carbon dioxide 
frost.  First seen in 1998 in the north polar region, the same 
features have been seen this year on dunes near the south pole.

     "These pictures look like aerial photographs of dunes on 
Earth," Malin said.  "They are so unusual in this context that we 
thought for a while that we were looking at a process that 
involves small 'explosions,' but the new images showed that wind 
was responsible for the streaks we were seeing."  

     The dark spots on frost-covered dunes continue to grow and 
spread as spring approaches until, eventually, the entire dune 
field is frost-free.

     A variety of new images of Mars is available on the Internet 
at:

                      http://www.msss.com/

     Mars Global Surveyor is the first mission in a long-term 
program of Mars exploration, known as the Mars Surveyor Program, 
managed by the Jet Propulsion Laboratory 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.  JPL is a division of the California 
Institute of Technology, Pasadena, CA.

                            -end-