FOR RELEASE: 1:00 P.M. (EDT) JULY 15, 1997

PHOTO CAPTION NO.: STScI-PRC97-24b


Rapid Weather Changes Observed on Mars

NASA Hubble Space Telescope images of Mars, obtained over three consecutive
days between July 9 and 11, 1997, dramatically show that the behavior of
dust and water-ice clouds exhibit substantial daily variations.  The
full-disk images are shown along the top (Pathfinder landing site marked
by the green crosses), and maps of the North polar region are shown
along the bottom.  (The maps are oriented with 0 degrees longitude to
the bottom, and extend from 40 degrees North latitude to the pole; latitude 
circles are shown at 40, 60, and 80 degrees, and lines of longitude are 
shown every 45 degrees.)  About 24 hours separates each of the images. 

The polar maps document the movement of bright water-ice clouds as they
are seen to progress eastward, perhaps driven by a passing weather front
on Mars.  Between July 9 and 10,the polar cloud seen near 60 degrees 
N latitude is measured to have moved about 550 km eastward over a period
of about 24 hours, corresponding to a velocity of about 22 km/hour.  The
cloud has dissipated considerably by July 11.  

Clouds in the southern hemisphere seem to thicken considerably over this
three day period as well.  The bluish south polar hood, composed of 
water-ice clouds, is seen along the bottom of the images.  We
cannot see the south polar cap, since the north polar cap is tilted
toward us during this season and the south cap is in winter darkness.  

On the July 9 image and map, a streamer of dust is visible in the North
polar region, extending about 1200 km southward from the dark sand dunes
surrounding the polar ice cap; diffuse dust is visible over much of
Acidalia, the dark region to the north of the Pathfinder landing site. 
This dust cloud is apparently diffusing with time, as it seems to become
less distinct in each successive image.  This dust storm may be
supplying some of the dust seen overhead by the Pathfinder spacecraft. 

These HST images support the conclusion that Pathfinder landed during a
period when large changes in the regional distributions of dust and
clouds were taking place on Mars, a point not readily evident from the
localized Pathfinder site observations.  These and future simultaneous
observations between HST and Pathfinder will help place the detailed
observations made by Pathfinder into the global perspective offered by HST.

Photo Credit: Steve Lee (Univ. Colorado), Phil James (Univ. Toledo),
and  Todd Clancy (Space Science Inst.)

Image files in GIF and JPEG format and captions may be accessed on the
Internet via anonymous ftp from oposite.stsci.edu in /pubinfo.

                             GIF                 JPEG
PRC97-24b  Mars July 1997    gif/mars9707.gif    jpeg/mars9707.jpg   

Higher resolution digital versions (300 dpi JPEG) of the release photograph
are available in /pubinfo/hrtemp:  97-24b.jpg (color) and 97-24bbw.jpg (black
& white).

GIF and JPEG images, captions and press release text are available via the
World Wide Web at http://oposite.stsci.edu/pubinfo/PR/97/24.html and via
links in http://oposite.stsci.edu/pubinfo/Latest.html or
http://oposite.stsci.edu/pubinfo/Pictures.html.