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Hubble Watches Mars As Mars Global Surveyor Begins Aerobraking

FOR RELEASE:  September 17, 1997



This NASA Hubble Space Telescope picture of Mars was taken on 
Sept. 12, one day after the arrival of the Mars Global Surveyor 
(MGS) spacecraft and only five hours before the beginning of 
autumn in the Martian northern hemisphere.  (Mars is tilted on 
its axis like Earth, so it has similar seasonal changes, including 
an autumnal equinox when the Sun crosses Mars' equator from 
the northern to the southern hemisphere).

This Hubble picture was taken in support of the MGS mission.  
Hubble is monitoring the Martian weather conditions during the 
early phases of MGS aerobraking;  in particular, the detection of 
large dust storms are important inputs into the atmospheric 
models used by the MGS mission to plan aerobraking operations.

Though a dusty haze fills the giant Hellas impact basin south of the 
dark fin-shaped feature Syrtis Major, the dust appears to be localized 
within Hellas.  Unless the region covered expands significantly, the 
dust will not be of concern for MGS aerobraking.

Other early signs of seasonal transitions on Mars are apparent in the 
Hubble picture.  The northern polar ice cap is blanketed under a polar 
hood of clouds that typically start forming in late northern summer.  
As fall progresses, sunlight will dwindle in the north polar region 
and the seasonal polar cap of frozen carbon dioxide will start 
condensing onto the surface under these clouds. 

Hubble observations will continue until October 13, as MGS carefully 
uses the drag of the Martian atmosphere to circularize its orbit about 
the Red Planet.  After mid-October, Mars will be too close to the Sun, 
in angular separation, for Hubble to safely view.

The image is a composite of three separately filtered colored images 
taken with the Wide Field Planetary Camera 2 (WFPC2).  Resolution 
is 35 miles (57 kilometers) per pixel (picture element).  The Pathfinder 
landing site near Ares Valles is about 2200 miles (3600 kilometers) 
west of the center of this image, so was not visible during this 
observation.  Mars was 158 million miles (255 million kilometers) 
from Earth at the time.

An image of this region of Mars, taken in June 1997, is shown for 
comparison.  The Hellas basin is filled with bright clouds and/or 
surface frost.  More water ice clouds are visible across the planet 
than in the Sept. image, reflecting the effects of the changing season.  
Mars appears larger because it was 44 million miles (77 million 
kilometers) closer to Earth than in the September image.

Credit:  Phil James (Univ. Toledo) and Steve Lee (Univ. Colorado), 
and NASA


Images are available via the World Wide Web at
http://oposite.stsci.edu/pubinfo/PR/gif/mars0609.gif (GIF),
http://oposite.stsci.edu/pubinfo/PR/jpeg/mars0609.jpg (JPEG) and via links
in http://oposite.stsci.edu/pubinfo/PR/97/31.html.

Image files also may be accessed via anonymous ftp from oposite.stsci.edu
in /pubinfo:  gif/mars0609.gif (GIF) and jpeg/mars0609.jpg (JPEG),
tiff/1997/31a.tif and 31b.tif (TIFF).