EMBARGOED UNTIL: 9:00 A.M. (EDT) May 19, 1999

CONTACTS:  Ray Villard
           Space Telescope Science Institute, Baltimore, MD
           (Phone: 410-338-4514)
           (E-mail: villard@stsci.edu)

           David Brand
           Cornell University, Ithaca, NY
           (Phone: 607-255-3651)
           (E-mail: deb27@cornell.edu)


PRESS RELEASE: STScI-PR99-22


COLLOSSAL CYCLONE SWIRLS NEAR MARTIAN NORTH POLE

Astronomers using NASA's Hubble Space Telescope have discovered an
enormous cyclonic storm system raging in the northern polar regions of
the planet Mars. Nearly four times the size of the state of Texas, the
storm is composed of water ice clouds like storm systems on Earth,
rather than dust typically found in Martian storms.

The system is similar to so-called "spiral" storms observed more than 20
years ago by NASA's Viking Orbiter spacecraft, but it is nearly three
times as large as the largest previously-detected Martian spiral storm
system. The storm is nearly 1100 miles across in the east-west direction
and 900 miles in the north-south direction. The eye of the storm is
nearly 200 miles in diameter. The system is larger than the planet's
residual north polar ice cap, and is comparable in size to
similarly-shaped terrestrial hurricanes.

The storm consists of at least three, and perhaps more, bands of clouds
organized into a spiral structure and wrapped counterclockwise around a
hollow central core or "eye". The storm appeared in the middle of the
Martian northern hemisphere's summer season, after the planet's seasonal
carbon dioxide polar cap has completely sublimated away, leaving only
the underlying residual water ice cap.  The smaller spiral storms seen
previously by Viking were also detected during the northern summer
season, and also at high northern latitudes.

Apparently, this type of cyclonic circulation, though rare on Mars, must
be related to specific climatic conditions unique to the planet's
northern polar regions at this season. Similar storms, some comparable
in size to the Martian storm, have been seen in Earth's polar regions.
On Earth, these polar cyclones appear to be low pressure systems fueled
by strong contrasts in oceanic versus atmospheric temperatures. In some
cases, winds within Earth's polar cyclones can reach hurricane force.

The general appearance of the Martian storm seems consistent with an
intense low pressure vortex with rising air causing cloud formation,
possibly with a small core that is cloud-free, like the eye of a
hurricane.  The storm may have been initiated by an unstable frontal
system, and then amplified by the strong temperature contrast between
the relatively warm high-latitude Martian dark regions and the much
colder and stable polar atmosphere.

The storm was detected using the Hubble's Wide Field Planetary Camera-2
by a team of astronomers observing Mars near its closest approach to
Earth in nearly eight years. When first imaged by Hubble on April 27,
1999, the storm was located near 65 deg. N latitude and 85 deg. W
longitude. When next imaged about 6 hours later, the storm appeared to
have moved only slightly eastward, but seemed to be in the process of
dissipating. The storm has not been seen again in subsequent Hubble
telescope images, but these have been centered on the opposite side of
the planet. The storm may have been a short-lived phenomenon, then,
which would be consistent with the short lifetimes (several days)
observed for smaller spiral storms by Viking.

The team of astronomers obtaining and analyzing these Hubble images
includes Jim Bell (Cornell University, Principal Investigator), Michael
Wolff and R. Todd Clancy (Space Sciences Institute), Steven Lee
(University of Colorado), Philip James (University of Toledo), and
Michael Ravine (Malin Space Science Systems, Inc.).