Colossal Cyclone Swirling Near Martian North Pole Is Observed By Cornell-Led Team On Hubble Telescope

The system is similar to so-called "spiral" storms observed morethan 20 years ago by NASA's Viking Orbiter spacecraft, but it is nearlythree times as large as the largest previously detected Martian spiralstorm system. The storm is nearly 1,100 miles across from east to west and900 miles from north to south. The eye of the storm is nearly 200 miles indiameter. The system is larger than the planet's residual north polar icecap, and is comparable in size to similarly shaped terrestrial hurricanes.

The storm was detected using the Hubble's Wide Field PlanetaryCamera-2 by a team of astronomers observing Mars near its closest approachto Earth in nearly eight years. Jim Bell, assistant professor of astronomyat Cornell University, principal investigator on the team of astronomersobtaining and analyzing the Hubble images, says the observations weresomewhat serendipitous in that they were made during the same season thatthe storms were first detected by Viking.

"These rapidly growing and decaying systems do appear to be typicalof the Martian polar weather at this season, which is mid northern summer,"says Bell. "The storm we detected from Hubble appears anomalous because ofits size, but I wouldn't be surprised if the Mars Global Surveyorspacecraft sees other storms of this size eventually as well."

The storm consists of at least three, and perhaps more, bands ofclouds organized into a spiral structure and wrapped counterclockwisearound a hollow central core or "eye." The storm appeared in the middle ofthe Martian northern hemisphere's summer season, after the planet'sseasonal carbon dioxide polar cap had completely sublimated away, leavingonly the underlying residual water-ice cap. The smaller spiral storms seenpreviously 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'snorthern polar regions at this season. Similar storms, some comparable insize to the Martian storm, have been seen in Earth's polar regions. OnEarth, these polar cyclones appear to be low pressure systems fueled bystrong contrasts in oceanic versus atmospheric temperatures. In somecases, winds within Earth's polar cyclones can reach hurricane force.

The general appearance of the Martian storm seems consistent withan 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 thenamplified by the strong temperature contrast between the relatively warmhigh-latitude Martian dark regions and the much colder and stable polaratmosphere.

When first imaged by Hubble on April 27, 1999, the storm waslocated near 65 degrees north latitude and 85 degrees west longitude. Whennext imaged about six hours later, the storm appeared to have moved onlyslightly eastward, but seemed to be in the process of dissipating. Thestorm has not been seen again in subsequent Hubble telescope images, butthese have been centered on the opposite side of the planet. The stormmight have been a short-lived phenomenon, which would be consistent withthe short lifetimes (several days) observed for smaller spiral storms byViking.

"It seems that our knowledge of the high-latitude weather on Marsis limited by the fact that even Viking, from orbit 20 years ago, could notobserve the polar regions very frequently," says Cornell's Bell. "So ourHST finding, and forthcoming results from the Mars Global Surveyorspacecraft, may simply be revealing all this activity up there because it'sthe first time we've been able to look in a detailed and dedicated way withhigh-resolution instruments during this season."

The team of astronomers, besides Bell, operator of HST, includedMichael Wolff and R. Todd Clancy (Space Telescope Science Institute),Steven Lee (University of Colorado), Philip James (University of Toledo),and Michael Ravine (Malin Space Science Systems, Inc.).

Related World Wide Web sites: The following sites provideadditional information on this news release. Some might not be part of theCornell University community, and Cornell has no control over their contentor availability.

-- Pictures of the Mars cyclone from the Hubble site, with detailedcaptions: http://oposite.stsci.edu/pubinfo/pr/1999/22

-- Hubble Space Telescope latest observations:http://oposite.stsci.edu/pubinfo/latest

-- Hubble page with links to higher resolution digital versions (300 dpiJPEG and TIFF):

http://oposite.stsci.edu/pubinfo/pr/1999/22/extra-photos