The storm, three times larger than any previously detected Martian spiral storm, is nearly 1100 by 900 miles in dimension. The eye of this colossal storm is approximately 200 miles in diameter. The system is larger than the planet's residual north polar ice cap, and is comparable in size to the hurricanes found on Earth.

The storm appeared in the middle of the Martian northern hemisphere's summer season; the smaller spiral storms discovered by Viking also occurred during the northern summer, at high northern latitudes. This type of cyclonic circulation appears to be related to climatic conditions that are unique to the planet's northern polar region during this time of year.

The storm was detected on April 27, 1999 using the Hubble's Wide Field Planetary Camera 2. The team of astronomers acquiring and analyzing these 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.).

For more information:
NASA
Space Telescope Science Institute Press Release
Malin Space Science Systems Animation