Stormy Weather on Mars

The Hubble Space Telescope photographed an enormous storm swirling near the Martian north pole on April 27, 1999.

May 19, 1999: Astronomers using NASA's Hubble Space Telescope (HST) during Mars's recent close approach to Earth have discovered an enormous cyclonic storm system raging in the northern polar regions of the Red Planet. 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.

Right: This view of the north polar region on Mars shows the relative size and location of the storm (smoke-ring-like structure at left) compared to the polar cap and other classical bright and dark features in this area. The bright surface region beneath the eye of the storm can be seen clearly. This map covers the region north of 45 degrees latitude and is oriented with 0 degrees longitude at the bottom. Image credit: Space Telescope Science Institute and NASA.

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 1,100 miles across from east to west and 900 miles from north to south. 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 was detected using the Hubble's Wide Field Planetary Camera-2 by a team of astronomers observing Mars on April 27, 1999, just a few days after its closest approach to Earth in nearly eight years. Jim Bell, assistant professor of astronomy at Cornell University, principal investigator on the team of astronomers obtaining and analyzing the Hubble images, says the observations were somewhat serendipitous in that they were made during the same season that the storms were first detected by Viking.

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

Three days after Hubble photographed the giant storm system, the Mars Global Surveyor spacecraft flew past the north pole and captured the image shown here (right). The cyclone had vanished, leaving behind what appeared to be normal cloud patterns for this time of year. In the north polar region, temperature differences between bright areas of year-round ice and dark areas of sand and rock create strong winds that mix the atmosphere and create waves of clouds that swirl around the polar cap. The motion of the clouds viewed in the images is typical for this season on Mars, and shows forms often seen on Earth.

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The April 27th storm consisted 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 had 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.

Right: This dramatic three-dimensional visualization of Mars' north pole is based on elevation measurements made by an orbiting laser. During the Spring and Summer of 1998 the Mars Orbiter Laser Altimeter (MOLA) flashed laser pulses toward the Martian surface from the Global Surveyor spacecraft and recorded the time it took to detect the reflection. This timing data has now been translated to a detailed topographic map of Mars' north polar terrain. More information.

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.

"It seems that our knowledge of the high-latitude weather on Mars is limited by the fact that even Viking, from orbit 20 years ago, could not observe the polar regions very frequently," says Cornell's Bell. "So our HST finding, and forthcoming results from the Mars Global Surveyor spacecraft, may simply be revealing all this activity up there because it's the first time we've been able to look in a detailed and dedicated way with high-resolution instruments during this season."

The team of astronomers, besides Bell, operator of HST, included Michael 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.).

Hubble Views Collossal Polar Cyclone on Mars - STScI Press Release

The Planet Mars - from the SEDS Nine Planets web site

Life on Mars - A review of evidence of signs of life in the Allen Hills meteorite

Mars Global Surveyor - home page

Mars at opposition - 1995 images from the Hubble Space Telescope

Mars - by Percival Lowell, 1895

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