Observations of the largest dust storm to hit the planet Mars in 30 years may shed new light on why such global dust storms form, scientists announced Thursday.

These comparison images shows how a global dust storm engulfed Mars with the onset of Martian spring in the Southern Hemisphere. When NASA's Hubble Space Telescope imaged Mars in June, the seeds of the storm were caught brewing in the giant Hellas Basin (oval at 4 o'clock position on disk) and in another storm at the northern polar cap. When Hubble photographed Mars in early September, the storm had already been raging across the planet for nearly two months obscuring all surface features. The fine airborne dust blocks a significant amount of sunlight from reaching the Martian surface. Because the airborne dust is absorbing this sunlight, it heats the upper atmosphere. Credit: NASA, James Bell (Cornell Univ.), Michael Wolff (Space Science Inst.), and the Hubble Heritage Team (STScI/AURA)

Data from NASA's Hubble Space Telescope and Mars Global Surveyor (MGS) spacecraft have given scientists an unprecedented view of a massive dust storm that arose on Mars in late June and only recently subsided, and have also helped prepare the 2001 Mars Odyssey spacecraft for its arrival at the Red Planet.

"This is an opportunity of a lifetime," James Bell of Cornell University, who studied Mars using Hubble. "We have a phenomenal, unprecedented view from these two spacecraft."

The storm started in late June as a localized storm within the giant Hellas impact basin in the planet's southern hemisphere. This storm, in turn, spawned three separate storm centers elsewhere on the planet in a matter of days. Dust thrown into the atmosphere by those storms soon spread over nearly the whole planet and stayed there for weeks.

"What began as a local event stimulated separate storms many thousands of kilometers away," said Michael Malin, principal investigator of the Mars Orbiter Camera on MGS. "We saw the effects propagate very rapidly across the equator -- something quite unheard of in previous experience -- and move with the southern hemisphere jet stream to the east."

Comparison of Martian atmospheric activity from 1997 and 2001. Credit: J. Bell (Cornell University)

The effects of the dust storm were made clear in a pair of Hubble Space Telescope images. The first image, taken on June 26, when the storm was just starting, clearly shows fine detail, down to a scale of about 15 kilometers, on the Martian surface. The second, taken on September 4, around the height of the storm, shows that those features are now masked behind all the dust in the atmosphere, giving the planet a hazy reddish appearance. Only the polar caps are clearly visible in the September 4 image.

The dust pushed dust up to 80 km into the atmosphere, blown by winds that averaged about 100 km/h, a very high wind speed for Mars. All the dust in the atmosphere absorbed sunlight, raising temperatures there about 50 degrees to around -20 Celsius.

The storm has only recently started to subside, when the Martian surface, shielded from the Sun by the atmospheric dust, began to cool. This has caused winds to subside, allowing the dust to settle out of the atmosphere. However, scientists caution that the conditions are ripe for another major storm to develop in the coming weeks, as the Sun warms the surface again.

"We're still in the middle of what we would call 'dust storm season' on Mars, and it's quite likely that this could blow up again into another storm," said Philip Christensen, principal investigator for the thermal emission spectrometer (TES) instrument on MGS.

Mars Global Surveyor's Thermal Emission Spectrometer data. Credit: Philip Christensen (Arizona State University) and the TES Team

This could pose an issue for the 2001 Mars Odyssey spacecraft, scheduled to enter Mars orbit on October 23 and spend two months dipping into the Martian upper atmosphere -- a process known as aerobraking -- to circularize its orbit. If another dust storm develops, the increased temperature of the upper atmosphere will cause it to expand out, increasing the atmospheric density at a given altitude and thus complicating the aerobraking process. NASA officials said they will monitor conditions in the Martian atmosphere using MGS and adjust Mars Odyssey's aerobraking accordingly.

Such a dust storm would pose a greater concern to a lander mission, which would have few options other than to ride out a trip through a stormy atmosphere. A similar large dust storm in 1971 may have contributed to the demise of two Soviet landers, one of which failed before landing and the second just 20 seconds after landing. However, there are no missions that will land on Mars this year, and the next landers, twin rovers planned by NASA and the Beagle 2 lander that is part of ESA's Mars Express mission, will arrive outside of the planet's typical dust storm season.

For now, scientists plan to use the data collected by MGS and HST to understand why a global dust storm, the largest since the 1971 storm, developed this year, but in past years only small local storms developed. "The most important question is what is the difference between a year with and one without this dust storm," said Richard Zurek, a planetary scientist at JPL. "We don't know that yet, but for the first time we have a data base that's going to cover that period so that we can see the onset of these storms and try to understand."