Giant storm shrouds Mars

Mars in June and, hidden by dust, September 2001
Image: Nasa, J Bell, M Wolff, Hubble Heritage Team

By BBC News Online science editor Dr David Whitehouse

A giant dust storm, larger than any seen on Earth, is ravaging the face of Mars.

It is being monitored by the Mars Global Surveyor (MGS), which is in orbit about the Red Planet, and by the Hubble Space Telescope, which is stationed just above the Earth.

We have a phenomenal, unprecedented view from these two spacecraft

James Bell
Cornell University

The storm is one of the most intense ever seen on Mars, and has engulfed the entire planet for the past three months.

The airborne dust is being warmed by the Sun and is raising upper-atmospheric temperatures, scientists say. But, at the same, the planet's surface is being chilled as it is deprived of solar radiation.

The storm is also being closely watched by the team operating the American space agency's (Nasa) 2001 Mars Odyssey spacecraft. The latest probe to visit the Red Planet will start to take up an orbit later this month.

'Unprecedented view'

Both Hubble and MGS caught the storm erupting in late June - an unusually early start in the Martian northern hemisphere's spring for such a large storm.

Hubble doesn't have continuous Mars coverage, but it does show the whole planet in a single snapshot. It can also reveal the full range of dust activity from sunrise to sunset.

"This is an opportunity of a lifetime," said Hubble observer James Bell, of Cornell University, US. "We have a phenomenal, unprecedented view from these two spacecraft."

Richard Zurek, of Nasa's Jet Propulsion Laboratory, added: "The beauty of Mars Global Surveyor is that we have almost two Martian years of continuous coverage, and this is the first time during the mission that we have seen such a storm."

Grows and spreads

The Thermal Emission Spectrometer on the MGS has been tracking the growing dust storm by measuring temperature changes that trace the amount and location of dust in the atmosphere.

Planetary scientists photograph the entire planet every day using MGS. This has allowed them to pinpoint the places where dust is being raised, and see it migrate and interact with other Martian weather phenomena and surface topography.

MGS can map the temperature and amount of dust in the Martian atmosphere

This has also provided them with an unprecedented, detailed look at how storms start and how they grows as they spreads across the planet.

"What we have learned is that this is not a single, continuing storm, but rather a planet-wide series of events that were triggered in and around the Hellas basin," said Mike Malin, of Malin Space Science Systems Inc., the lead investigator on the camera.

"What began as a local event stimulated separate storms many thousands of kilometres away. 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," he added.

High winds

"By the time the first tendrils of dust injected into the stratosphere by the initial events circumnavigated the southern hemisphere, which took about a week, separate storms were raging in three main centres.

"The most intriguing observation is that the regional storm in Claritas/Syria has been active every day since the end of the first week of July," Dr Malin said.

After three months, the storm is beginning to wane. The planet's shrouded surface has cooled, and this has allowed the winds to die down and the fine dust to begin settling.

However, Mars is approaching the point in its orbit where it is closest to the Sun. Once the atmosphere begins to clear, the return of unfiltered solar radiation may trigger additional high winds and kick up the dust all over again. This effect has been seen for centuries in previous Mars storms.

Puffing up

"Understanding global dust storms, such as that which we have witnessed this year, is a vital part of the science goals of the Mars Exploration Program," said James Garvin, Nasa's lead scientist for Mars exploration.

Hubble can stand back and take spectacular views of Mars

"Such extreme climate events could potentially provide clues to how climate changes operate on Mars, now and in the past, and provide linkages to the record of sediments on the planet."

When it gets to the Red Planet in a few weeks time, the 2001 Mars Odyssey team plans to dip the probe into the Martian atmosphere. The latest spacecraft to go to Mars will gradually deepening its pass through the atmosphere until the desired drag levels are found and the spacecraft can enter orbit.

The dust storm will affect this tricky procedure as a warm atmosphere "puffs up", creating more drag on the spacecraft.