"The Mars atmosphere is a dynamic place," said Michael Smith, a rover scientist from Goddard Space Flight Center. "You see things that look like weather fronts. You see these dust storms, and you have seasons." Martian dust storms can consume the planet. Measuring dust has become a critical part of planning the rover landings.
A large regional dust storm that passed through Opportunity's landing site also sent dust around the globe, to the sky above Gusev Crater. It reduced the density of the air and forced the landing team to tweak its timeline for opening the lander's parachute.
"This dust is like a little blanket, a thermal blanket," landing manager Rob Manning said, that "warms up that layer of the atmosphere that the dust lives in, which is quite high up."
The challenge for Mars forecasters is the lack of data on the planet's weather, despite some measurements by orbiting spacecraft.
"We have the equivalent of three Mars years' worth of data," said David Kass, an expert on Mars' atmosphere at JPL whose research helped the engineers design their landing sequence. "When you compare that to hundreds of years' worth of climate history on the Earth, we're still not quite there."
The thermal emission spectrometer on the orbiting Mars Global Surveyor can deduce temperatures at the upper levels of the atmosphere. During its 90-day mission, each rover can look up with its own spectrometer and take similar measurements at the lower levels.
"This is the first time we've had a thermal instrument at the lower part of the Martian atmosphere," said Diana Blaney, a research scientist for the rovers' mini-thermal emission spectrometer at JPL.
Winds, which are critical during a landing, are more elusive. They are driven by temperature differences, seasons and the reflectivity of the surface, Blaney said.
"Things like winds are much more difficult to measure from orbit," Kass said, "and therefore we have a much worse understanding of them. We have to rely a lot more on the models to get that data."
As a result, he said, forecasting for the rovers is more about understanding the climate at a particular time of year and knowing what extremes they must withstand during landing and on the surface.
"The rover has to survive an incredibly large swing in temperature range each day," project scientist Joy Crisp said.
The temperature a few feet above the surface plummets to minus 100 degrees Fahrenheit before dawn, Crisp said.
"On the Earth, this corresponds to an exceptionally cold night at the South Pole," she said.
Those temperatures will get colder as Gusev moves into autumn -- perhaps minus 123 degrees, Crisp said.
At least the rover will stay dry.
There are high clouds of carbon dioxide ice and water ice, and there's ice on parts of the planet, but there are no rivers or lakes. The rovers are seeking mineral evidence that water once flowed there.
It seems clear Mars has experienced some climate change, Kass said, possibly because the tilt of its axis has undergone extreme wobbles over millions of years.
A tilted axis causes seasons on Mars and on Earth. Now, the two planets have a similar tilt.
They have something else in common: Dust devils. Only the devils on Mars are monsters.
"The Martian dust devils are bigger than tornadoes on Earth," science team member Mark Lemmon said.
In geologic time, many of the wide dust-devil tracks criss-crossing the area around Spirit could be relatively young -- that is, centuries old, Blaney said.
Pathfinder snapped a low-contrast photo of dust devils in Ares Vallis in 1997, and the mission team hopes the rovers might get as lucky.
Gusev Crater has 10 times the dust devil activity of the Pathfinder site, Lemmon said.
Still, the mission team isn't really worried about them.
"The pressure is so much lower," Smith said. Mars' atmospheric surface pressure is less than one percent of Earth's.
Lower pressure adds up to devils with less power. Even if a dust devil shook the rover, Kass said, "the rover is kind of built to handle a certain amount of buffeting and shaking. I mean, it had to go through launch."
Of more concern is the dust that the devils or other winds may deposit on solar panels.
"That's one of the potential major limits to the life of the mission, is too much dust building up," Kass said. "On the other hand, something like a dust devil, maybe it's got enough winds that it actually picks dust back off the panels for you."
Published under license from FLORIDA TODAY. Copyright 2004 FLORIDA TODAY. No portion of this material may be reproduced in any way without the written consent of FLORIDA TODAY.
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