Top: Mars as it appeared to the Mars Reconnaissance Orbiter’s Mars Color Imager on June 22, 2007. The first in a series of regional dust storms has sprung up, to the west of Opportunity.
Bottom: By July 17, nearly the entire planet was obscured.

Weathering the Storm

July and August here in Pasadena are usually some of the most predictable days of sunshine the year has to offer. Scientists on the Mars Exploration Rover (MER) team at JPL, however, spent the better part of their summer battling the largest dust storm in the solar system, which enshrouded nearly the entire planet in a dark haze. Project managers were forced to pull back the reins on rovers Opportunity and Spirit and hunker down for the storm.

In the biggest threat to the mission since their landings on Mars three and a half years ago, the twin rovers faced the risk of losing power and shutting down indefinitely. The situation was particularly hazardous for Opportunity, which at the storm’s peak was receiving less than 1 percent of the normal amount of sunlight on its solar panels. A heater switch in Opportunity’s arm that had been stuck in the “on” position since landing provided an additional energy challenge, draining one-third of the diminished solar-generated electricity. Both rovers parked themselves and went into a low-power mode in order to conserve as much energy as possible. While on standby, communications opportunities were limited to once every three days, and all but the most basic functions necessary for the rovers’ survival were turned off. But the heat generated by the rovers’ electronics helps keep the insulated boxes housing them warm, and with much of their circuitry inactive, concern grew that the rovers might not be able to maintain their normal operating temperatures during the cold Martian nights. Damaged circuitry or a depleted battery could have spelled doom for even the most intrepid robotic explorer.

The storm came at an inopportune time. Spirit was poised near a plateau known as Home Plate, ready to study Mars’s volcanic history. Opportunity was waiting out the storm at the rim of Victoria Crater, into which it is slated to descend to study the billions of years of geological history chronicled in the walls of the 70-meter-deep crater. Victoria Crater exposes significantly more strata than any other feature studied by the MER team, who are eagerly awaiting the chance to look further into Mars’s geological past than ever before.

This false-color view of Cape St. Vincent, a quarter of the way around Victoria Crater from Duck Bay, shows the band of bright rock just below the rim that is visible all around the crater. The band, thought to be Mars’s surface just before the crater was created, will be one of Opportunity’s first stops on Duck Bay’s much gentler slope.

While the rovers were stymied on the surface, the Mars Reconnaissance Orbiter (MRO) had a field day imaging the storm and its evolution. As Richard Zurek, JPL’s project scientist on the MRO mission put it, “When you get lemons you make lemonade, and when you get a dust storm you study the dust storm.” Dust storms are common on Mars, but storms of this magnitude only flare up every five or six years, rather like the El Niño cycle here on Earth. With months of data from this storm, the MRO team will be in a position to answer some fundamental questions about Martian weather patterns: What triggers such a global storm? Why do they occur some years and not others? How does dust get distributed over the planet and alter surface features? This in turn will help us interpret the evolution of Mars’s surface geology with more confidence.

Zurek is often asked just what it would be like to experience this dust storm. “Visibility is still a few miles,” he says, “like a hazy day in L.A., but quite a bit darker. It is significantly cooler during the day since the majority of the sun’s energy is absorbed or scattered by the dust, but warmer at night since the remaining heat is trapped, leaving average temperatures essentially unchanged over the course of a martian day.”

Both rovers resumed driving and doing science in late August. Spirit climbed onto Home Plate the week after Labor Day. And favorable gusts of wind have removed some dust from Opportunity’s solar panels almost as quickly as it settled. As E&S went to press, Opportunity was cautiously beginning its descent into Victoria Crater via a scallop on its edge known as Duck Bay. —EQ

Elijah L. Quetin is a graduate student in astronomy, working on galaxy evolution with Richard Ellis, the Steele Family Professor of Astronomy.