NASA's Spirit rover currently exploring Mars completed one full swing around the Sun Monday, giving researchers a year-long look at the Martian seasons.
"We feel like, weather-wise, we've just about seen it all," said Sharon Laubach, the rover's integrating sequence team chief, in a telephone interview. "We've gone through all the seasons, we've survived Martian winter and gone through conjunction...yes, we're having a party."
While both Spirit and its robotic twin Opportunity hit the one Earth year mark of their mission in January, researchers said the Nov. 21 Martian anniversary holds far more significance for the long-lived rovers.
"It's a big, important milestone," said Steve Squyres, principal investigator of the rover's science mission at Cornell University in Ithaca, New York in an earlier interview. "We'll have acquired an entire year's worth of observations."
One Mars year is longer than Earth's (about 687 Earth days), with Spirit hitting its anniversary on the 670th sol - or Martian day - of its mission. Spirit has rolled across 3.3 miles (5.4 kilometers) of Martian terrain at its landing site inside the planet's Gusev crater.
Opportunity will complete its first Martian year exploring the plains of Meridiani Planum on Dec. 11, mission scientists said. Both rovers touched down on Mars in January 2004 on a primary mission that spanned 90 days.
Spirit and Opportunity are currently experiencing the final days of Martian summer and preparing for the onset of autumn, mission managers said.
During its first Martian year, Spirit found dust devils swirling across the planet's landscape and frost settling on the its surface, researchers said, adding that the fringes of a major dust storm may have brushed Opportunity last month.
"So the planet, though it seems dead, does have a vibrant atmosphere," explained Amitabha Ghosh, an atmospheric scientist and rover science team member with Gaithersburg, Maryland's Tharsis, Inc.
Ghosh is using the mast-mounted Miniature Thermal Emissions Spectrometer (Mini-TES) aboard both rovers to study atmospheric changes depending on the Martian season.
"There are some really seasonal phenomena, like the dust devils," Squyres said.
Mars researchers initially hoped to find many dust devils roaming across the Martian surface, but had to wait until early spring on the planet before the first whirlwinds were caught by Spirit's cameras.
The infrared Mini-TES instrument, which is also used by geologists to determine Martian rock and soil minerals, records the temperature changes, dust levels and water vapor in Mars' atmosphere. Mars researchers can also use each rover's panoramic camera, which sits next to Mini-TES on the mast, to record dust devils, clouds and wind - which has apparently brushed the robot's solar arrays clean from time to time.
"[We're] trying to continue these routine measurements and, more importantly, look for patterns," Ghosh told SPACE.com.
In general, Opportunity's landing site seems to run a bit warmer than Spirit's - about 4 to 6 degrees Celsius - while the surface temperatures at both sites hit their peak at noon local time, he said, citing rover observations.
Building a comprehensive understanding of Martian weather will be critical for future red planet expeditions, since wind, dust storms and other atmospheric phenomena can factor into a mission's landing, Ghosh added.
A large dust storm that cropped up at Meridiani Planum in 2003 flung dust all the over to Spirit's Gusev Crater landing site and reduced the density of the air there, forcing mission planners to alter the rover's timeline for opening its parachute during descent.
Meanwhile, the weather on Mars should begin to turn as the landing sites for Spirit and Opportunity head further into fall.
In about 104 Martian days, or sol 775, rover handlers hope to have both rovers on Sun-facing inclines to maximize the amount of sunlight striking their solar arrays, Laubach said. The technique helped the rover mission weather the last Martian winter.
Spirit is currently rolling down Haskin Ridge as it descends from the summit of Husband Hill and makes its way toward a feature dubbed 'Home Plate.' Opportunity, meanwhile, continues to creep around Erebus Crater.
A far-off record
Despite their accomplishments, Spirit and Opportunity have a long way to go to set an endurance record on Mars. Both mission times pale in comparison with NASA's twin Viking missions.
Viking 1 and Viking 2, both of which set down on Mars in the summer of 1976, spent several years recording Mars from their stationary landing spots. Viking 2's mission ended in April of 1980 about 1,281 Martian days after landing when its batteries failed. Viking 1, however, continued to function until Nov. 13, 1982, more than four Earth years after arriving on Mars.
While their longevity is impressive, the Viking missions had a powerful - literally - advantage over Spirit and Opportunity. Both landers relied on radioisotope thermal generators (RTGs) that converted heat from decaying plutonium into electricity instead of solar arrays.
Dust buildup has gradually degraded the amount of power available from the solar arrays aboard Spirit and Opportunity. Both rovers were initially able to generate about 900 watt hours of power from the available sunlight, though dust has clouded their solar panels.
Spirit's solar arrays currently produce about 650 watt hours while Opportunity's generate up to 700 watt hours, Laubach said. A minimum of 300 watt hours is required for Spirit to function, though Opportunity can operate on slightly less, she added.
If Spirit and Opportunity continue to perform as they have for the last few weeks, it's entirely possible that they could make it through January 2006 and their second Earth anniversary exploring the red planet, mission managers said.
"Though certainly they could die tomorrow, that's just a fact," Laubach said, adding that she and other rover team members have been amazed at the mission's science output so far and hope for continued success. "We certainly haven't seen everything there is on Mars."