By Steve Bartlett
NASA has just reached the milestone of operating a robotic craft on the surface of another planet for a remarkable ten years. John Callas, the Project Manager of the Mars Exploration Rover (MER) program at the Jet Propulsion Laboratory, described the achievements of the twin craft in his talk, "A Decade on Mars: The Mars Exploration Rover Program," on February 22nd. Callas showed his rapt audience stunning views of the Red Planet and shared the excitement of scientific discovery in his OASIS lecture at the Long Beach City Planetarium.
He opened his talk with some background on what we knew and speculated about Mars prior to the start of the Space Age. Through most of human civilization, what we knew of the planet Mars was learned through naked eye astronomy and ground-based telescopic observation. We had determined the size and approximate mass of the planet, its orbit, the length of its day, that it had polar ice caps that expanded and contracted over the course of its year, and that it had two moons.
But ground-based telescopes were limited in what they could see and, in many cases, human imagination filled in the gaps where good information was lacking. There were those who looked at the fourth planet and thought they saw signs of a network of channels. Some, including the American Percival Lowell, speculated that Mars might have a civilization. A large number of books, movies, and radio and television shows imagined all sorts of life forms inhabiting the planet.
It wasn't until 1965, when Mariner 4 made a flyby of the world, that we got our first close-up look at Mars. It and subsequent probes in the 1960's and 70's found that Mars wasn't the hospitable place we'd expected: the Red Planet was cold and cratered like the Moon. It possessed an atmosphere, but only 1% as thick as Earth's and consisting mostly of carbon dioxide. The two Viking landers went there specifically to search for signs of life but found nothing conclusive. And public interest in Mars waned for over a decade.
Then, in 1996, researchers investigating a meteorite discovered in the Allan Hills of Antarctica, ALH84001, found that it had been knocked from the surface of the Red Planet in a cosmic collision and, over several millennia, made its way to Earth. Their studies suggested that the materials in the space rock showed biologically based chemical properties. They found that ALH84001 had polycyclic aromatic molecules, which on Earth are typically associated with living things. Further bolstering their belief that Mars may have once had life, they discovered tiny structures just a tenth of a micron (or one ten-millionth of a meter) in size that were very reminiscent of bacteria, only smaller.
Biologists also discovered on Earth a number of different organisms that existed under extreme conditions. These organisms lived in the driest deserts, or inside Antarctic lakes buried under miles of ice, or in sulfurous volcanic vents at the bottom of the ocean at very high temperatures and pressures. "By comparison," Callas said, "Mars isn't so bad!"
Suddenly the interest in Mars was renewed. The general view among scientists was that, while Mars might be cold and dry now, perhaps sometime in its distant past conditions were much warmer and wetter and may have given rise to life. And if it existed once, perhaps there might be isolated places on Mars where some life forms still survived.
To find out, NASA sent the orbiting Mars Global Surveyor spacecraft to that world to photograph its surface. The probe's results further whetted speculation that Mars may have once hosted life. It discovered that Mars had a low, smooth northern hemisphere: a possible location for a great ocean at one time. Further, they found channels that may have been carved by water that may have been on the surface of Mars for an extended period. Callas pointed out that at Mars' current low atmospheric pressure, water would be either a vapor or a solid. For water to have been a liquid for a long time, the atmospheric pressure must have been much higher. This was another indicator that conditions there had been more hospitable for life.
In 1997, the space agency landed the Mars Pathfinder probe on the planet. The lander and its toaster oven-sized rover, Sojourner, were intended to demonstrate that a rover could be operated on the planet's surface and collect valuable scientific data. Its findings further suggested that Mars had had a warmer, wetter climate in its past. The Sojourner rover, which was only designed to operate for a week, lasted for 31 days. In that time, it explored to the limit of its 10-12 meter range around the lander.
These results made researchers want to go back to Mars for a longer stay, with a more capable rover that could look at a larger area on the planet. Thus, the Mars Exploration Rover program started. The plan was to have two rovers land on opposite sides of the Martian globe. These would be mobile robotic field geologists about the size of a lawn tractor with multiple cameras and tools. They would have robotic arms and instruments to dig beneath the surfaces of Martian rocks to determine their composition and their history.
The rovers, dubbed "Spirit" and "Opportunity," were designed for a 3-month mission—three times as long as the Mars Pathfinder rover had operated. JPL engineers hoped that the two craft could operate on the surface for that long before the temperature extremes on Mars made the electronics fail, or the dust on the surface jammed their drive motors or covered the rovers' solar arrays and starved them of power.
In June and July of 2003, the craft were launched to the Red Planet aboard two Delta II rockets. One rover, Spirit, was targeted for the Gusev crater, which researchers believed might have been a flooded crater in the planet's past. Opportunity was aimed at the Meridiani Planum, a flat plain that may have had water for an extended period.
During the interplanetary cruise phase of the mission, the craft were covered by their circular heat shields and cruise stages. This gave them a saucer-like appearance and Callas joked, "We figured that after all those years of Mars sending flying saucers to Earth, it was high time that we send some back to them!"
The craft came screaming into the Martian atmosphere at 25,000 miles per hour in early January 2004. Their heat shields absorbed most of the enormous kinetic energy and slowed them to 700 miles per hour in under a minute. Then, parachutes further slowed them until the point where airbags surrounding them could be deployed. The craft released their parachutes and dropped to the ground, where they bounced around until coming to a stop on the surface. The landers then deflated the airbags, and the rovers deployed and rolled off their landing ramps.
Soon after landing, Spirit suffered an anomaly in its flash memory. The rover stopped sending data and barely communicated with controllers on Earth. The controllers were able to fix the problem by updating the software and reformatting the memory. It was then able to resume normal operations.
Furthering its run of bad luck, when Spirit began examining the area of Gusev crater, instead of finding the hoped-for signs of rocks that were formed in the water, it found a field of volcanic rocks and debris. Undaunted by their initial negative results, controllers sent it to the nearby Bonneville Crater, where they hoped for better luck.
Opportunity, on the other side of the planet, scored a geologic "hole-in-one": it immediately found sedimentary rocks and conclusive evidence that they were formed in an acidic, evaporated environment. It also found spherical rocks—dubbed "blueberries"—made of hematite, which on Earth forms only in the presence of water. Controllers then began cautiously performing a detailed study of the diverse geology in the area, including an in-sequence stratigraphic survey of the layers of sedimentary rocks in the area to understand their makeup and history. Observations indicated that a kilometer-scale body of water had covered the area.
The work of the rovers was aided by the fact that their solar arrays weren't collecting dust as fast as JPL scientists had feared. The Martian equivalent of "dust devils" – the spinning vortexes of air that move around the desert – were occasionally moving across the surfaces of the rovers and sweeping the solar arrays clean of dust. So the rovers continued to generate power for longer than expected.
As Spirit performed its explorations, it descended into the Columbia Hills, an older area of Mars, where it finally found the water-formed rocks it was designed to ferret out. These rocks were both formed in and altered by water, implying that water had been present on the Martian surface for a very long period of time. These included carbonate minerals which, on Earth, are remnants of an ancient carbon dioxide atmosphere and a neutral pH water environment.
But Spirit's bad luck resumed after the first winter on Mars when one of its motor wheels failed and controllers were forced to drive the rover backwards to keep the wheel from digging into the ground. Thus, it was dragging the wheel in the dirt for much of its operation.
Despite the setbacks, the rovers continued to operate far beyond anyone's expectations. In fact, the software had to be updated just prior to the 1,000th day of operation because the computers onboard were only programmed to operate up to 1,000 days. As Callas noted, "We had to do our own version of a Y2K update to the rovers' software."
A global dust storm further complicated the rovers' work. According to Callas, the sky became VERY dark, almost black: "We had to hunker down and turn off equipment just to save power." Controllers would keep equipment turned off for up to 2 weeks and then briefly turn it back on just to keep it alive.
After the dust storm, Opportunity explored Victoria Crater, a sandstone crater with a rich geologic history. Meanwhile, Spirit's dragging wheel revealed signs of pure olivine silica on the ground. On Earth, pure olivine silica only forms in water with a hydrothermal system. "This was the greatest discovery of the mission," said Callas, "because it showed that there was a source of energy that could drive an ecosystem." So the failed wheel turned out to be a blessing in disguise because the discovery of the silica showed that Mars was once very much like early Earth.
But time continued to take its toll on the Spirit rover. After 6 years on Mars, dust covered its solar arrays and its wheels became stuck in the loose, fine dirt. Efforts to free it were unsuccessful, so it remained in place. It continued to explore the area immediately around it, where it found a scientific bonanza of fine materials, including unconsolidated sulfates and other materials formed relatively recently in geologic terms. Spirit sent its last signal on March 22, 2010. Just before that, it sent its last image—a view of the Martian sunset with a pink sky and a blue sunset.
Opportunity continued working, though, thus far for 13 kilometers. Along the way, it found a large meteorite on the surface, which could only have gotten there if it came down to Mars through a much thicker atmosphere than we see today. It is currently at the rim of the Endeavor Crater, an area formed during what is known as the Noachian Era of Mars history, where clay minerals formed in the presence of neutral pH water – another sign of a benign early environment on the Martian surface.
The rover also found a seam of gypsum, a material on Earth used in drywall on houses. Gypsum is hydrated calcium sulfate which, on Earth, only forms in the presence of water. Additionally, the area has a "mother lode" of clay philosilicate minerals. One rock recently overturned, dubbed the "Jelly donut" for its shape and color, has a substantial content of manganese, which according to Callas suggests water alteration.
Dust has been accumulating on the surfaces of Opportunity's solar arrays. The ground controllers had to position it to face the sun and remain stationary during the last Martian winter so that it would collect enough power to operate. This also allowed researchers on Earth to use the craft's radio signal to collect data on the makeup of the interior of the planet and its motions.
Otherwise, the rover is in "extremely good health," according to Callas, but he cautioned, "It's going through tremendous thermal cycling every day, like going from the Sahara desert to Antarctica and back." He said that there's no redundancy on the rover and "one component could fail at any moment and it's game over." But, until then, he and his team will do their best to keep it running.