In more than 14 years of roaming Mars, NASA’s Opportunity rover took more than 210,000 pictures. (Its twin, Spirit, snapped an additional 125,000 on the other side of the planet.) Some were sweeping 360-degree panoramas of the reddish landscape. Others were microscopic close-ups of rocks. Through the images, mission scientists were able to decipher some of the geological history of Mars.
“Spirit and Opportunity were robotic field geologists,” said Steven W. Squyres, principal investigator for the mission, during a NASA news conference Wednesday. “A geologist is like a detective at the scene of a crime. Something happened at this place on Mars billions of years ago. What was it? What was it like there back then? And you’re looking for clues, and the clues are in the rocks.”
Sticking the Landing
Opportunity was sent to Meridiani Planum, a plain just south of the Martian equator.
When the first images popped up on the monitors, the scientists were flabbergasted. Opportunity, cocooned in a sphere of protective air bags, had by pure chance, rolled into one of the few craters in Meridiani Planum — an interplanetary hole-in-one.
Along the sides of the crater were exposed bedrock, all ready for the scientists to examine as they worked to produce a natural history of how this corner of Mars had formed. At first, the rocks were nicknamed the “Great Wall,” but that turned out to be a trick of perspective. The crater was tiny — only about 70 feet wide — and the exposed bedrock was about the height of a sidewalk curb. People quickly stopped calling it the Great Wall.
Blueberries Everywhere
Meridiani Planum was selected for two primary reasons. For engineers, a flat place is a safer, easier place to land. For the scientists, the Mars Odyssey orbiter had detected an intriguing signature of an iron oxide mineral known as gray hematite, spread out over a region the size of Oklahoma. On Earth, hematite typically forms in the presence of liquid water. (It is also possible for hematite to come out of lava without water.)
“The orbital data told us it was going to be there,” said Raymond E. Arvidson, deputy principal investigator. “It didn’t tell us where.”
On the surface, Opportunity found a multitude of BB-size spheres, which the scientists called blueberries, and the blueberries were made of hematite, which formed as acidic water flowed through the sediments.
“Sort of the way an oyster forms a pearl,” Squyres said.
As the surrounding rock weathered, the blueberries, which were harder, rolled onto the surface.
Greater Craters
As it became apparent that Opportunity would last much longer than the 90 days they had originally expected, scientists moved onto bigger craters with larger swaths of exposed bedrock.
While the years of exploration added detail to the story of how Meridiani Planum formed, “we got most of the story in the first six weeks,” Squyres said. After they finished up at Victoria Crater, a half-mile-wide feature, they wondered what to do next.