This story was updated at 3:15 p.m. ET
NASA's Opportunity rover has eyed an odd-shaped, dark rock on the surface of Mars ? scientists think it could be a meteorite.
Opportunity's handlers spotted the rock, which measures about 2 feet (0.6 meters) across, in images sent by the rover on July 18 in the opposite direction from which the rover was driving. The rock, dubbed "Block Island," is unusual for its size, mission scientists said.
"The images came down after we had already passed," said planetary scientist Albert Yen of NASA's Jet Propulsion Laboratory in Pasadena, Calif.
Mission managers decided the rock was worth a closer look and had the rover then backtrack some 820 feet (250 meters).
Over the weekend, scientists used the rover's alpha particle X-ray spectrometer to get composition measurements and to confirm it was a meteorite.
"It's pretty clear now that it is," Yen told SPACE.com.
The rock has a similar composition to another meteorite that Opportunity found in Jan. 2005. That meteorite was the first to be found on another planet. It was lying just over half a mile from its landing site in Mars' Meridiani Planum. Since landing on Mars in 2004, Opportunity has driven across 10.7 miles (17.2 km) of the red planet?s surface.
The 1-foot (31-cm) diameter slug of iron and nickel gained the moniker "Heat-Shield Rock" due to the rover's discarded heat shield having come to rest only 20 feet (6 meters) from the meteorite.
There are some differences between the two meteorites though and scientists plan to use some of Opportunity's other instruments to learn more about Block Island.
"We're likely to spend a good amount of time here," Yen said, at least through the upcoming weekend.
The rover team has also taken color and microscopic images of the meteorite. Next up are measurements with the rover's M?ssbauer spectrometer, which can tell scientists more about the mineralogy of the rock.
Studying the meteorite could tell scientists more about the surface conditions in place when the meteorite fell: The pitted surface of the rock shows millimeter-sized granules of what scientists think is hematite, which suggest the meteorite was once buried under the Martian sand, Yen said. The meteorite could also tell the scientists about the atmosphere at the time, since the chunk that is left is relatively large.
Opportunity and its sister rover Spirit have been rolling along opposite ends of the red planet for more than five years now. While Opportunity passed the 10-mile mark, Spirit has been mired in a Martian sand trap for more than a month. Rover engineers back on Earth are working with a test rover to come up with ways to free the mired robot.
In its five years on Mars, Opportunity has spent two years exploring Victoria Crater, revealing that the whole region the crater lies in was likely shaped by water and winds that blew up huge sand dunes. Its twin Spirit is on the other side of Mars in a region known as Gusev Crater.
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