NASA’s Opportunity rover has discovered a peculiar rock on Mars that scientists think originated deep within the red planet.

The stone could reveal new secrets about the makeup of Mars’ interior.

Dubbed “Marquette Island,” the rock is a dark boulder not much bigger than a basketball that sits on a rippled martian plain.

“Marquette Island is different in composition and character from any known rock on Mars or meteorite from Mars,” said Steve Squyres, the Cornell University professor who serves as principal investigator for the Mars Exploration Rover mission. “It is one of the coolest things Opportunity has found in a very long time.”

Opportunity and its twin rover, Spirit, landed on Mars in 2004, and have drastically outlived their original 90-day mission plan. While Spirit is currently stuck in a sand trap with two broken wheels, Opportunity is still roving free.

In all of its nearly 18 kilometers’ worth of traveling, Opportunity has found only one other rock of comparable size to Marquette Island and scientists think it was ejected from a distant crater. Called “Bounce Rock,” that stone closely matched the composition of a meteorite that landed on Earth, but was thought to have originated on Mars.

The coarse-grained texture and basalt composition of Marquette Island indicate that it cooled slowly from molten rock, allowing crystals time to grow. That means that it likely originated deep in the crust, not at the surface where it would cool quicker and have finer-grained texture, scientists say.

“It is from deep in the crust and someplace far away on Mars, though exactly how deep and how far we can’t yet estimate,” Squyres said.

In contrast, most martian basalt rocks that Spirit and Opportunity have encountered have different textures and composition.

At first, scientists thought Marquette Island could be a meteorite, but it appears to have a much lower nickel content than other meteorites Opportunity has found. And Marquette Island’s interior contains more magnesium than typical martian basalt rocks.

“It’s like having a fragment from another landing site,” said Ralf Gellert of the University of Guelph, in Ontario, Canada. Gellert is lead scientist for the alpha particle X-ray spectrometer on Opportunity’s robotic arm. “With analysis at an early stage, we’re still working on some riddles about this rock.”

The rover team used Opportunity’s rock abrasion tool to grind away some of Marquette Island’s weathered surface and expose the interior.

This was the 38th rock target Opportunity has ground into, and one of the hardest. The tool was designed to grind into only one martian rock, and this rock may not be its last.

“We took a conservative approach on our target depth for this grind to ensure we will have enough of the bit left to grind the next hard rock that Opportunity comes across,” said Joanna Cohen of Honeybee Robotics Spacecraft Mechanisms Corp. in New York, which built and operates the tool.

While Marquette Island is intriguing, Opportunity could not stop too long to investigate — it left the site Jan. 12. The rover is midway on a journey toward a much larger crater, Endeavour, that scientists think will offer a host of scientific prospects.

“We’re on the road again,” said Mike Seibert, a rover mission manager at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “The year ahead will include lots more driving, if all goes well. We’ll keep pushing for Endeavour crater but watch for interesting targets along the way where we can stop and smell the roses.”