"Mackinac Island, Mars" is a meteorite discovered and examined last year by scientists working on NASA's Mars Exploration Rover Mission. Scientists are naming rocks of scientific interest after islands on earth. Mackinac Island was named by meteorite expert James Ashley, a former Michigan resident, who worked on the mission and is with the School of Earth and Space Exploration at Arizona State University. (Photograph courtesy of D. Savransky and J. Bell (Cornell) / JPL / NASA) Two large rocks discovered in a sea of sand dunes on the planet Mars have been named Marquette Island and Mackinac Island. Existing worlds away from the Michigan islands that inspired their names, the two rocks with significant and unique characteristics have been named and studied by scientists who are working on NASA's Mars Exploration Rover Mission.

To most people, Mackinac and Marquette islands are rocky islands in Lake Huron off the Eastern Upper Peninsula shoreline. To scientists, they are a window to Mars' past climate and hydrology. The planet could have once supported life, they say.

Opportunity, NASA's Mars Exploration rover, performing an analysis of Marquette Island November 2009. The photograph is taken by the rover's front hazard-avoidance camera. (Photograph courtesy of NASA/JPL-Caltech) Two exploration rovers, Spirit and Opportunity, have been probing Mars for about six years. Opportunity is on the Cobble Campaign, where rocks are tested as the rover treks across the Meridiani Plains to its ultimate destination, Endeavour Crater.

On Earth, mission scientists command the rovers toward rocks and other objects of interest. The rovers then perform on-site geological tests, including investigating the rocks using a microscopic camera that sends photographs to earth. The rovers also have an abrasive tool that exposes the inside of a rock, allowing scientists to evaluate a rock's composition.

"The area we are exploring is very smooth and very flat and almost devoid of rocks," said scientist Steven Squyres. "We see lots of sand and lots of big, flat exposure of bedrock, but very few loose rocks sitting around on the plain. When we do come across them, we pay particular attention because there is almost always something unusual and interesting."

Dr. Squyres, the Goldwin Smith Professor of Astronomy at Cornell University, is the scientific principal investigator for the Mars Exploration Rover Project.

He recalls last year when scientists who were steering Opportunity along the plain spotted a large rock in the distance. Working on the team that day was James Ashley, an expert in meteorites from the School of Earth and Space Exploration at Arizona State University.

Mr. Ashley suggested the rock be named Mackinac Island. Mackinac Island turned out to be a big meteorite.

"I now live near Phoenix, Arizona, but have very fond memories of visiting Mackinac Island on summer trips with my parents and sister while growing up in Grand Rapids," he said.

Upon examination, said Mr. Ashley, the rock turned out to be "a most extraordinary looking meteorite on Mars."

Other people working that day were unfamiliar with Mackinac Island and had a hard time understanding the spelling with the silent 'c' ending, recalled Dr. Squyres, who also is familiar with the Straits region.

He said the family of his mother in-law, Marnie Johnson, has had a home on Marquette Island in the Les Cheneaux Islands for decades. He decided to suggest Marquette Island as the name for the next rock the rover encountered. This rock turned out to be most unusual.

"Sure enough, we left Mackinac Island and came across another rock," he said. "When we got to it, it turned out it wasn't a meteorite at all, it was something very much more interesting than that. We spent almost two months working on Marquette Island, the rock on Mars, and it turned out, I think, actually to be one of the more significant science findings for the whole mission."

Marquette Island, the scientists learned, was a rare and unusual piece of Mars.

"We've never seen anything like it anywhere," said Dr. Squyres. "It is what geologists call a crater ejector. When a huge meteorite from space comes crashing into the planet, making a big hole, stuff gets thrown up into the air. That stuff is call ejecta. Some of these pieces of ejecta travel very long distances. This piece of ejecta came from some far away crater. We don't know what crater. It is from somewhere far away, from deep in the crust of Mars. What it does is provides us with access to the kind of material we would never be able to see otherwise. We can't drive to that crater, it's too far away. We can't go down that deep into the crust. We didn't bring a big drill. But nature dug some big holes for us in the form of these impact craters, and if you can find one of these pieces of far away ejecta, you can learn stuff that you can't learn any other way."

Naming Mars rocks after islands is a way for scientists to share the exploration adventure with the public. Coming up with the island theme was logical, he said, as the scientists looked out over the sea of sand and saw an occasional rock, which looked like an island. The naming process, said Dr. Squyres, also serves as a learning tool for students and piques interest in the Mars project. The NASA Web site has a section specifically designed for youngsters, found at www.marsrovers. nasa.gov.

"I remember when we first landed with the rover Spirit," he recalled. "We'd literally been on Mars three days and nobody was sleeping. We'd been up all night, every night, all exhausted. There were all these little craters around the lander and the closest one we decided to name Sleepy Hollow because we were just so tired. Within 12 hours I got a phone call from a reporter from the Sleepy Hollow Gazette."

After years of exploring Mars, devising a naming convention or some kind of theme is also helpful to scientists.

"We learned a long time ago you can't just call them rock one, rock two, rock three, then you get up into the hundreds and the thousands and it is pretty much impossible," he said. "So we needed to give them a memorable name."

The Mars Exploration Rover Project is a $900 million mission. The rovers were designed to last about six months.

"They are getting old and creaky," said Mr. Squyres, "but we're going to keep pushing them as long as we can."