Scientists Get A Look At Mercury's Innards

Meteors that smashed into the planet Mercury 3.9 billion years ago are giving scientists a glimpse deep into the tiny planet's interior and providing clues to how it has evolved in the eons since.

The 430-mile-wide Rembrandt impact basin, first seen by NASA's Maryland-built Messenger spacecraft during two flybys last year, preserves cracks created during ancient upheavals from beneath the basin, as well as ridges formed like wrinkles as the planet cooled and shrank.

"This is really exciting, because this pattern of tectonic land forms is different than anything we see anywhere in the solar system," said Thomas Watters, a scientist with the Smithsonian Institution in Washington who is part of the Messenger team.

Messenger was designed and built at the Johns Hopkins University's Applied Physics Laboratory, and its operations center is on the lab's campus near Laurel. Another flyby is planned for Sept. 29. If all goes well, the spacecraft in March 2011 will become the first to orbit the planet.

Messenger's new discoveries were published Friday in the journal Science.

What's remarkable about the Rembrandt impact basin, Watters said, is that its geological history is still visible on the surface. Similar basins in the solar system have been buried by subsequent volcanic activity.

One striking feature is a huge fault that runs across the basin. More than 600 miles long, it is the longest ever seen on Mercury, comparable to California's San Andreas fault.

Another impact basin imaged by Messenger has a smaller crater at its center. Brett Denevi, an imaging team member from Arizona State University in Tempe, called it "a natural drill hole" that blasted rocks rich in iron and titanium to the surface.

"It's uplifted from great depths, up to [six miles], giving us a look deep down into the basin," she said.

Marilyn Lindstrom, a NASA program scientist, said the Messenger findings show that Mercury is "just an amazingly dynamic planet, both in the past and in the present."