NASA: Mercury shrinking

Mercury is not just the solar system's shrimpy kid brother, at least since Pluto was kicked out of the planetary club two years ago. It's shrinking.

Measurements taken by NASA's Messenger spacecraft earlier this year show that the innermost planet has shrunk by more than a mile in diameter over its history. Scientists attribute that to the gradual cooling of the planet's core.

Messenger is the first spacecraft to study Mercury up close since Mariner 10 in 1975. It made its first close flyby in January, whisking to within 125 miles of the surface. It will swing back for a second encounter in October before settling into a final orbit in 2011.

The first comprehensive results from the January flyby are being published in today's issue of the journal Science.

Mercury has long been considered little more than a hot rock, with daytime surface temperatures ranging up to 1,100 degrees Fahrenheit. But Messenger has uncovered a more surprising place, with peaks reaching up to 15,000 feet and vast basins stretching hundreds of miles.

“When you look at the planet in the sky, it looks like a simple point of light,” said Messenger project scientist Ralph McNutt of the Applied Physics Laboratory at Johns Hopkins University in Baltimore. “But when you experience Mercury close up, you perceive a complex system and not just a ball of rock and metal. We are all surprised by how active that planet is.”

Scientists had long debated the origin of the planet's mostly smooth surface. Messenger results indicate it is a result of volcanic activity throwing material into the atmosphere and gradually filling up the craters made by a bombardment of meteorites and comets during the planet's formation.

Besides the smooth surface, the dominant structures on Mercury are called lobate scarps. These are cliffs pushed upward by the planet's contraction, according to Sean Solomon, the mission's principal investigator from the Carnegie Institution for Science in Washington.

Mercury is the only other terrestrial planet besides Earth in the solar system with a global magnetic field. On Earth, this field provides a bubble protecting us from dangerous solar particles.