Mercury is shrinking, NASA craft finds; a cool clue: ‘lobate scarps’

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.

New 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, which stands for MErcury Surface, Space ENvironment, GEochemistry and Ranging, 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 before cruising off on a highly elliptical orbit. It will swing back for a second encounter in October before settling into a final close 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 across the planet’s surface.

“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.

“They tell us how important the cooling core has been to the evolution of the surface,” Solomon said.

Besides Earth, Mercury is the only other terrestrial planet in the solar system with a global magnetic field. On Earth, this field provides a bubble protecting us from dangerous solar particles. Earth’s magnetic field is believed to be produced by the flow of liquid iron in its core.

The cause of Mercury’s magnetic field had been a mystery, since scientists believed the planet’s iron core had long ago cooled and solidified. In fact, the new measurements from Messenger seem to indicate the core is still active, scientists said.

By measuring the contents of the magnetic field around the planet, Messenger also discovered a variety of elements that must have come from the planet’s surface, including an abundance of silicon, sodium and sulfur.

“This observation means that this flyby got the first-ever look at surface composition,” according to Thomas Zurbuchen, associate professor of space science at the University of Michigan.

 john.johnson@latimes.com