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 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 fly-by in January, whisking to within 125 miles of the surface of the planet 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 data from the January fly-by 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 of up to 1,100 degrees Fahrenheit. But Messenger has uncovered a more surprising place, with peaks as high as 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's data indicate it is the result of volcanic activity that threw material into the atmosphere and gradually filled up the craters caused by the bombardment of meteorites and comets during Mercury's formation.
Besides the smooth surface, the dominant features on Mercury are lobate scarps. These are cliffs pushed upward by the planet's contraction, said mission investigator Sean Solomon of 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 forms a bubble that protects us from dangerous solar particles. Earth's magnetic field is believed to be produced by the flow of liquid iron in its core.
The source of Mercury's magnetic field had been a mystery, because scientists believed the planet's iron core had long ago cooled and solidified. The new measurements from Messenger seem to indicate the core is still active, scientists said.
By measuring the contents of the magnetic field around Mercury, 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 fly-by got the first-ever look at surface composition," said Thomas Zurbuchen, associate professor of space science at the University of Michigan.