Mercury is shrinking, NASA craft finds; a cool clue: 'lobate scarps'
Measurements by the Messenger spacecraft show the innermost planet has lost more than a mile in diameter over its history because of the cooling of its core.
By John Johnson Jr., Los Angeles Times Staff Writer
July 4, 2008
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.
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This image of the planet Mercury, made during the January 2008 flyby of the planet by NASA's Messenger spacecraft was released Thursday. The image shows that volcanoes were involved in plains formation and suggest that its magnetic field is actively produced in the planet's core.
Water traces on Mercury
Instruments aboard a Maryland-built spacecraft that soared past the planet Mercury in January have provided a real surprise: traces of water molecules in the hot little world's extremely thin atmosphere, scientists reported yesterday.
It's not clear where they came from yet, but astronomers suspect that the water molecules are being blasted from the planet's surface by the solar wind, along with ions of sodium, calcium and magnesium - all clues to the chemical composition of surface material.
"This water is clearly there," said Thomas H. Zurbuchen, a member of the Messenger science team from the University of Michigan, Ann Arbor.
The discovery is among the first formal findings from Messenger's initial flyby of Mercury, on Jan. 14. They're contained in 11 papers published today in the journal Science.
Although surface temperatures on the planet closest to the sun can soar to 750 degrees Fahrenheit, Zurbuchen said, some observations have suggested that water ice may persist in the cold, shaded recesses of polar craters. If the observations are right, the data are the first to confirm the presence of water on Mercury.
"It's very interesting," Zurbuchen said.
Messenger scientists said their data are also revealing a planet with a solid iron core, and an enormous outer core of molten iron. Motion in that outer core is generating a planet-wide magnetic field much like the one that protects life on Earth from deadly radiation from the sun and outer space.
But Messenger's data suggest the little planet is being raked by solar particles anyway, blowing surface material into a cometlike tail that extends far into space, away from the sun.
"Even though it appears the solar wind was relatively quiescent [during the flyby] we still see a strong signature in the tail," said William McClintock, a team member from the University of Colorado. "It's very exciting. I can't wait until orbital observations begin and we're able to monitor the tail's response ... on a daily basis."
As Mercury's core cools, and the molten iron condenses, the planet has been shrinking like a dried apple. And that has caused the titanic cliffs and faults Messenger photographed on its rocky surface. The shrinkage may be continuing today, scientists said.
Messenger has also found evidence that at least some of the bright, smooth plains visible on the planet's surface are volcanic lava flows rather than material ejected from meteor impact craters as some had theorized.
One ancient volcanic vent inside the vast Caloris Basin of Mercury is twice the size of Washington state, they said. It has piled lava as deep as the Washington Monument in a flow that would extend as far as Baltimore.
The volcanic activity is "testimony to a long history of volcanic flooding of the interior of the Caloris Basin," said James W. Head III, a Brown University geologist on the team. He said the discovery "adds new life to what many had thought might be a dead planet."
The $446 million Messenger mission was designed and built at the Johns Hopkins University's Applied Physics Laboratory near Laurel and is being operated from a control center on the APL campus.
Launched by NASA in 2004, the spacecraft is the first to visit Mercury since Mariner 10 made a series of flybys in 1974 and 1975.
Messenger will zip past the planet again Oct. 6, and a third time in September next year. If all goes well, it will slip into orbit in March 2011 for at least a year of close-up observations.
Discoveries from Mercury are contributing to scientists' understanding of the early history of all the "terrestrial" planets, including Venus, Earth and Mars.
The disclosure of water molecules in Mercury's tenuous "exosphere" was made by the spacecraft's Fast Imaging Plasma Spectrometer, part of a seven-pound instrument developed by the APL and the University of Michigan.
It's a kind of nose, designed to identify low-energy ions encountered as they're lifted off the planet's surface and swept up in the solar wind.
"The magnetic protection layer of Mercury is not as impenetrable as the one on Earth," Zurbuchen said. "The solar wind is filling the entire volume around Mercury ... and even touches the planet at multiple locations," tossing surface material into space where Messenger scooped them up.
"Every single element seen here is really unprecedented," he said. But the "real surprise" was the water ions. Much of the planet may be sizzling so close to the sun, but in shaded craters, surface temperatures can dip to minus-256 degrees Fahrenheit, and water might be preserved.
Messenger's finding fits with observations from Earth that have found highly reflective regions near the planet's poles that theorists suspected might be water ice. Zurbuchen said small comets or meteors may have carried water to Mercury during the early history of the solar system.
There has been similar evidence of water ice in deep, shaded craters at the poles of the Earth's moon, too. Such water resources could help sustain manned lunar bases, supplying drinking water, breathing oxygen and hydrogen fuel for rocket engines.
