Scientists 'astonished' by signs of water on Mercury

WASHNGTON -- A flyby by a NASA probe has settled a decades-old debate among scientists about Mercury's surface and led to a startling discovery about the planet.The trip has provided scientists with evidence that lava flows and other volcanic activity helped shape the planet's surface and revealed the possible existence of water.In January, NASA's Messenger spacecraft performed a close pass over Mercury, coming within about 124 miles of the surface and giving researchers never-before-seen views of the closest planet to our sun.Science magazine details the probe's findings in their July 4 issue.Since the NASA Mariner 10 spacecraft's 1975 visit to Mercury, volcanism on Mercury has been a point of contention among scientists, many of whom concluded that the planet's series of plains were created solely by meteor and comet impacts.More than 30 years after Mariner, more detailed pictures and a favorable angle of the sun revealed distinctly different geological features resting next to one another and evidence of small craters erased by lava flows.The probe mapped 20 percent of the planet unseen by Mariner.Messenger -- an acronym for Mercury Surface, Space Environment, Geochemistry and Ranging -- helped scientists conclude that the planet's core makes up about 60 percent of its total mass, much more than Venus and the Earth. Scientists can now say with certainty that Mercury's dense molten core fuels its magnetic field, just like on Earth.The core has been cooling considerably since the end of a period of heavy meteor bombardment more than 3.8 billion years ago, contributing to steady contraction of the planet."The total amount of contraction has been greater than we appreciated from Mariner 10," said Sean Solomon, principal investigator at the Carnegie Institution of Washington, who contributed to the Science magazine report.Mercury has most likely shrunk about 2.5 miles in diameter since its formation or about one percent of its total size.Studying the planet's exosphere for expected ions like sodium and calcium produced another startling discovery -- large amounts of water."Nobody expected that. I don't know a single person that did. We were astonished, just astonished," said Thomas Zurbuchen, associate professor in the Department of Atmospheric, Oceanic and Space Sciences at the University of Michigan.The planet's exosphere is its outermost particle layer, as its proximity to the sun destroys almost any atmosphere because of constant bombardment by solar wind.The solar wind assault knocks particles into the exosphere, giving scientists an indication of the surface's chemical composition. If there are water molecules in the exosphere, it's possible there is water on the surface."This is very interesting because the temperature on the surface of Mercury can range to over 400 degrees Celsius. Water can't really sit there," Zurbuchen said.But it may, leaving scientists to question how it can exist in the extreme heat. One possibility is the existence of reservoirs of ice in small permanently shadowed areas at Mercury's poles. Water could have also come from comets.Or, Zurburchen said, the water could have formed only in the planet's exosphere in a process called "chemical sputtering." Hydrogen and oxygen atoms knocked into the exosphere by solar wind could have reacted to form water molecules.Scientists hope to collect more useful data when Messenger maps another 30 percent of the planet in another flyby on October 6.(Distributed by Scripps Howard News Service, http://www.scrippsnews.com)