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Mercury's secrets revealed during fleeting flybys


In 2011 NASA’s MESSENGER spacecraft will insert itself into Mercury’s orbit, ending a nearly seven-year journey spanning billions of miles. The real work begins, though, once MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) settles in as Mercury’s lone satellite and provides the first prolonged look at the dense little planet. Until that time, planetary scientists have to settle for tantalizing but fleeting glimpses of Mercury from MESSENGER’s three flybys, the second of which took place on October 6.

Some of the results of that up-close peek were announced at a NASA news conference today, providing a taste of what revelations will come once MESSENGER takes up residence in Mercury’s orbit. (The flybys are gravity-assist procedures to bring MESSENGER’s trajectory into line for the future orbit insertion; any scientific data collected during them are icing on the cake.) Relatively little is known about the planet, because its proximity to the sun largely precludes ground-based study; before MESSENGER’s first pass, less than half the planet had even been imaged.

Among the findings: hints about the nature of Mercury’s magnetic field. MESSENGER’s flyby permitted the first measurements of the field on the planet’s western hemisphere. As it turned out, they were “almost identical to the other side” captured during the first flyby, said Brian Anderson of the Johns Hopkins University Applied Physics Laboratory (JHUAPL) in Laurel, Md. That symmetry means that Mercury’s magnetic field is likely aligned quite closely to its axis of rotation, unlike that of the earth, where the two are separated by roughly 11 degrees of tilt.

JHUAPL's Ronald Vervack noted that MESSENGER's atmospheric spectrometer confirmed the presence of magnesium in Mercury’s tenuous atmosphere, or exosphere. (Its existence had been predicted but not demonstrated.) The presence of magnesium in the exosphere indicates that the element is “definitely part of the Mercury surface," he said. Adding magnesium to the list of known exospheric constituents—joining hydrogen, helium, oxygen, sodium, potassium and calcium—helps refine researchers’ understanding of the nature of the planet and its thin atmosphere.

Another view of the surface comes from MESSENGER’s laser altimeter, which provides investigators with a three-dimensional view of Mercury’s crater-scarred exterior. Maria Zuber, a geophysicist at the Massachusetts Institute of Technology (MIT), described several findings from the altimetry data, including a so-called wrinkle ridge (a feature thought to form during planetary cooling and contraction) twice as large as similar ridges discovered on Mars.

MESSENGER also uncovered evidence of substantial volcanic activity; two neighboring craters of similar size were found—one largely empty and the other filled with solidified lava. By comparing them, the MESSENGER team estimated that the filled crater was 3,600 cubic miles (15,000 cubic kilometers) in size and, so, contained enough lava, according to Zuber, to blanket the entire Washington, D.C.–Baltimore metropolitan area in a layer twelve times the height of the Washington Monument. She called the volcanic activity necessary for such lava output “an awful lot for such a little planet,” adding that “we now have a better understanding that volcanism is quite an important process” on Mercury. (In previous images, such as those from Mariner 10 in the 1970s, differentiating between volcanic and cratering effects was more difficult, Zuber said.)

With so many pieces of the Mercury puzzle starting to fit together, investigators will continue to evaluate the new data from MESSENGER while eagerly awaiting further insights from the spacecraft’s next pass. As NASA’s Marilyn Lindstrom put it in looking forward to the next flyby in September, “It’s sad to leave, but we’re going to be back.” In 2011, of course, MESSENGER will finally settle down, never to leave again.



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