BY DR EMILY BALDWIN
Posted: 5 May, 2009
A previously unknown 690 kilometre wide impact basin and evidence that Mercury’s atmosphere and the interaction of its magnetic field with the solar wind are more active than previously thought are the latest offerings from NASA’s MESSENGER mission.
The new results stem from the 6 October 2008 flyby and are presented in four papers featured in this week’s edition of the journal Science. “One of the biggest surprises was how strongly the dynamics of the planet’s magnetic field-solar wind interaction changed from what we saw during the first Mercury flyby in January 2008,” says Sean Solomon, the probe’s principal investigator from the Carnegie Institution of Washington. “The discovery of a large and unusually well preserved impact basin shows concentrated volcanic and deformational activity.”
The Rembrandt impact basin was discovered by MESSENGER during its second flyby of Mercury in October 2008. Image: NASA/Johns Hopkins University Applied Physics Laboratory/Smithsonian Institution/Carnegie Institution of Washington.
The 690 kilometre wide basin, known as Rembrandt, presents some of the freshest surfaces seen on the planet. “This basin formed about 3.9 billion years ago, near the end of the period of heavy bombardment of the inner Solar System,” says Thomas Watters from the Smithsonian Institution in Washington. “Although ancient, the Rembrandt basin is younger than most other known impact basins on Mercury.”
The crater’s age can be inferred by the number per area and size distribution of impact craters superposed on Rembrandt’s rim. Landforms such as those revealed on the floor of Rembrandt are usually buried completely by volcanic flows. Some volcanic material does fill the central portion of the basin and was subsequently deformed by tectonic forces into a complex pattern of ridges and troughs. The longest thrust fault scarp yet discovered on Mercury, over 1000 kilometres long and on the scale of the San Andreas Fault in California, cuts across the rim and floor of the basin (visible in the upper left portion of the crater in the image shown here).
From the surface to the exosphere, differences observed in Mercury’s magnetosphere support the idea that day to day changes occur due to a result of changes in the shielding provided by the magnetosphere. “During the first flyby, important discoveries were made, but scientists didn’t detect any dynamic features. The second flyby witnessed a totally different situation,” says James Slavin from NASA’s Goddard Space Flight Center. In the second flyby MESSENGER measured large magnetic fields passing through the dayside magnetopause and observed evidence for magnetic reconnection, a process that determines the degree of connectivity between the interplanetary magnetic field and a planet’s magnetic field. At the time MESSENGER made the observations, the rate of reconnection at Mercury was ten times the rate typical at Earth.
A global view of Mercury in enhanced colour (left) and at right, a preliminary interpretive map of Mercury's smooth plains (shades of yellow) and low-reflectance material (blues). The white area indicates areas not yet viewed by MESSENGER with resolution and Sun angles suitable for colour analysis. The Caloris impact basin can be seen as a distinctive circular feature near the top centre. Image: NASA/Johns Hopkins University Applied Physics Laboratory/Arizona State University/Carnegie Institution of Washington.
Just over a year ago only half of Mercury was known, now scientists have observed some 90 percent of the planet at high resolution, giving fresh insight into surface processes that have dominated the planet. Planetary geologists reveal that approximately 40 percent is covered by smooth plains that are probably volcanic in origin, and that much of Mercury’s crust may have formed through repeated volcanic eruptions in a manner more similar to the crust of Mars than to that of the Moon.
A third flyby of the planet is scheduled for 29 September this year. “The third Mercury flyby is our final dress rehearsal for the main performance of our mission, the insertion of the probe into orbit around Mercury in March 2011,” says Solomon. “The orbital phase will be like staging two flybys per day and will provide the continuous collection of information about the planet and its environment for one year. Mercury has been coy in revealing its secrets slowly so far, but in less than two years the innermost planet will become a close friend.”
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