Photonbelt

A second fly-by of Mercury in October 2008 by the U.S. space probe MESSENGER has revealed the Solar System’s smallest planet to be far more active than previously thought, four studies have found.

Cameras aboard the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) probe took more than 1,200 images of the surface, including details of a mammoth well-preserved 692-km impact basin that shows signs of a volcanic past.

The so-called Rembrandt basin is the first such geological feature observed on Mercury where the ground is well exposed and not covered by a thick layer of volcanic ash, like most of the planet’s other features.

“This basin formed about 3.9 billion years ago, near the end of the period of heavy bombardment of the inner Solar System,” said Thomas Watters from the Smithsonian Institution in Washington DC, a lead author of one of the studies, all of which are published in today’s edition of the U.S. journal, Science.

“This second Mercury flyby provided a number of new findings,” said Sean Solomon, the probe’s principal investigator from the Washington-based Carnegie Institution. “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,” he added. “The discovery of a large and unusually well preserved impact basin shows concentrated volcanic and deformational activity.”

“This second Mercury flyby provided a number of new findings,” said Sean Solomon, the probe’s principal investigator from the Carnegie Institution of Washington. “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. The discovery of a large and unusually well preserved impact basin shows concentrated volcanic and deformational activity.”

The spacecraft also made the first detection of magnesium in Mercury’s thin atmosphere, known as an exosphere. While finding it was not surprising, the amounts and distribution was unexpected. The instrument also measured other exospheric constituents, including calcium and sodium.

“This is an example of the kind of individual discoveries that the science team will piece together to give us a new picture of how the planet formed and evolved,” said William McClintock, co-investigator and lead author of one of the four papers. McClintock, who is from the Laboratory for Atmospheric and Space Physics at the University of Colorado at Boulder, suspects that additional metallic elements from the surface, including aluminum, iron and silicon, also contribute to the exosphere.

The variability that the spacecraft observed in Mercury’s magnetosphere, the volume of space dominated by the planet’s magnetic field, so far supports the hypothesis that the great day-to-day changes in Mercury’s atmosphere may be a result of changes in the shielding provided by the magnetosphere.

“The spacecraft observed a radically different magnetosphere at Mercury during its second flyby compared with its earlier January 14 [2009] encounter,” said James Slavin from NASA’s Goddard Space Flight Centre in Greenbelt, Maryland. “During the first flyby, important discoveries were made, but scientists didn’t detect any dynamic features. The second flyby witnessed a totally different situation.”

Using revolutionary image-capturing technology and a laser altimeter to survey the ground, MESSENGER revealed like never before 30 per cent of the mysterious planet.

In a grand feat of engineering, the probe soared past the innermost planet’s equator at an altitude of 201 km at a speed of 23,818 km/h.

Combined with data from the first flyby and from Mariner 10, which made three passes in 1974 and 1975, the latest coverage means scientists have now seen about 95 per cent of the planet. The probe is on course to make its third flyby on 29 September 2009.

Mercury is the closest of all the planets to the Sun, and because of the high-risks of its proximity – the Sun’s enormous gravitational pull, and massively high levels of radiation – it is one of the most mysterious bodies in the Solar System, even though it is relatively close to Earth.

The January 2008 flyby showed scientists that volcanic eruptions produced many of Mercury’s expansive plains, littered with meteor craters, and that its magnetic field appears to be actively generated in a molten iron core.

Via Cosmos Magazine