The Messenger Mission was launched on 3 August 2004 and moved into orbit around Mercury in March 2011, after a rather circuitous trip, involving a flyby of Earth, two of Venus and three of Mercury itself. It has been transmitting a steady stream of information back to Earth for six months now. This week NASA published the first findings derived from this information in a series of papers in the journal Science.
An animation showing the route Messenger took to Mercury.
The first two papers concerned the composition of rocks on the surface of Mercury, and the inferences that could be made about the origins of the planet from this. The first of these papers, by a team led by Larry Nittler of the Department of Terrestrial Magnetism at the Carnegie Institution in Washington DC focuses on the abundance of common elements in the crust. This reveals that the surface of Mercury has an extremely rich in magnesium and poor in aluminium and calcium compared to other terrestrial planets, and that it is also extremely rich in sulphur. The second paper, by a team led by Patrick Peplowski of the Applied Physics Laboratory at Johns Hopkins University concentrated on radioactive elements, finding the surface of Mercury to have an unusual abundance of radioactive potassium, relative to the radioactive elements thorium and uranium. These findings both suggest that the surface of Mercury has a highly evolved surface; that is to say that it is likely to have been formed from a medium with a more usual balance of elements, which was then separated out by some process, similar to the separation of elements that occurs in lavas cooling slowly within a volcano.
The third paper, by a team led by James Head of the Department of Geological Sciences at Brown University, studied the physical geology of Mercury's northern hemisphere. This revealed that a vast area of the planet, in excess of 6% of the total surface, was covered by vast areas of flood basalt. Flood basalts are areas where vast amounts of volcanic rock have erupted onto the surface of a planet at one time. On Earth these have been associated with mass extinction events; the Deccan Traps in India are associated with the end of the Cretaceous, and the larger Siberian Traps in Russia (less reliably) with the end of the Permian. However the extent of flood basalts far outstrips anything seen on Earth, and implies a catastrophic event on a scale far beyond anything Earth has ever seen (though rather less likely to have caused a mass extinction, given Mercury's lack of an atmosphere, and therefore life).
The Deccan Traps; layer after layer of volcanic rock covering a vast area of eastern India.
The fourth paper is by a team led by David Blewit, also of the Applied Physics Laboratory at Johns Hopkins University. This deals with a feature seen on the surface of Mercury that has never been seen on any other planet or moon. Many craters on Mercury are dotted with mysterious, highly reflective blue cavities, or hollows, ranging in size from a few tens of meters to several kilometers across. Blewit et al. theorize that these have formed where minerals have sublimated (passed from a solid to a gaseous phase, without ever being a liquid) in the intense heat at Mercury's surface.
Mysterious blue hollows on the surface of Mercury.
The fourth paper is by a team led by Brian Anderson, again of the Applied Physics Laboratory at Johns Hopkins University, and concerns Mercury's magnetic field. This is very week compared to that of Earth - about 1.1% the strength - but far stronger than those of Mars or Venus. The magnetic poles of Mercury are very close to the geographical poles (the points on the end of a hypothetical line about which the planet spins), only about 3% off. This suggests that the field is being generated by the spinning of a liquid iron core within the planet.
In the absence of an atmosphere this week magnetic field is the only protection that the planet has from the solar winds (a stream of charged particles constantly emitted by the sun) - and it isn't quite enough, as these winds actually touch the planet at the poles. This leads to the ionization of atoms at the surface, generating a planetary field of charged ions which is the subject of the fifth paper in the journal. In this paper a team led by Thomas Zurbuchen of the Department of Atmospheric, Oceanic and Space Sciences at the University of Michigan analyse this field, finding it to be rich in positively charged sodium and oxygen ions (i.e. sodium and oxygen atoms that have been stripped of one or more electrons), which on the side of the planet away from the sun rival the density of free protons (ionized hydrogen particles). There is also a considerable density of ionized helium atoms, though these are more evenly distributed.
The sixth paper also concerns Mercury's magnetic field. In this study a team led by George Ho of the Applied Physics Laboratory at Johns Hopkins University study the (intense) electron field about Mercury and come to the conclusion that the planet's magnetic field is to week to generate Van Allen radiation belts; torus shaped belts of charged particles that surround the Earth and other planets with strong magnetic fields, leading to Mercury having fields with less well defined shapes.