Messenger brings news

All eight of the major planets have had spacecraft at least swing by them now and again. Perhaps unsurprisingly, much of the attention has focused on our nearest neighbors in the Solar system, the "terrestrial" (rocky) planets Mercury, Venus, Earth and Mars. Earth has been the focus of most of the attention, since we have a vested interest here. Mars has been a popular tourist destination for satellites and landers, like the current Phoenix mission. Venus, too, has over twenty spacecraft visits, including a few landers, though the toxic surface conditions tended to make those short-lived.

Yet poor Mercury had had only one spacecraft visit - Mariner 10, in the mid-1970s. While this satellite mapped most of the surface, and sent back a great deal of interesting data, there have remained a number of open questions about this small neglected planet. Enter NASA's Messenger spacecraft, which was fired off in 2004 and did its first swing by Mercury in January of this year.

Follow up:

Now a passel of scientific papers based on Messenger data have appeared in the July 4 issue of Science, and it's confirming some notions about little Mercury - and startling us on a few other points.

For example, take the long cliffs on Mercury that wind for hundreds of kilometers across the surface. These escarpments, or "scarps", can be from a couple hundred meters to a couple of kilometers in height. We knew that these scarps were there, from the Mariner 10 maps. We even have a good hypothesis for what happened: as Mercury is a fairly small planet, it would have cooled down relatively quickly, geologically speaking, after its formation. As it cooled, it shrank; as it shrank, the outer crust wrinkled up, a bit like a rasin's skin as the fruit dries. Now Messenger finds that these scarps cover much more of the surface than we realized from the Mariner 10 maps, which means Mercury's shrinkage was significantly greater than we'd expected. (It still comes out to only a tenth of a percent of the planet's radius, but that's enough for a lot of wrinkles to form!)

Yet although Mercury's size and these scarps indicate its interior has cooled significantly, Mariner 10 found signs that the planet had a weak magnetic field. This was a surprise, because as we understand it, a planet's magnetic field is formed by the circulation of some conducting material. For instance, Earth's iron core has a liquid outer layer. As Earth spins, currents are built up in that core, and the motion of charges through the conducting iron generates Earth's magnetic field. Now, Mercury has a whopper of an iron core - it takes up a large percentage of the planet's interior. However, if Mercury had cooled completely, the iron core should be completely solid. A fully solid core means no internal motion, which means no circulating conducting material, which means no magnetic field. Mariner 10, however, measured such a magnetic field, and Messenger confirmed that the field is similar in structure to Earth's, although much weaker. Apparently something active must still be going on in Mercury's core. It may even be the continuing cooling of the core that's driving a lot of the motion that generates the magnetic field. Nifty!

More news came as Messenger studied the extremely thin excuse for an atmosphere that surrounds Mercury. It's really just a few stray wisps of gas that come and go. Gas is released from the Mercurian surface when particles from the sun (the "solar wind" or just solar radiation) knock surface atoms up into orbit around Mercury, or when a meteorite hits Mercury's surface and throws out more gas. Those "atmospheric" atoms, eventually, will either settle back to Mercury's surface or be knocked free entirely by solar winds and radiation. Messenger was able to study what materials were up there, including some trapped in Mercury's magnetic field, and therefore get a better idea about what materials are present on Mercury's surface.

This is just the first Messenger swing-by; there will be two more in the next fourteen months, and in 2011 Messenger will settle into orbit around Mercury to study it more closely. Stay tuned!

More coverage of this topic from NASA, Sky&Tel, Space.com and Universe Today.