When the Messenger spacecraft swooped low past the planet Mercury on October 6 2008, it gathered up a wealth of data that will have planetary scientists puzzling for years. As researchers sort through findings regarding Mercury’s volcanic past, meteor impacts, and the effect of the solar wind on the innermost planet’s magnetosphere, one broad conclusion stands out: Mercury isn’t just a boring chunk of rock. Marilyn Lindstrom, a NASA program scientist, said the Messenger findings show that Mercury is “just an amazingly dynamic planet, both in the past and in the present” [Baltimore Sun].
Superficially, Mercury looks a lot like the moon: small, grayish-brown and pockmarked with craters. Some scientists assumed that Mercury’s surface formed the same way the moon’s did, with lighter rocks rising to the surface of a magma ocean and congealing into a brittle crust early on. But the new observations reveal that 40 percent of the surface was formed by volcanoes. “Up until before Messenger’s arrival, we weren’t even sure that volcanism existed on Mercury” [Wired], says researcher Brett Denevi. The presence of titanium oxide also suggests that the planet was hot enough in its first 100 million years to be covered in magma oceans.
NASA’s Messenger also discovered the second largest impact crater on the planet. The 430-mile-wide Rembrandt impact basin is thought to have been formed 3.9 billion years ago at the end of what’s known as the Late Heavy Bombardment, a tumultuous time in our solar system’s history. The impact that formed the crater broke through the planet’s crust, allowing magma to seep out and partially fill the crater. But researchers cannot yet explain some of the features etched in that volcanic material: a spoke-like pattern of troughs and ridges emanating from the centre of the basin. Troughs and ridges, which are thought to form through very different processes, are not expected to be found lying side by side. So-called “wrinkle ridges” are caused when the crust compresses, while troughs are formed when it is stretched, causing the surface to separate [New Scientist].
These findings and others were revealed in four separate papers emanating from the second flyby, and were all published in Science. The Messenger probe also made a startling discovery about the solar wind’s interactions with Mercury. The first time it flew by the planet, the magnetic field carried by the solar wind (the constant stream of charged particles that spew from the sun) aligned with Mercury’s magnetic field. But the second flyby revealed a different configuration, in which the solar wind’s magnetic field opposed that of the planet. In that second configuration, the two fields connected in a magnetic handshake that dumped about 10 times more energy from the solar wind into Mercury’s magnetosphere than astronomers have observed in past interactions between the wind and Earth’s magnetosphere…. The magnetic handshake generated giant waves that set Mercury magnetosphere pulsating with magnetic twisters dancing on its surface [Science News].
Messenger will make one last flyby past Mercury on September 29, and will finally settle into orbit around the planet in March of 2011.
80beats: Mercury Close-Ups Reveal the Planet’s Ancient Volcanic Eruptions
Bad Astronomy: The colors of Mercury
80beats: Brand New Postcards From Mercury, Courtesy of Messenger Space Probe
80beats: Mercury Is Shrinking and Cooling, Space Probe Reveals
Image: NASA/Johns Hopkins University Applied Physics Laboratory/Smithsonian Institution/Carnegie Institution of Washington