CRATER CLOSE-UP: Rachmaninoff Basin, photographed during MESSENGER's third and final flyby of Mercury en route to a planned orbital insertion in 2011, appears to have been shaped by volcanism in the relatively recent past. Image: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington
NEW YORK—The prevailing view among astronomers used to be that Mercury, the smallest and innermost planet in the solar system, was a static and unevolving world, baked to infernal temperatures by its proximity to the sun. Mariner 10, the first spacecraft to fly by Mercury, offered some tantalizing evidence of volcanic activity in 1974 and 1975. Now MESSENGER, a NASA spacecraft that has passed by Mercury in the past year and will go into orbit in 2011, has confirmed Mariner 10's observations by imaging young lava plains indicative of recent volcanic activity on the planet. MESSENGER has also detected rapid variations in Mercury's magnetic shield and found a surprising distribution of elemental atoms and ions in its exosphere, the extremely tenuous atmosphere of gases that surrounds the planet.
"Mercury is definitely not what we thought it was even two and a half years ago," said Sean Solomon of the Carnegie Institution for Science's Department of Terrestrial Magnetism in Washington, D.C., the principal scientist of the MESSENGER mission, who was in town to present the spacecraft's latest results July 26 at the 73rd annual meeting of The Meteoritical Society.
MESSENGER (for MErcury Surface, Space ENvironment, GEochemistry and Ranging) launched in 2004 on a mission to fly by, then orbit Mercury and study its characteristics and environment in detail. Getting MESSENGER into orbit is a tricky endeavor, however, since the spacecraft has to slow down enough to be captured by the planet's weak gravity.
In addition, it has to be able to survive the tremendously high temperatures near the sun. "It is difficult to send a spacecraft to Mercury," Solomon explained. "It is even more difficult to get it into orbit around Mercury and have it survive. You have to protect against the heat."
MESSENGER is covered by a thick ceramic cloth that acts as a protective sunshade. On the sun-facing side, temperatures are typically 350 degrees Celsius (about the temperature inside a pizza oven). Inside the spacecraft, however, the sunshade maintains room temperature, protecting the sensitive electronics. MESSENGER's solar panels are two-thirds mirrors to reflect heat away for the same reason.
After seven years spent following a very complicated route meant to slow it down, MESSENGER is finally supposed to enter orbit around Mercury next March. In the meantime, it has completed three flybys, the most recent of which was on September 29, 2009. It was data collected during this third and last flyby that has now been analyzed, and has provided these surprising discoveries.
MESSENGER photographed a 290-kilometer-diameter double-ring impact crater, now named the Rachmaninoff Basin, that seems to have flat and very smooth plains that look like hardened lava flows. The small number of craters in Rachmaninoff imply that the plains have not existed for very long, Solomon said, guessing that they were anywhere between one billion and two billion years old. (Although that is still relatively old by terrestrial standards, it tells planetary scientists that volcanic activity continued well after Mercury formed about 4.5 billion years ago.) Researchers also found other examples of volcanic activity in the spacecraft's images. They showed two craters side by side, with one filled in, which must be from volcanic activity, according to Solomon. "Mercury has gone from being a planet where volcanism was questioned to one whose surface is dominantly shaped by volcanism," he said.
In its latest flyby, MESSENGER also found that Mercury's magnetosphere—the magnetic sheath that covers the planet—is susceptible to magnetic storms that cause large variations in a matter of minutes. These "substorms," which are produced by changes in the solar wind, are also seen on Earth, but the variations are much more gradual than on Mercury. "We saw periods lasting a few minutes when the front side of the magnetosphere was stripped bare," said Solomon.
Observations of both the volcanism and the magnetosphere were written up as separate scientific papers that were recently published online by Science. A third paper, also published at the same time by Science, focused on observations of Mercury's thin exosphere, composed of atoms ejected from the surface of the planet by the sun's rays as well as ions captured from the solar wind and dust grains.
MESSENGER also revealed during its last flyby a highly variable exosphere, owing to Mercury's eccentric orbit and the effects of a constantly changing space environment. The flyby produced the first detailed maps of elements and ions in Mercury's exosphere, and showed that elements such as calcium, magnesium and sodium are each distributed at different altitudes. One puzzling observation is that concentrations of calcium were higher near sunrise than around sunset. (Theorists had assumed that hours of exposure to sunlight would lead to more calcium from Mercury’s surface finding its way into the exosphere.)
Taken together, the three papers offer a picture of a dynamic Mercury, one that is at odds with the theoretical picture of the planet.
"What we have here is a common situation in science, with eloquent theories but limited information," Solomon said—people thought that they understood Mercury, more or less. "What Mercury is telling us is that it, too, has lessons in how planets are assembled," he added.
As Jack Lissauer, a planetary theorist at NASA Ames Research Center at Moffett Field, Calif., summed up the new observations, "Mercury is not the dead world that people believed it to be before we started getting evidence from spacecraft."
To date, that evidence has come only from fleeting flybys, but that will soon change if MESSENGER becomes the first spacecraft to take up long-term residence at Mercury next year as planned. "We will surely learn a lot more when it goes into orbit next year," Lissauer said.