Sean Solomon – Only six months into its Mercury orbit, the tiny MESSENGER spacecraft has shown scientists that Mercury doesn’t conform to theory. Its surface material composition differs in important ways from both those of the other terrestrial planets and expectations prior to the MESSENGER mission, calling into question current theories for Mercury’s formation. Its magnetic field is unlike any other in the Solar System, and there are huge expanses of volcanic plains surrounding the north polar region of the planet and cover more than 6% of Mercury’s surface. These findings and other surprises are revealed in seven papers in a special section of the September 30, 2011, issue of Science.
Two of the seven papers indicate that the surface material is more like that expected if Mercury formed from similar, but less oxidized, building blocks than those that formed its terrestrial cousins, perhaps reflecting a variable proportion of ice in the initial accretionary stages of the planets. Measurements of Mercury’s surface by MESSENGER’s X-Ray and Gamma-Ray Spectrometers also reveal substantially higher abundances of sulfur and potassium than previously predicted. Both elements vaporize at relatively low temperatures, and their abundances thus rule out several popular scenarios in which Mercury experienced extreme high-temperature events early in its history.
“Theorists need to go back to the drawing board on Mercury’s formation,” remarked the lead author of one of the papers, Carnegie’s Larry Nittler. “Most previous ideas about Mercury’s chemistry are inconsistent with what we have actually measured on the planet’s surface.” …
some impact craters are very bright and have a blue color relative to other areas of Mercury. These deposits were considered to be unusual because no craters with similar characteristics are found on the Moon. But without higher-resolution images, the bright crater deposits remained a curiosity.
Now MESSENGER’s orbital mission has provided close-up, targeted views of many of these craters. The bright areas are composed of small, shallow, irregularly shaped depressions that are often found in clusters said David T. Blewett, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory (APL) and lead author of one of the Science reports. “The science team adopted the term ‘hollows’ for these features to distinguish them from other types of pits that are found on Mercury.”
Hollows have been found over a wide range of latitudes and longitudes, suggesting that they are fairly common across Mercury. Many of the depressions have bright interiors and halos, and Blewett says the ones detected so far have a fresh appearance and have not accumulated small impact craters, indicating that they are relatively young. …
Earth, Mercury, Jupiter, Saturn, Uranus, and Neptune all have intrinsic magnetic fields, but MESSENGER found that Mercury’s weak field is different. So too are particle acceleration processes in Mercury’s magnetosphere, as described in a paper by lead author George Ho of APL. MESSENGER’s observations of energetic electrons indicated that their distribution is not consistent with what are known as Van Allen radiation belts. These belts are bands of charged particles that interact with the magnetic field and surround the planets.
Mercury’s magnetic equator is also well to the north of the planet’s geographic equator. The best-fitting internal dipole magnetic field is located about 480 km (298 miles), northward of the planet’s center.
The team found that sodium is the most important plasma ion contributed by the planet to the magnetosphere. “We had previously observed neutral sodium from ground observations, but up close we’ve discovered that charged sodium particles are concentrated near Mercury’s polar regions where they are likely liberated by solar wind ion sputtering, effectively knocking sodium atoms off Mercury’s surface” notes the University of Michigan’s Thomas Zurbuchen, author of one of the Science reports. “We were able to observe the formation process of these ions, one that is comparable to the manner by which auroras are generated in the Earth atmosphere near polar regions.”
MESSENGER’s Fast Imaging Plasma Spectrometer detected helium ions throughout the entire volume of Mercury’s magnetosphere. “Helium must be generated through surface interactions with the solar wind,” says Zurbuchen. “We surmise that the helium was delivered from the Sun by the solar wind, implanted on the surface of Mercury, and then fanned out in all directions.”
“Our results tell us is that Mercury’s weak magnetosphere provides very little protection of the planet from the solar wind,” he continued. “Extreme space weather must be a continuing activity at the surface of the planet closest to the Sun.” …