Mercury is Not Like Other Planets
Mercury not like other planets MESSENGER finds
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.”
For decades scientists had puzzled over whether Mercury had volcanic deposits on its surface. MESSENGER’s three flybys answered that question in the affirmative, but the global distribution of volcanic materials was not well constrained. New data from orbit show a huge expanse of volcanic plains surrounding the north polar region of Mercury. These continuous smooth plains cover more than 6% of the total surface of Mercury.
Scientists have also discovered vents, measuring up to 25 kilometers (km) (15.5 miles) in length, that appear to be the source of some of the tremendous volumes of very hot lava that have rushed out over the surface of Mercury and eroded the substrate, carving valleys and creating teardrop-shaped ridges in the underlying terrain.
MESSENGER revealed an unexpected class of landform on Mercury and suggests that a previously unrecognized geological process is responsible for its formation. Images collected during the Mariner 10 and MESSENGER flybys of Mercury showed that the floors and central mountain peaks of 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.
“Analysis of the images and estimates of the rate at which the hollows may be growing lead to the conclusion that they are actively forming today,” Blewett says. “The old conventional wisdom was that ‘Mercury is just like the Moon.’ But from its vantage point in orbit, MESSENGER is showing us that Mercury is radically different from the Moon in just about every way we can measure.”
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.”
“In the history of exploration of our planetary system, the first spacecraft to orbit a planet has always yielded stunning surprises, and MESSENGER has been true to that pattern,” notes Carnegie’s Sean Solomon, MESSENGER Principal Investigator. “Our first good views of the polar regions, our first high-resolution images, our first continuous observations of the exosphere and magnetosphere, and our first opportunity to collect time-consuming measurements of surface composition have all returned unexpected results. Mercury is not the planet described in the textbooks. Although a true sibling of Venus, Mars, and Earth, the innermost planet has had a much more exciting life than anyone predicted.”