Thursday, September 29, 2011

Mercury Not Like Other Planets MESSENGER Finds

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.

Moon–Mercury image comparison. (Left) The near side of the Moon, showing the dark volcanic areas (maria) composed of lava flows and the bright, heavily cratered highland crust. (Right) Mariner 10 view of Mercury showing that, unlike the Moon, there is no brightness contrast between the cratered terrain and the smooth plains. The Moon is about one-quarter of the diameter of Earth; Mercury is about one-third of the diameter of Earth.
image 1.1
Credit: Lick Observatory (left); NASA/Jet Propulsion Laboratory/U.S. Geological Survey (right)

Surface Surprises

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.

The distribution of Mercury’s northern high-latitude smooth plains superposed on the United States and North America to compare scales. The area of these plains on Mercury equals almost 60% that of the continental United States.
image 1.4
Credit: Brown University/Jennifer Whitten/James Dickson
"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."

Volcanism

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.

Estimating lava flow deposit thicknesses. (Left) A fresh impact crater on Mercury (Hokusai, 114 km diameter, 57.8°N, 343.1°E) provides the opportunity to measure the depth of its interior and the height of its rim above the surrounding terrain. (Right) A "ghost" crater (90 km diameter, 74.3°N, 335.7°E), a circular feature in the volcanic plains outlined by ridges, suggests that the outpouring of the volcanic plains has buried a preexisting impact crater. From an estimate of the height of the rim of the buried crater, the thickness of the lava covering the crater may be determined. Both images were obtained with the Mercury Dual Imaging System (MDIS) during the orbital phase of the MESSENGER mission.
image 1.5
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Another lead author, James Head of Brown University, said that the deposits appear typical of flood lavas, like those found in the few-million-year-old Columbia River Basalt Group on Earth. "Those on Mercury appear to have poured out from long, linear vents and covered the surrounding areas, flooding them to great depths and burying their source vents,"

Features interpreted to be lava flow fronts associated with the northern smooth plains deposits on Mercury. (Upper left) Steep flow margin within a flooded impact crater (thick arrows); narrow arrow points to possible flooding of a smaller crater. (Lower left) Smooth plains (left) embaying rough plains (right), with flow front indicated by arrows. (Right) Candidate lava flow fronts (arrows) facing each other. The western flow unit is embaying a crater (E) and flooding the hills (H) between flow fronts to form kipukas (K), islands of pre-existing terrain flooded by lava.
image 1.6
Credit: Courtesy of Science/AAAS

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.

New landforms

MESSENGER revealed an unexpected class of landform on Mercury and suggest 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."

Sander crater, viewed from orbit at about 10 times better pixel resolution than during the flyby (Image 2.1). The high-resolution, targeted image reveals that the bright portions of the floor consist of large numbers of shallow irregular depressions ("hollows"), giving an etched appearance to the surface.
Credit: Courtesy of Science/AAAS
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.

Portion of the interior of the Raditladi basin. Inset shows the location of the main image. The resolution of this image is about 30 times better than that of the flyby image in Image 2.2. Yellow arrows indicate hollows on the peak-ring mountains; white arrows point to hollows on the basin floor.
Credit: Courtesy of Science/AAAS
"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."

Magnetic Field

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."

Visualization of measurements of sodium-group ion flux (blue) and magnetic field (red) during MESSENGER’s orbit on 14 April 2011. Sodium-group ions are measured throughout the entire magnetosphere, but they strongly peak near the magnetic cusps, where the magnetic protection of the planet is least effective. Measurements from more than 120 orbits were used to assemble a global picture of Mercury's three-dimensional exosphere.
Credit: University of Michigan; inset courtesy of Science/AAAS
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."

Theories of Mercury’s formation have been developed to explain its unusually large metal-to-silicate ratio compared with Venus, Earth, and Mars. These theories generally fall into one of two categories: physical removal of silicates or differences in the composition material from which Mercury formed compared with other solar system bodies. Two of the physical models invoke one or more giant impacts (left) or the vaporization of surface by a hot solar nebula to remove the planet’s original crust and outer mantle. Chemical models describe the material from which Mercury formed, for example, refractory condensates or primitive precursory material (right). The abundances of potassium, thorium, and uranium on the surface of Mercury measured by the MESSENGER Gamma-Ray Spectrometer rule out the giant impact, vaporization, and refractory condensation models. Formation from primitive material, similar to some forms of chondritic meteorites, is consistent with the GRS measurements.
Credit: NASA/JPL/Caltech (left); Reprinted by permission from Macmillan Publishers Ltd: Nature [473(7348):460–461, © 2011] (right)
MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and entered orbit about Mercury on March 18, 2011, to begin a one-year study of its target planet. Dr. Sean C. Solomon, of the Carnegie Institution of Washington, leads the mission as Principal Investigator. The Johns Hopkins University Applied Physics Laboratory built and operates the MESSENGER spacecraft and manages this Discovery-class mission for NASA.

The Carnegie Institution for Science (carnegiescience.edu) is a private, nonprofit organization headquartered in Washington, D.C., with six research departments throughout the U.S. Since its founding in 1902, the Carnegie Institution has been a pioneering force in basic scientific research. Carnegie scientists are leaders in plant biology, developmental biology, astronomy, materials science, global ecology, and Earth and planetary science.


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