From: Planetary Science Institute
Posted: Saturday, June 18, 2011
The Mercury MESSENGER spacecraft has discovered a wealth of new information, including a few scientific surprises, after orbiting the planet closest to the Sun for almost three months.
After MESSENGER's historic entry into orbit around Mercury on March 18, instruments aboard the spacecraft have provided researchers with new data on the planet's geochemistry, geophysics, geologic history, atmosphere, magnetosphere, and plasma environment.
The magnetometer instrument on the spacecraft has shown that the magnetic field of Mercury is much like that of the Earth in that it has a north pole and south pole, each approximately aligned with the corresponding geographic pole. However, unlike on Earth, the magnetic equator on Mercury is systematically offset about 480 kilometers northward of Mercury's geographic equator.
"This is an exciting result that suggests something fundamentally different about what processes play a key role in the generation of Mercury's magnetic field compared with those important to Earth's magnetic field," said Catherine Johnson, Planetary Science Institute senior scientist, University of British Columbia professor of geophysics and MESSENGER mission participating scientist. "The result may have important implications for the internal dynamics of the planet and how the planet cools today."
Other PSI researchers working on the MESSENGER mission include Deborah Domingue Lorin, William Feldman, Robert Gaskell, Faith Vilas and Elizabeth Jensen.
With MESSENGER'S instruments performing the first complete reconnaissance of Mercury, major features on the planet -- previously seen only at comparatively low resolution -- are now in sharp focus. Measurements of the chemical composition of Mercury's surface are providing important clues to the origin of the planet and its geological history. Maps of the planet's topography and magnetic field are revealing new clues to Mercury's interior dynamical processes. And scientists now know that bursts of energetic particles in Mercury's magnetosphere are a continuing product of the interaction of Mercury's magnetic field with the solar wind.
"MESSENGER has passed a number of milestones just this week," said MESSENGER principal investigator Sean Solomon of the Carnegie Institution of Washington. "We completed our first perihelion passage from orbit on Sunday, our first Mercury year in orbit on Monday, our first superior solar conjunction from orbit on Tuesday, and our first orbit-correction maneuver on Wednesday. Those milestones provide important context to the continuing feast of new observations that MESSENGER has been sending home on nearly a daily basis."
As part of MESSENGER's global imaging campaign, the Mercury Dual Imaging System (MDIS) is acquiring global monochrome and stereo base maps with an average resolution of 250 meters per pixel and a global color base map at an average of 1.2 kilometer per pixel. These base maps are providing the first global look at the planet under optimal viewing conditions.
The broad expanses of plains near Mercury's north pole seen in orbital imaging confirm that volcanism shaped much of Mercury's crust and continued through much of Mercury's history. MESSENGER's new orbital images show that the plains are likely among the largest expanses of volcanic deposits on Mercury, with thicknesses of up to several kilometers
The X-Ray Spectrometer (XRS) -- one of two instruments on MESSENGER designed to measure the abundances of many key elements on Mercury -- has made several important discoveries since the orbital mission began. The magnesium/silicon, aluminum/silicon, and calcium/silicon ratios averaged over large areas of the planet's surface show that, unlike the surface of the Moon, Mercury's surface is not dominated by feldspar-rich rocks.
XRS observations have also revealed substantial amounts of sulfur at Mercury's surface, lending support to prior suggestions from ground-based telescopic spectral observations that sulfide minerals are present. This discovery suggests that the original building blocks from which Mercury was assembled may have been less oxidized than those that formed the other terrestrial planets, and it has potentially important implications for understanding the nature of volcanism on Mercury.
MESSENGER's Gamma-Ray and Neutron Spectrometer has detected the decay of radioactive isotopes of potassium and thorium and has allowed a determination of the bulk abundances of these elements.
MESSENGER's Mercury Laser Altimeter has been systematically mapping the topography of Mercury's northern hemisphere. After more than 2 million laser-ranging observations, the planet's large-scale shape and profiles of geological features are both being revealed in high detail. The north polar region of Mercury, for instance, is a broad area of low elevations. The overall range in topographic heights seen to date exceeds 9 kilometers.
Two decades ago, Earth-based radar images showed that near both Mercury's north and south poles are deposits characterized by high radar backscatter. These polar deposits are thought to consist of water ice and perhaps other ices preserved on the cold, permanently shadowed floors of high-latitude impact craters. MESSENGER's altimeter is testing this idea by measuring the floor depths of craters near Mercury's north pole. To date, the depths of craters hosting polar deposits are consistent with the idea that those deposits occupy areas in permanent shadow.
One of the major discoveries made by Mariner 10 during the first of its three flybys of Mercury in 1974 were bursts of energetic particles in Mercury's Earth-like magnetosphere. Four bursts of particles were observed on that flyby, so it was puzzling that no such strong events were detected by MESSENGER during any of its three flybys of the planet in 2008 and 2009. With MESSENGER now in near-polar orbit about Mercury, energetic events are being seen almost like clockwork.
With Mercury's smaller magnetosphere and with the lack of a substantial atmosphere, both the generation of these energetic electrons and their distribution are different than at Earth. One candidate mechanism for the generation of these energetic electrons is the formation of a "double layer," a plasma structure with large electric fields along the local magnetic field. Another is induction brought about by rapid changes in the magnetic field, a process that follows the principle used in generators on Earth to produce electric power. Which of these mechanisms, if either, predominates in their acceleration will be the subject of study over the coming months.
"We are assembling a global overview of the nature and workings of Mercury for the first time and many of our earlier ideas are being cast aside as new observations lead to new insights," Solomon said. "Our primary mission has another three Mercury years to run, and we can expect more surprises as our solar system's innermost planet reveals its long-held secrets."
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The Planetary Science Institute is a private, nonprofit 501(c)(3) corporation dedicated to solar system exploration. It is headquartered in Tucson, Arizona, where it was founded in 1972. PSI scientists are involved in numerous NASA and international missions, the study of Mars and other planets, the Moon, asteroids, comets, interplanetary dust, impact physics, the origin of the solar system, extra-solar planet formation, dynamics, the rise of life, and other areas of research. They conduct fieldwork in North America, Australia and Africa. They also are actively involved in science education and public outreach through school programs, children's books, popular science books and art. PSI scientists are based in 15 states, the United Kingdom, Switzerland, Russia, Australia and Canada
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