Mercury Comes into Better Focus, Offers Surprises
After nearly three months in orbit about Mercury, MESSENGER’s payload is providing a wealth of new information about the planet closest to the Sun, as well as a few surprises. The spacecraft entered orbit around Mercury on March 18, 2011, becoming the first spacecraft ever to do so. Its instruments are performing the first complete reconnaissance of the planet’s geochemistry, geophysics, geologic history, atmosphere, magnetosphere, and plasma environment.
Courtesy of NASA
Tens of thousands of images of major features on the planet — previously seen only at comparatively low resolution — are now available 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,” offered 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.”
A Surface Revealed in Unprecedented Detail
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. Orbital images reveal broad expanses of smooth plains near Mercury’s north pole. Flyby images from MESSENGER and from Mariner 10 in the 1970s indicated that smooth plains may be important near the north pole, but much of the territory was viewed at unfavorable imaging conditions.
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 broad expanses of plains confirm that volcanism shaped much of Mercury’s crust and continued through much of Mercury’s history, despite an overall contractional stress state that tended to inhibit the extrusion of volcanic material onto the surface.
Among the fascinating features seen in flyby images of Mercury were bright, patchy deposits on some crater floors. Without high-resolution images to obtain a closer look, these features remained only a curiosity. New targeted MDIS observations at up to 10 meters per pixel reveal these patchy deposits to be clusters of rimless, irregular pits varying in size from hundreds of meters to several kilometers. These pits are often surrounded by diffuse halos of higher-reflectance material, and they are found associated with central peaks, peak rings, and rims of craters.
“The etched appearance of these landforms is unlike anything we’ve seen before on Mercury or the Moon,” said Brett Denevi, a staff scientist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., and a member of the MESSENGER imaging team. “We are still debating their origin, but they appear to have a relatively young age and may suggest a more abundant than expected volatile component in Mercury’s crust.”
Mercury’s Surface Composition
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. “The abundance of potassium rules out some prior theories for Mercury’s composition and origin,” said Larry Nittler, a staff scientist at the Carnegie Institution of Washington. “Moreover, the inferred ratio of potassium to thorium is similar to that of other terrestrial planets, suggesting that Mercury is not highly depleted in volatiles, contrary to some prior ideas about its origin.”
Mapping of Mercury’s Topography and Magnetic Field
MESSENGER’s Mercury Laser Altimeter has been systematically mapping the topography of Mercury’s northern hemisphere. After more than two 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.
The geometry of Mercury’s internal magnetic field can potentially discriminate among theories for how the field is generated. An important finding is that Mercury’s magnetic equator, determined on successive orbits as the point where the direction of the internal magnetic field is parallel to the spin axis of the planet, is well north of the planet’s geographic equator. The best-fitting internal dipole magnetic field is located about 0.2 Mercury radii, or 480 km, northward of the planet’s center. The dynamo mechanism in Mercury’s molten, metallic outer core responsible for generating the planet’s magnetic field therefore has a strong north-south asymmetry.
As a result of this north-south asymmetry, the geometry of magnetic field lines is different in Mercury’s north and south polar regions. In particular, the magnetic “polar cap” where field lines are open to the interplanetary medium is much larger near the south pole. This geometry implies that the south polar region is much more exposed than in the north to charged particles heated and accelerated by solar wind–magnetosphere interactions. The impact of those charged particles onto Mercury’s surface contributes both to the generation of the planet’s tenuous atmosphere and to the “space weathering” of surface materials, both of which should have a north-south asymmetry given the different magnetic field configurations at the two poles.
Energetic Particle Events at Mercury
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, said MESSENGER Project Scientist Ralph McNutt, of APL. “While varying in strength and distribution, bursts of energetic electrons — with energies from 10 kiloelectron volts (keV) to more than 200 keV — have been seen in most orbits since orbit insertion,” McNutt said. “The Energetic Particle Spectrometer has shown these events to be electrons rather than energetic ions, and to occur at moderate latitudes. The latitudinal location is entirely consistent with the events seen by Mariner 10.”
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 the acceleration of energetic electrons will be the subject of study over the coming months.
“One mystery has been answered, only to be replaced by another, but that is how science works,” McNutt said. “In the coming months as MESSENGER’s orbit swings around the planet, we will be able to observe the overall geometry of these events, providing yet more clues to their production and interaction with the planet.”
“We are assembling a global overview of the nature and workings of Mercury for the first time,” added Solomon, “and many of our earlier ideas are being cast aside as new observations lead to new insights. Our primary mission has another three more Mercury years to run, and we can expect further surprises as our solar system’s innermost planet reveals its long-held secrets.”