Messenger spacecraft has been orbiting Mercury for 3 months. The instrument payload is providing a wealth of information.
“MESSENGER has passed a number of milestones just this week,” offers 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.”
Some excerpts from the June 16 press conference
A Surface Revealed in Unprecedented Detail
The Mercury Dual Imaging System (MDIS) is getting global monochrome and stereo base maps with an average resolution of 250 meters per pixel and a 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.
Images reveal broad expanses of smooth plains near Mercury’s north pole. The plains are likely among the largest expanses of volcanic deposits on Mercury and confirm that volcanism shaped much of Mercury’s crust and continued through much of Mercury’s history.
MDIS observations at up to 10 meters per pixel reveal light patchy deposits to be clusters of rimless, irregular pits varying in size from hundreds of meters to several kilometers 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,” says Brett Denevi, a staff scientist at the Johns Hopkins University Applied Physics Laboratory (APL). “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) revealed the magnesium/silicon, aluminum/silicon, and calcium/silicon ratios averaged over large areas of the planet’s surface show that Mercury’s surface is not dominated by feldspar-rich rocks. XRS observations have also revealed substantial amounts of sulfur at Mercury’s surface.
The Gamma-Ray and Neutron Spectrometer has detected the decay of radioactive isotopes of potassium and thorium.
“The abundance of potassium rules out some prior theories for Mercury’s composition and origin,” says 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
The Mercury Laser Altimeter has been systematically mapping Mercury’s northern hemisphere. More than two million observations are revealing the shape and profiles of geological high detail. The north polar region of Mercury, for instance, is a broad area of low elevations.
Tests for polar ice deposits preserved on the cold, permanently shadowed floors of high-latitude impact craters are being done. To date, the depths of craters hosting polar deposits show they are in permanent shadow.
Mercury has an asymmetrical magnetic field. The geometry of magnetic field lines is different in Mercury’s north and south polar regions. The magnetic “polar cap” is much larger near the south pole and exposes that region to more 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.
Energetic Particle Events at Mercury
Mariner 10, during the first of its three flybys of Mercury in 1974, showed bursts of energetic particles in Mercury’s Earth-like magnetosphere. With MESSENGER now in near-polar orbit about Mercury, energetic events are being seen almost like clockwork, says 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 says. “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.” The mechanisms of 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 says. “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,” adds 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.”
If you are interested,
further details about these findings and others are provided by 4 presenters at the news conference.
Original Diary is here.