MESSENGER scientists presented 30 papers and posters with the latest results from observations made during the spacecraft’s first six months in orbit around Mercury at a special session of the joint meeting of the European Planetary Science Congress and the Division for Planetary Sciences of the American Astronomical Society in Nantes, France.
“This is the first major scientific meeting at which MESSENGER orbital observations are being presented to the scientific community,” said Principal Investigator Sean Solomon of the Carnegie Institution of Washington. “As the first spacecraft to orbit our solar system’s innermost planet, MESSENGER continues to reveal new surprises every week. It is timely to sum up what we’ve learned so far and to seek feedback from our international colleagues across planetary science on our interpretations to date.”
After three successful flybys of Mercury, the MESSENGER spacecraft entered orbit about the innermost planet on March 18, 2011. The orbital phase of the mission is enabling the first global perspective on the planet’s geology, surface composition, topography, gravity and magnetic fields, exosphere, magnetosphere, and solar-wind interaction.
This image shows a map of smooth plains during a solar flare on 16 June 2011. Plains of volcanic origin are mapped in green while yellow denotes plains of uncertain origin.
Credit: NASA/Johns Hopkins University Applied Physics Lab.
The Evolution of Mercury’s Geological and Surface Composition
MESSENGER has nearly completed two of its primary global imaging campaigns: a monochrome map at 250 meters per pixel and an eight-color, 1 kilometer per pixel color map. Apart from small gaps, which will be filled in during the next orbits, these maps cover the entire planet under uniform lighting conditions ideal for assessing the form of Mercury’s surface features as well as the color and compositional variations across the planet.
Flybys of Mercury by the MESSENGER and Mariner 10 spacecraft showed broad expanses of plains across the planet. There was strong evidence for a volcanic origin of many of these plains, indicating that volcanism played an important role in shaping Mercury’s crust; but large regions of the planet remained unmapped, and the origin of many plains units had until now remained ambiguous.
“With images from MESSENGER’s orbital mapping campaigns, as well as targeted high-resolution images, we can now begin to assess the origin of plains on a global basis, and — when combined with data from MESSENGER’s X-ray spectrometer — their compositional variation,” says Brett Denevi, a planetary scientist in APL’s Space Department. “We find that volcanic rocks dominate much of Mercury’s crust, even in regions that are geologically complex and where impact cratering has destroyed many of the original surface features.”
The X-ray spectrometer collects compositional information averaged over relatively large regions on Mercury’s surface. Signals that identify the heavier elements are received only during times of high solar activity. For regions where geologic mapping and detailed compositional information are both available, many of the large-scale volcanic units on Mercury are seen to be basaltic. Basalts are common volcanic rocks on Earth and the Moon.
Mercury’s Global Magnetic Field
The magnetic and gravity fields of Mercury are the primary clues scientists have on the structure deep in the interior of the planet, which in turns helps develop general theories for how planets form and evolve. Orbital data reveal that Mercury’s magnetic field is offset far to the north of the planet’s center, by nearly 20% of Mercury’s radius. Relative to the planet’s size, this offset is much more than in any other planet, and accounting for it will pose a challenge to theoretical explanations of the field.
“Although we don’t know how to explain that yet — it is no doubt an important clue to the workings of Mercury’s dynamo,” said Brian Anderson, MESSENGER Deputy Project Scientist and a space physicist at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, MD.
This finding has several implications for other aspects of Mercury, said Anderson, who co-authored several of the presentations in the MESSENGER session. “This means that the magnetic field in the southern hemisphere should be a lot weaker than it is in the north. At the north geographic pole, the magnetic field should be about 3.5 times stronger than it is at the south geographic pole.
“The big difference in northern and southern surface field strengths means that energetic particles, solar wind, and high-energy electrons will preferentially impact the surface in the south, and this situation should lead to asymmetries both in sources of atoms, ions, and molecules for Mercury’s exosphere and in the discoloration of the surface by charged particle bombardment,” he continued. “Both should occur more strongly in the south.”
Read the full press release with all the findings.