[meteorite-list] MESSENGER Team Presents New Mercury Findings at Planetary Conference

[Ron Baalke]

The Johns Hopkins University Applied Physics Laboratory
Office of Communications and Public Affairs
Laurel, Maryland

Media Contact: Paulette W. Campbell. JHU/APL
(240) 228-6792; Paulette.Campbell at jhuapl.edu
Anita Heward, European Planetology Press Officer
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Vishnu Reddy, Division for Planetary Sciences, American Astronomical Society
+49 555 15787 579623; reddy at space.edu

October 5, 2011

FOR IMMEDIATE RELEASE

MESSENGER TEAM PRESENTS NEW MERCURY FINDINGS AT PLANETARY CONFERENCE

MESSENGER scientists will highlight the latest results on Mercury 
from MESSENGER observations obtained during the first six months (the 
first Mercury solar day) in orbit. These findings will be presented 
Oct. 5 in 30 papers and posters as part of 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, Frances.

Scientists will also look ahead to MESSENGER observations still to 
come and to the dual-spacecraft BepiColombo mission of the European 
Space Agency and the Japan Aerospace Exploration Agency's later this decade.

"This is the first major scientific meeting at which MESSENGER 
orbital observations are being presented to the scientific 
community," says MESSENGER 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.

** 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 percent 
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," says 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, 
says 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 continues. "Both should 
occur more strongly in the south."

** The Dynamics of Mercury's Exosphere **

Mercury is surrounded by a tenuous exosphere of gas generated and 
maintained by the interaction of the space environment with the 
planet's surface. Measuring the composition and structure of the 
exosphere provides insight into how the space environment modifies 
the outermost layers of the planet's surface materials.

MESSENGER's observations during the flybys and orbit show that the 
current understanding of the nature of Mercury's exosphere is 
incomplete, says William McClintock, a MESSENGER mission 
co-investigator and senior scientist at the Laboratory for 
Atmospheric and Space Physics, University of Colorado, Boulder.

"They show that distinctly different source and loss processes 
control the populations of the major constituents of sodium, 
magnesium, and calcium atoms in the exosphere," says McClintock.

Before MESSENGER, the prevailing theory suggested that material was 
released from the dayside by solar wind and radiation. In this 
picture, lofted material then was carried to the nightside by solar 
radiation pressure. MESSENGER measurements show that magnesium and 
calcium in the tail region are substantially more abundant than would 
be expected if they were produced in this way, he points out.

New magnetic field models, derived from MESSENGER's Magnetometer 
data, indicate that the planet's intrinsic field can couple with the 
interplanetary field to direct solar wind ions to the nightside, 
sputtering material from non-illuminated surfaces. But that source is 
too weak to explain the observed concentrations. Calcium also 
exhibits an unexplained enhanced concentration at the equator near 
dawn, a pattern that appears to be a persistent feature in the 
exosphere. Such dawn enhancements are not observed for magnesium, 
which is chemically similar to calcium.

** The Evolution of Mercury's Geological and Surface Composition **

After its first Mercury solar day in orbit, 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 solar day, 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, and signals 
diagnostic of 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.

** Variations in Surface Reflectance Spectra **

Over the course of the first solar day in orbit, the Visible and 
Infrared Spectrograph (VIRS) channel of the Mercury Atmospheric and 
Surface Composition Spectrometer (MASCS) obtained over one million 
spectra of the surface from near one pole to the other and spanning 
all longitudes. VIRS observed all the major geologic units and 
structures, from large basins to small fresh-looking craters, and 
from average pains to hollows and possible pyroclastic materials. 
Whereas the Mercury Dual Imaging System highlights the morphology and 
broad color characteristics of these materials, VIRS reveals greater 
details of the reflective properties of surface materials.

"One surprise that's been with us since the flybys is the apparent 
lack of iron in the silicate minerals of the rocks on the surface of 
the planet," says APL's Noam Izenberg, instrument scientist for the 
MASCS instrument on MESSENGER.

"In rock-forming silicates, the primary materials of most planetary 
crusts, iron shows up as a characteristic absorption at infrared 
wavelengths, but such features have been completely absent in spectra 
from Mercury," says Izenberg. The infrared continues to show very 
little spectral variation indicative of distinct mineralogies, and we 
are working hard to tease out what we can."

An important chapter of the story, however, appears to be at 
ultraviolet wavelengths, he says. "Iron in rocks also has effects in 
this region of the spectrum as well, but those effects are less well 
studied and understood. However it is here that we see variations 
among, for example, fresh-looking craters, plains, hollows, 
pyroclastic deposits, and low-reflectance units."

According to Izenberg, the variations in the ultraviolet may reflect 
both iron content and the type of rocks that hold it and may provide 
hints at other materials, such as sulfur, which have characteristic 
ultraviolet reflectance signatures as well. "Evidence from other 
instruments on MESSENGER, such as the X-Ray Spectrometer, 
corroborates low iron abundance near the surface and the presence of 
sulfur, so as our analyses advance we'll be working to correlate the 
findings across all instruments."

** Looking Ahead **

MESSENGER continues to send back data that illuminate Mercury's 
mysteries. The knowledge gained is already sharpening the mission 
goals of the dual-spacecraft BepiColombo mission, scheduled to launch 
to Mercury in 2014.

Members of the MESSENGER team met with BepiColombo scientists at 
Kyoto University in Japan last month to review the state of knowledge 
about Mercury and to present initial MESSENGER orbital results.

"We discussed many of the new findings that will be covered in the 
MESSENGER sessions today," says MESSENGER project scientist Ralph 
McNutt. "BepiColombo team members presented new perspectives on 
surface mineralogy from recent high-temperature laboratory 
measurements and new theories for Mercury's formation and for the 
generation of Mercury's magnetic field. This meeting continued a 
dialogue, begun more than a decade ago, on the synergies of the two 
investigations and how ongoing MESSENGER measurements are informing 
the planning for BepiColombo operations."

Related images are available online at: 
http://messenger.jhuapl.edu/news_room/presscon10.html.
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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.
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