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

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 5 Oct 2011 11:23:10 -0700 (PDT)
Message-ID: <201110051823.p95INArR009416_at_zagami.jpl.nasa.gov>

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
+44 (0) 7756 034243; anita.heward at europlanet-eu.org
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.
________________________________________

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.
________________________________________
Received on Wed 05 Oct 2011 02:23:10 PM PDT