[meteorite-list] MESSENGER's Data Offers Insights on Inner Planet

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 17 Jun 2011 10:58:18 -0700 (PDT)
Message-ID: <201106171758.p5HHwImn022675_at_zagami.jpl.nasa.gov>

Alan Fischer
Public Information Officer
Planetary Science Institute
fischer at psi.edu

MESSENGER's Data Offers Insights on Inner Planet
June 16, 2011

The Mercury MESSENGER spacecraft has discovered a
wealth of new information, including a few scientific surprises, after
orbiting the planet closest to the sun for almost three months.

After MESSENGER's historic entry into orbit around Mercury on March 18,
instruments aboard the spacecraft have provided researchers with new
data on the planet's geochemistry, geophysics, geologic history,
atmosphere, magnetosphere, and plasma environment.

The Magnetometer instrument on the spacecraft has shown that the
magnetic field of Mercury is much like that of the Earth in that it has
a north pole and south pole, each approximately aligned with the
opposite geographic pole. However, unlike on Earth, the magnetic equator
on Mercury is systematically offset about 480 kilometers northward of
Mercury's geographic equator.

"This is an exciting result that suggests something fundamentally
different about what processes play a key role in the generation of
Mercury's magnetic field compared with those important to Earth's
magnetic field," said Catherine Johnson, Planetary Science Institute
senior scientist, University of British Columbia professor of geophysics
and MESSENGER mission participating scientist. "The result may have
important implications for the internal dynamics of the planet and how
the planet cools today."

Other PSI researchers working on the MESSENGER mission include Deborah
Domingue Lorin, William Feldman, Robert Gaskell, Faith Vilas and
Elizabeth Jensen.

With MESSENGER'S instruments performing the first complete
reconnaissance of Mercury, major features on the planet - previously
seen only at comparatively low resolution - are now 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," said
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."

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

The broad expanses of plains near Mercury's north pole seen in orbital
imaging confirm that volcanism shaped much of Mercury's crust and
continued through much of Mercury's history. 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 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.

MESSENGER's Mercury Laser Altimeter has been systematically mapping the
topography of Mercury's northern hemisphere. After more than 2 million
laser-ranging observations, the planet's large-scale shape and profiles
of geological features are both being revealed in high detail. The
northpolar 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 polesare 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

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.

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 their acceleration will be the
subject of study over the coming months.

"We are assembling a global overview of the nature and workings of
Mercury for the first time and many of our earlier ideas are being cast
aside as new observations lead to new insights," Solomon said. "Our
primary mission has another three Mercury years to run, and we can
expect more surprises as our solar system's innermost planet reveals its
long-held secrets."

Catherine Johnson
Senior Scientist
cjohnson at psi.edu
Received on Fri 17 Jun 2011 01:58:18 PM PDT

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