[meteorite-list] Surprises Stream Back From Mercury's MESSENGER

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 30 Jan 2008 13:41:42 -0800 (PST)
Message-ID: <200801302141.NAA24533_at_zagami.jpl.nasa.gov>

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

Media Contacts:
Paulette Campbell
(240) 228-6792 or (443) 778-6792, paulette.campbell at jhuapl.edu

Tina McDowell, Carnegie Institution of Washington
202-939-1120, tmcdowell at .ciw.edu
January 30, 2008

FOR IMMEDIATE RELEASE

SURPRISES STREAM BACK FROM MERCURY'S MESSENGER

After a journey of more than 2.2 billion miles and three and a half
years, NASA's MESSENGER spacecraft made its first flyby of Mercury
just after 2 p.m. EST on Jan. 14, 2008. All seven scientific
instruments worked flawlessly, producing a stream of surprises that
is amazing and delighting the science team. The 1,213 mages
conclusively show that the planet is a lot less like the Moon than
many previously thought, with features unique to this innermost
world. The puzzling magnetosphere appears to be very different from
what Mariner 10 discovered and first sampled almost 34 years ago.

"This flyby allowed us to see a part of the planet never before
viewed by spacecraft, and our little craft has returned a gold mine
of exciting data," stated Sean Solomon, principal investigator and
the director of the Department of Terrestrial Magnetism at the
Carnegie Institution of Washington. "From the perspectives of
spacecraft performance and maneuver accuracy, this encounter was
near-perfect, and we are delighted that all of the science data are
now on the ground. The science team appreciates that this mission
required a complex flight trajectory and a spacecraft that can
withstand the intense thermal environment near the Sun. Without the
hundreds of engineers and technicians at the Applied Physics
Laboratory and all of the partner organizations who designed,
assembled, tested, and now operate the spacecraft, we would not have
been able to make any of the scientific observations now in hand."

"MESSENGER has shown that Mercury is even more different from the
Moon than we'd thought," said Science Team co-investigator James
Head, professor at Brown University and chair of the mission's
Geology Discipline Group. The tiny spacecraft discovered a unique
feature that the scientists dubbed "The Spider." This type of
formation has never been seen on Mercury before, and nothing like it
has been observed on the Moon. It is in the middle of the Caloris
basin and consists of over a hundred narrow, flat-floored troughs
(called graben) radiating from a complex central region. "The Spider"
has a crater near its center, but whether that crater is related to
the original formation or came later is not clear at this time.

Unlike the Moon, Mercury also has huge cliffs or scarps, structures
snaking up to hundreds of miles across the planet's face, tracing
patterns of fault activity from early in Mercury's -- and the solar
system's -- history. The high density and small size of Mercury
combine to provide a surface gravity about 38 percent that of Earth
and almost exactly the same as that of Mars, which is some 40
percent larger than Mercury in diameter (2.7 times Mercury's
volume). Because gravity is stronger on Mercury than on the Moon,
impact craters appear very different from lunar craters; material
ejected during impact on Mercury falls closer to the rim and many
more secondary crater chains are present.

"We have seen new craters along the terminator on the side of the
planet viewed by Mariner 10 where the illumination of the MESSENGER
images revealed very subtle features. Technological advances that
have been incorporated in MESSENGER are effectively revealing an
entirely new planet from what we saw over 30 years ago," said Science
Team co-investigator Robert Strom, professor emeritus at the
University of Arizona and the only member of both the MESSENGER and
Mariner 10 science teams.

Now that MESSENGER has shown scientists the full extent of the
Caloris basin, its diameter has been revised upward from the Mariner
10 estimate of 800 miles to perhaps as large as 960 miles (about
1,550 kilometers) from rim crest to rim crest. The plains inside the
Caloris basin are distinctive and have a higher reflectance -- albedo
-- than the exterior plains, the opposite characteristics from many
lunar impact basins such as the Imbrium basin on the Moon, yet
another new mystery for Mercury. This finding could be the result of
several processes -- when the basin was formed by a large impact,
deeper material may have been excavated that contributed to impact
melt now preserved on the basin floor; alternatively, the basin
interior may have been volcanically resurfaced by magma produced deep
in Mercury's crust or mantle subsequent to the impact. The science
team is eagerly exploring the possibilities.

The magnetosphere of Mercury during the MESSENGER flyby appears to be
very different from what Mariner 10 saw. MESSENGER found that the
planet's magnetic field was generally quiet but showed several
signatures indicating significant plasma pressure within the
magnetosphere. Although the Energetic Particle Spectrometer (EPS) did
not find any of the energetic particles -- signatures of the solar
wind -- that were detected by Mariner 10, the Fast Imaging Plasma
Spectrometer (FIPS) did detect lower energy plasma ions in the
magnetosphere coincident with the plasma pressure signatures in the
magnetic field. Scientists are working to understand how the solar
wind plasmas gain entry, how they evolve, and how they might weather
the surface and contribute to the planet's exosphere.

"MESSENGER found that Mercury's intrinsic magnetic field is almost
identical to what it was 30 years ago. After correcting for the
contribution from the solar wind interaction, the mean dipole has the
same intensity to within a few percent and has the same slight tilt.
The search is now on for structure in the internal field to identify
its source," said Brian Anderson, the Magnetometer (MAG) instrument scientist.

Magnetic fields like Earth's, and their resultant magnetospheres, are
generated by electrical dynamos operating deep in the planet in a
liquid metallic outer core. Of the four terrestrial planets, only
Mercury and Earth -- the smallest and largest -- exhibit such a
structure. The magnetic field stands off the solar wind from the Sun,
in effect producing a protective bubble around Earth that, with the
Earth's thick atmosphere, shields the surface of our planet from
sporadic energetic particles from the Sun and the more constant and
more energetic cosmic rays from farther out in the galaxy. Earth's
magnetic field does not stay the same; it moves around and the poles
periodically flip, over geologic ages, changing the exposure of the
surface to these dangerous particles. Similar variations are expected
for Mercury's field, but the nature of its field-producing dynamo and
the times between the corresponding changes are unknown at this time.

The next two flybys and the yearlong orbital phase will shed more
light on this surprise. Mercury's global magnetic field has been a
particular puzzle to scientists. The planet's small size should have
resulted in the cooling and solidification of a liquid core long ago,
quenching any dynamo activity. How this small world continues to
maintain a magnetic field has been a major conundrum to planetary
scientists. Solving this puzzle will help understand the history of
Earth's magnetic field and why there are no modern global magnetic
fields at Venus and Mars.

Ultraviolet emissions detected by the Ultraviolet and Visible
Spectrometer (UVVS) segment of the Mercury Atmospheric and Surface
Composition Spectrometer (MASCS) clearly showed sodium, calcium, and
hydrogen in Mercury's exosphere (an atmosphere that is so thin that
atoms comprising it rarely, if ever, collide). There is an abundance
of sodium in an exospheric "tail" extending in an approximately
antisunward direction from the planet by over 25,000 miles (40,000
km). During the MESSENGER flyby, there was a strong north-south
asymmetry in the density of both sodium and hydrogen in Mercury's
tail, perhaps a signature of the dynamic state at the time of the
interaction of the solar wind with Mercury's magnetosphere and surface.

The suite of instruments that measured, for the first time, the
elemental and mineralogical composition of Mercury's surface include
the X-Ray Spectrometer (XRS), the Gamma-Ray and Neutron Spectrometer
(GRNS), and the Visible and Infrared Spectrograph (VIRS) portion of
MASCS. They all operated as planned. Despite the fast flyby, the GRNS
acquired observations vital to the interpretation of measurements
that will be made during the orbital phase. XRS relies on the Sun's
X-ray output to produce fluorescence in Mercury's surface elements,
so the increase in solar activity when MESSENGER nears and enters the
orbital phase of the mission will improve the resolution of the XRS
for elemental remote sensing. Detailed analysis of spectra from VIRS,
along with the color images, has just begun to provide insight into
the mineralogical makeup of surface materials along the spacecraft's
ground track.

The Mercury Laser Altimeter also worked flawlessly, providing a
topographic profile of craters and other geological features along
the spacecraft's flight path at all altitudes less than about 930
miles (1,500 km) on the night side of the planet. Precise tracking
and signal acquisition following the occultation of the spacecraft by
the planet, in the minutes just prior to closest approach, enabled
the acquisition of new information on the long-wavelength variations
in the planet's gravitational field. In turn, these results will shed
light on the size of Mercury's dense metallic core.

"But," says project scientist Ralph McNutt of APL, "we should keep
this treasure trove of data in perspective. With two flybys yet to
come and an intensive orbital mission to follow, 'You ain't seen nothing yet.'"

***

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Received on Wed 30 Jan 2008 04:41:42 PM PST