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

[Ron Baalke]

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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