[meteorite-list] MESSENGER Provides New Look at Mercury's Landscape, Metallic Core, and Polar Shadows

[Ron Baalke]

FOR IMMEDIATE RELEASE
March 21, 2012

Media Contacts:
Paulette Campbell
(240) 228-6792
Paulette.Campbell at jhuapl.edu

Michael Buckley
(240) 228-7536
Michael.Buckley at jhuapl.edu

MESSENGER Provides New Look at Mercury's Landscape, Metallic Core, and Polar Shadows 
Spacecraft concludes primary mission, looks toward second year at innermost planet

MESSENGER completed its one-year primary mission on March 17. Since moving into 
orbit about Mercury a little over one year ago, the spacecraft has captured 
nearly 100,000 images and returned data that have revealed new information 
about the planet, including its topography, the structure of its core, 
and areas of permanent shadow at the poles that host the mysterious polar 
deposits.

The latest findings are presented in two papers published online 
in Science Express today, and in 57 papers presented this week at the 
43rd Lunar and Planetary Science Conference in The Woodlands, Texas. Team 
members at the meeting will also preview MESSENGER's extended mission, 
set to run to March 2013. The event, scheduled for 12:30 p.m. CDT (1:30 
p.m. EDT), will be streamed live on the Web at http://www.livestream.com/lpsc2012. 
Presentation materials are available online at http://messenger.jhuapl.edu/news_room/presscon11.html.
 
"The first year of MESSENGER orbital observations has revealed many surprises," 
says MESSENGER Principal Investigator Sean C. Solomon, of the Carnegie 
Institution of Washington. "From Mercury's extraordinarily dynamic magnetosphere 
and exosphere to the unexpectedly volatile-rich composition of its surface 
and interior, our inner planetary neighbor is now seen to be very different 
from what we imagined just a few years ago. The number and diversity of 
new findings being presented this week to the scientific community in 
papers and presentations provide a striking measure of how much we have 
learned to date."

Mercury's Landscape 

Ranging observations from MESSENGER's Mercury Laser Altimeter (MLA) have provided 
the first-ever precise topographic 
model of the planet's northern hemisphere and characterized slopes and 
surface roughness over a range of spatial scales. From MESSENGER's eccentric, 
near-polar orbit, the MLA illuminates surface areas as wide as 15 to 100 
meters, spaced about 400 meters apart.

The spread in elevations is considerably 
smaller than those of Mars or the Moon, notes MESSENGER Co-investigator 
Maria T. Zuber, author of one of the papers published in Science Express 
According to Zuber, of the Massachusetts Institute of Technology, the 
most prominent feature is an extensive area of lowlands at high northern 
latitudes that hosts the volcanic northern plains. Within this lowland 
region is a broad topographic rise that formed after the volcanic plains 
were emplaced.

At mid-latitudes, the interior of the Caloris impact basin 
- 1,500 kilometers wide - has been modified so that part of the basin 
floor now stands higher than the rim, Zuber says. "The elevated portion 
of the floor of Caloris appears to be part of a quasi-linear rise that 
extends for approximately half the planetary circumference at mid-latitudes," 
she writes. "Collectively, these features imply that long-wavelength changes 
to Mercury's topography occurred after the earliest phases of the planet's 
geological history."

A Surprising Core

Scientists have also come up with the first precise model of Mercury's gravity 
field which, when combined with the topographic data and earlier information 
of the planet's spin state, shed light on the planet's internal structure, 
the thickness of its crust, the size and state of its core, and its tectonic 
and thermal history.  

Mercury's core is huge for the planet's size, about 85% of the 
planetary radius, even larger than previous estimates. The planet is sufficiently 
small that at one time many scientists thought the interior should have 
cooled to the point that the core would be solid. However, subtle dynamical 
motions measured from Earth-based radar combined with parameters of the 
gravity field, as well as observations of the magnetic field that signify 
an active core dynamo, indicate that Mercury's core is at least partially 
liquid. "MESSENGER's observations of the gravity field have let us peer 
inside Mercury and get the first good look at its largest component - 
the core," says Case Western Reserve University's Steven A. Hauck II, 
coauthor of one of the papers published in Science Express.

Scientists sought to unravel the mystery of the size and state of Mercury's core 
by studying its effect on long-wavelength variations in the planet's gravity 
field, and recent results point to a much different interior structure 
for Mercury from that expected.

"Mercury's core may not look like any other terrestrial planetary core," Hauck 
says. "The structure certainly is different from that of Earth, which has a 
metallic, liquid outer core sitting above a solid inner core. Mercury appears 
to have a solid silicate crust and mantle overlying a solid, iron sulfide outer 
core layer, a deeper liquid core layer, and possibly a solid inner core."

These findings will have implications for how Mercury's magnetic field is 
generated and for understanding how the planet evolved thermally, Hauck adds.

Polar Shadows

A chief goal of MESSENGER's primary mission was to understand the nature 
of the radar-bright deposits at the poles of Mercury. The leading proposal 
since the deposits were discovered has been that radar-bright material 
consists dominantly of frozen water ice.

"We've never had the imagery available before to see the surface where these 
radar-bright features are located," says Nancy L. Chabot, instrument scientist 
for MESSENGER's Mercury Dual Imaging System (MDIS) at the Johns Hopkins 
University Applied Physics Laboratory (APL). "MDIS images show that all the 
radar-bright features near Mercury's south pole are located in areas of permanent 
shadow, and near Mercury's north pole such deposits are also seen only in shadowed 
regions, results consistent with the water-ice hypothesis."

This finding is not definitive proof that those deposits are water ice, says Chabot, 
who is presenting her results at LPSC. And some of the radar-bright deposits 
are located in craters that provide thermally challenging environments 
to the water-ice theory. For instance, for the radar-bright material in 
many of the craters to be water ice would require that there be a thin 
layer of insulation to keep it colder than the surface, Chabot says.

But the MDIS images, combined with ongoing analysis of data from MESSENGER's 
Neutron Spectrometer and the MLA, will provide a more complete picture 
of the nature of the deposits.

Extending the Discoveries

MESSENGER's second year at Mercury will build upon these and other results 
from the primary mission phase, emphasizes MESSENGER Project Scientist Ralph L. 
McNutt Jr., of APL. "The second year of orbital operations will not be 
a simple continuation of the primary mission," he says. "Extended mission 
themes will include more comprehensive measurement of the magnetosphere 
and exosphere during a period of more active Sun, greater focus on observations 
at low spacecraft altitudes, and a greater variety of targeted observations."

"MESSENGER has already fundamentally changed our view of this innermost planet," 
he adds. "With the extension of the MESSENGER mission, many more discoveries 
can be expected."


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 (UTC), to begin its primary mission 
- a yearlong study of its target planet. MESSENGER's extended mission 
began on March 18, 2012. 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.