Gliding over the battered surface of Mercury for the
second time this year, NASA's
MESSENGER spacecraft has revealed even more previously
unseen real estate on the innermost
planet, sending home hundreds of photos and measurements of
its surface, atmosphere and magnetic
The probe flew by Mercury shortly after 4:40 a.m. on
Oct. 6, completing a critical gravity assist
to keep it on course to orbit Mercury in 2011 and unveiling
30 percent of Mercury's surface never
before seen by spacecraft.
"The region of Mercury's surface that we viewed at
close range for the first time this month is
bigger than the land area of South America," said Sean
Solomon, the mission's principal investigator
and director of the Department of Terrestrial Magnetism at
the Carnegie Institution of Washington.
"When combined with data from our first flyby and from
Mariner 10, our latest coverage means that
we have now seen about 95 percent of the planet."
The spacecraft's science instruments worked feverishly
through the flyby; cameras snapped
more than 1,200 pictures of the surface, while topography
beneath the spacecraft was profiled with
the laser altimeter. "We have completed an initial
reconnaissance of the solar system's innermost
planet, enabling us to gain a global view of Mercury's
geological history and internal magnetic field
geometry for the first time," Solomon said.
The comparison of magnetosphere observations from the
probe's first flyby in January with
data from its second pass has provided key new insight into
the nature of the planet's internal
magnetic field and revealed new features of Mercury's
Deputy project scientist Brian Anderson, of Johns
Hopkins' Applied Physics
"The previous flybys by MESSENGER and Mariner 10 provided
data only on Mercury's eastern
hemisphere. The most recent flyby gave us our first
measurements on Mercury's western hemisphere,
and with them we discovered that the planet's magnetic
field is highly symmetric.
"This seemingly simple result is significant for the
planet's internal field because it implies that
the dipole is even more closely aligned with the planet's
rotation axis than we could conclude before
the second flyby," Anderson said. "Even though the rigorous
analyses of these data are ongoing, we
expect that this result will allow us to limit the theories
of planetary magnetic field generation to
those that predict a strongly rotationally aligned
The Mercury Atmospheric and Surface Composition
Spectrometer, known as MASCS, observed
the extended tail and night side and day side regions of
Mercury's thin atmosphere — known as an
exosphere — searching for emission from sodium,
calcium, magnesium and hydrogen atoms.
Ron Vervack of APL, MESSENGER participating scientist,
said, "The MASCS observations of
magnesium are the first-ever detection of this species in
Mercury's exosphere." Preliminary analysis
of the sodium, calcium and magnesium observations suggests
that the spatial distributions of these
three species are different and that the distribution of
sodium during the second flyby is noticeably
different from that observed during the first flyby.
"The spatial distributions of sodium, calcium and
magnesium are a reflection of the processes
that release these species from Mercury's surface," Vervack
said. "Now that we were finally able to
measure them simultaneously, we have an unprecedented
window into the interaction of Mercury's
surface and exosphere."
The probe's Mercury Laser Altimeter measured the
planet's topography, allowing scientists, for
the first time, to correlate high-resolution topography
measurements with high-resolution images.
"During the last flyby, the Mercury Laser Altimeter
acquired a topographic profile in a
hemisphere of the planet for which there were no spacecraft
images," said Maria Zuber, the mission's
co-investigator and head of the Department of Earth,
Atmospheric and Planetary Sciences at MIT.
"During the second flyby, in contrast, altimetry was
collected in regions where images from
MESSENGER and Mariner 10 are available, and new images were
obtained of the region sampled by the
altimeter in January. These topographic measurements now
improve considerably the ability to
interpret surface geology."
Now that the spacecraft's cameras have imaged more
than 80 percent of Mercury, scientists
say it is clear that, unlike the moon and Mars, the planet
lacks hemispheric-scale geologic differences.
"On the moon, dark volcanic plains are concentrated on the
near side and are nearly absent from the
far side," said co-investigator Mark Robinson, of Arizona
State University. "On Mars, the southern
hemisphere consists of older, cratered highlands, whereas
the northern hemisphere consists of
younger lowlands. Mercury's surface is more homogeneously
ancient and heavily cratered, with large
extents of younger volcanic plains lying within and between
giant impact basins."
Color imaging also shows that Mercury's crust is
compositionally heterogeneous. "Although
definitive compositional interpretations cannot yet be
made, the distribution of different components
varies both across the surface and with depth. Mercury's
crust is more analogous to a marbled cake
than a layered cake," Robinson said. "Once MESSENGER's
suite of science instruments returns a host
of data from the orbital phase of the mission, compositions
will be determined for the newly
discovered color units."
Solomon said that the first two Mercury flybys have
returned a rich dividend of new
observations. "But some of the observations we are most
eager to make — such as the chemical makeup
of Mercury's surface and the nature of its enigmatic polar
deposits — will not be possible until
MESSENGER begins to orbit the innermost planet," he said.
"Moreover, the very dynamic nature of
Mercury's interaction with its interplanetary environment
has taught us that continuous observations
will be required before we can claim to understand our most
sunward sister planet."
The MESSENGER spacecraft launched on Aug. 3, 2004, and
after flybys of Earth, Venus and
Mercury will start a yearlong study of Mercury, its target
planet, in March 2011. The Applied Physics
Laboratory built and operates the spacecraft and manages
the Discovery-class mission for NASA.