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 on Jan. 14. All
seven scientific instruments worked
flawlessly, producing a stream of surprises that is amazing
and delighting the science team. The 1,213
images conclusively show that the planet is a lot less like
the moon than many scientists 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,"
said 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," he said. "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 [Johns Hopkins'] 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."
Science team co-investigator James Head, professor at
Brown University and chair of the
mission's Geology Discipline Group, said, "MESSENGER has
shown that Mercury is even more different
from the moon than we'd thought."
The tiny spacecraft discovered a unique feature that
the scientists dubbed "the Spider." This
type of formation has never been seen on Mercury, and
nothing like it has been observed on the moon.
It is in the middle of the Caloris basin and consists of
more than 100 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, that snake 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. 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," 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.
"Technological advances that have been
incorporated in MESSENGER are effectively revealing an
entirely new planet from what we saw over
30 years ago."
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
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 moon's Imbrium basin, 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 did not find any of the
energetic particles — signatures of the
solar wind — that were detected by Mariner 10, the
Fast Imaging Plasma Spectrometer 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," said Brian Anderson, the magnetometer
instrument scientist. "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."
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 segment of the
Mercury Atmospheric and Surface Composition Spectrometer,
or 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 anti-sunward direction from the planet by
more than 25,000 miles. 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, the Gamma-Ray and Neutron
Spectrometer, and the Visible and Infrared Spectrograph
portion of MASCS. They all operated as
planned. Despite the fast flyby, the Gamma-Ray and Neutron
Spectrometer acquired observations
vital to the interpretation of measurements that will be
made during the orbital phase. The X-Ray
Spectrometer, or 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 the Visible and Infrared Spectograph, 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 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," said 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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