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January
30, 2008: After a journey of more than 2 billion
miles and three and a half years, NASA's MESSENGER spacecraft
flew by Mercury on Jan. 14, 2008, and it has beamed back some
surprises.
"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, MESSENGER's
principal investigator at the Carnegie Institution of Washington.
The spacecraft's cameras and other sensors collected more
than 1,200 images and made the first up-close measurements
of Mercury since Mariner 10 visited the planet in the mid-1970s.
Above: The "Spider crater" located
on the floor and near the center of Mercury's giant Caloris
basin. [Larger
image] [More]
Researchers
once thought Mercury to be much like Earth's moon, but MESSENGER
has found many differences. For instance, unlike the moon,
Mercury has huge cliffs with structures snaking hundreds of
miles across the planet's face. The spacecraft also revealed
impact craters that appear very different from lunar craters.
One particularly curious crater has been dubbed "The
Spider."
This
formation never has been seen on Mercury before and nothing
like it has been observed on the moon. It lies in the middle
of a huge impact crater called the Caloris basin and consists
of more than 100 narrow, flat-floored troughs 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," said James Head, science
team co-investigator at Brown University, Providence, R.I.
When
Mariner 10 flew by Mercury in the 1970s, it saw only a portion
of Caloris basin. Now that MESSENGER has shown scientists
the basin's full extent, its diameter has been revised upward
from the Mariner 10 estimate of 800 miles to perhaps as large
as 960 miles from rim to rim. Researchers already knew that
Caloris was one of the largest impact craters in the solar
system; MESSENGER has shown it is even bigger than they thought!
 Turning
to Mercury's magnetic field, MESSENGER found it to be different
compared to Mariner 10 observations 30 years ago. While the
magnetic field was generally quiet (no magnetic storms) on
Jan. 14th, it showed several signs of significant internal
pressure. Additional flybys by MESSENGER in late 2008 and
2009 plus a yearlong orbital phase beginning in 2011 will
shed more light on the stability and dynamics of Mercury's
magnetic cocoon.
Right:
During the Jan. 14th flyby, MESSENGER made the first measurements
of Mercury's magnetospheric plasma. Click
to play a movie of data recorded by the spacecraft's Fast
Imaging Plasma Spectrometer (FIPS). [Movie]
[More]
MESSENGER's
suite of instruments also detected ultraviolet emissions from
sodium, calcium and hydrogen in Mercury's exosphere. (An exosphere
is a super-low-density atmosphere probably formed, in this
case, from atoms sputtering off Mercury's surface. The sputtering
may be caused by contact with hot plasma trapped in Mercury's
magnetic field.) MESSENGER encountered Mercury's sodium-rich
exospheric "tail" which extends more than 25,000
miles from the planet and also discovered a hydrogen
tail of similar dimensions.
"We
should keep this treasure trove of data in perspective,"
said project scientist Ralph McNutt of the Applied Physics
Laboratory, Laurel, Md. "With two flybys to come and
an intensive orbital mission to follow, we are just getting
started to go where no one has been before."
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Editor: Dr.
Tony Phillips | Credit: Science@NASA
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