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March
7, 2008: The surprises
continue. Scientists studying the harvest of photos from the
MESSENGER spacecraft's Jan. 14th flyby of Mercury have found
several craters with strange dark halos and one crater with
a spectacularly shiny bottom.
"The
halos are really exceptional," says MESSENGER science
team member Clark Chapman of the Southwest Research Institute
in Boulder, Colorado. "We've never seen anything like
them on Mercury before and their formation is a mystery."
Consider
the following:
![A picture of two craters in Caloris Basin](images/strangecraters/calorisbasin_strip.jpg)
The
two craters at the bottom of the frame are located in Mercury's
giant Caloris Basin, a thousand mile wide depression formed
billions of years ago when Mercury collided with a comet or
asteroid. For scale, the larger of the two is about 40 miles
wide. Both craters have dark rims or "halos" and
the one on the left is partially filled with an unknown shiny
material.
Chapman
offers two possible explanations for the halos:
1.
The Layer Cake Theory--There could be a layer of
dark material under the surface of Caloris Basin, resulting
in chocolate-colored rims around craters that penetrate to
just the right depth. If such a subterranean layer exists,
however, it cannot be unique to the Basin. "We've found
a number of dark halos outside of Caloris as well—for instance,
these
two near Mercury's south pole."
2.
The Impact Glass Model--Thermal energy from the impacts
melted some of Mercury's rocky surface. Perhaps molten rock
splashed to the edge of the craters where it re-solidified as
a dark, glassy substance. Similar "impact melts" are
found around craters on Earth and the Moon. If this hypothesis
is correct, future astronauts on Mercury exploring the crater
rims would find themselves crunching across fields of tiny glass
shards.
Chapman
notes that the Moon also has some dark haloed craters--"Tycho
is a well-known example." But lunar halos tend to be
subtle and/or fragmentary. "The ones we see on Mercury
are much more eye-catching and distinct."
The
difference may be gravity. Lunar gravity is low. Any dark
material flying out of a crater on the Moon travels a great
distance, spreading out in a diffusion that can be difficult
to see. The surface gravity of Mercury, on the other hand,
is more than twice as strong as the Moon's. On Mercury, debris
can't fly as far; it lands in concentrated form closer to
the impact site where it can catch the attention of the human
eye.
Right:
Another dark-haloed crater near Mercury's south pole. [More]
None
of this explains the shiny-bottomed crater: "That is
an even bigger mystery," says Chapman. Superficially,
the bright patch resembles an expanse of ice glistening in
the sun, but that's not possible. The surface temperature
of the crater at the time of the photo was around 400 degrees
Celsius. Perhaps the shiny material is part of another subsurface
layer, bright mixed with dark; that would be the Marbled
Layer Cake Theory. "I haven't heard any really convincing
explanations from our science team," he adds. "We
don't yet know what the material is, why it is so bright,
or why it is localized in this particular crater."
Fortunately,
MESSENGER may have gathered data researchers need to solve
the puzzle. Spectrometers onboard the spacecraft scanned the
craters during the flyby; the colors they measured should
eventually reveal the minerals involved. "The data are
still being calibrated and analyzed," says Chapman.
And
if those data don't yield an answer….?
There
are still two more flybys—one in Oct. 2008 and another in
Sept. 2009—before MESSENGER enters Mercury orbit in 2011.
In the fullness of time "we'll get to the bottom of this
mystery"—and probably many more mysteries yet to be revealed.
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Author: Dr.
Tony Phillips | Credit: Science@NASA
more
information |
A
note about firsts: This is the first time dark
halos have been seen on Mercury, but bright-bottomed
craters may have been observed before. "Bright
deposits were seen on some crater floors in Mariner
10 images of Mercury from the 1970s, but they weren't
so prominent and weren't given much attention,"
notes Chapman. "This one [in Caloris Basin] is
clearly the most spectacular example."
MESSENGER
mission home page
Surprises
from Mercury (Science@NASA)
Mercury
Flyby Sets Stage for Discovery (Science@NASA)
Mercury
flyby gallery
The
MESSENGER project is the seventh in NASA's Discovery
Program of low-cost, scientifically focused space missions.
The Applied Physics Laboratory designed, built and operates
the spacecraft and manages the mission for NASA. MESSENGER
stands for MErcury Surface, Space ENvironment, GEochemistry
and Ranging.
NASA's
Future: The
Vision for Space Exploration |
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