Brilliant New Images of Mercury From First Year of Orbit
After one Mercury year (88 Earth days) in orbit, NASA's Messenger spacecraft is getting comfortable around the first rock from the sun.
Since becoming the first spacecraft ever to orbit Mercury on March 17, Messenger (MErcury Surface, Space ENvironment, GEochemistry and Ranging) has sent back more than 20,000 high-resolution images, enough to include Mercury in Google Earth.
The orbiter's seven science instruments are also probing Mercury's chemical composition, topography, magnetic field and energetic particles in the planet's tenuous, ever-shifting atmosphere.
These measurements contain surprises that are sending planetary scientists scrambling for their pencils.
"What we are finding is that in many cases a lot of the original ideas about Mercury are just plain wrong," said planetary scientist Larry Nittler of the Carnegie Institute of Washington in a press conference June 16.
This gallery highlights some of the first science results from the first spacecraft to orbit Mercury.
Above: Crescent Mercury
This image, taken May 24, shows off Mercury's southern hemisphere. Once a week, Messenger takes an edge shot like this one to provide information about Mercury's shape.
More Ice Than the Moon
One of Mercury's most exciting mysteries is whether the solar system's hottest planet can support frozen water. 20 years ago, radar data showed bright deposits that scientists interpreted as water ice hiding deep in permanently shadowed craters at Mercury's poles, similar to ice deposits recently confirmed on the moon.
In its fist few months in orbit, Messenger checked to see if these craters truly never saw the sun. The crater circled in red (above) is about 15 miles wide and two miles deep, according to laser altimetry data from Messenger (below).
"This crater passes the test," said Messenger principal investigator Sean Solomon of the Carnegie Institute of Washington. "The portion of the floor that is in permanent shadow coincides with the portion where we see the radar bright deposits hypothesized to be water ice."
Messenger still needs to take more data on the composition of the deposits to see if they contain lots of hydrogen, a sign that they really are water ice. If all the deposits turn out to be water, Solomon said, "we have the irony that the planet closest to the sun has more ice even than our own moon."
I Can See Clearly Now
Until March, the only views scientists had of the tiny planet were caught on the fly. Messenger flew by Mercury three times before settling into orbit, and a spacecraft called Mariner 10 flew past in the 1970s.
Together, the spacecraft mapped 98 percent of the planet's surface -- but not always very well. The image above shows a region near Mercury's north pole as seen by Messenger in orbit. Below is the same view, glimpsed during the second flyby.
The sharp image from orbit reveals a region wracked with volcanic activity, including "ghost craters," pre-existing impact craters that were later covered over by lava.
Northern Plains
Mercury's north pole is one of the least well-studied parts of the planet. Messenger is filling in the gaps, revealing much more widespread volcanic activity than scientists expected. The region outlined in yellow above is thought to be a volcanic plain covering more than 1.5 million square miles, around half the size of the continental United States.
"We had some hints of volcanic plains from flybys, but we're seeing their full extent for the first time," said planetary scientist Brett Denevi of the Johns Hopkins University Applied Physics Laboratory. "We're seeing all kinds of evidence for volcanism."
What Happens in Degas
This image compares Mariner 10's view of the crater Degas (at left) to Messenger's high-resolution, colored view. Degas is 32 miles wide and located in Mercury's northern hemisphere. The new image shows that the crater's floor is coated in rock that melted in the heat and pressure of the impact that formed the crater. The melted rock fractured as it cooled, leaving cracks along the crater floor.
Data from Messenger's other instruments hint "that this may have a unique composition, distinct from its surroundings and different from most of the impact melts we see on Mercury," Denevi said.
Etched Rimlets
This image captures details of bright rings of depressions tracing the edge of impact craters. Planetary scientists aren't sure how these "rimlets" form, but they could mark spots where splotches of volcanic material landed after an explosive eruption.
"These features appear quite different form the typical vents we see on the moon and on Mercury," Denevi said. "They're different form anything we've seen before."
Wrinkle Ridge
The wavy wrinkle ridges to the upper left of this unnamed crater formed after the crater was filled with lava. The lava bunched up as it cooled, solidified and contracted.
Music and Literature
This image captures the double rings of Bach crater (named for composer Johann Sebastian Bach, bottom) and the bright crater Alencar (named for Brazilian novelist José de Alencar). Mercury's craters are all named for artists, poets or musicians.
Confusing Composition
This image shows the northern plains in enhanced color, highlighting the differences between eight color filters to make their variations easier to pick out. The variations in color give clues to the chemical composition of Mercury's surface.
Other instruments aboard Messenger, like the X-ray Spectrometer and Gamma-Ray and Neutron Spectrometer, can probe surface composition more directly by measuring the abundance of key elements.
These measurements are helping rule out certain theories of how Mercury formed, like the idea that the sun stripped layers of rock from the infant planet's surface. Mercury may have been Earth-sized when it formed, but lost material in a cataclysmic collision with another proto-planet.
Figuring out how Mercury formed can help planetary scientists learn about how planets form in general.
"We have in our own solar system four experiments on how Earth-like planets evolve once they form under different conditions," said Solomon -- Mars, Earth, Venus and Mercury. "Only one of those experiments made life. It really behooves us to understand in a general way how Earth-like planets form and evolve and operate. What Mercury is telling us is that we didn't understand how planets of the Earth-like class operate in detail. So we're learning new things every day."
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