Mercury Flyby Reveals Bright Craters, Long Rays

Anne Minard
for National Geographic News
October 7, 2008
A new look at the solar system's innermost planet is revealing bright young craters and an extensive pattern of rays, suggesting that Mercury undergoes weathering processes like those on the moon.

NASA's MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft turned toward Earth in the wee hours this morning and began transmitting images and data from its second planetary flyby.

A previous flyby in January was the first in a series of maneuvers designed to position MESSENGER in orbit around Mercury in 2011. That encounter imaged 20 percent of the planet's surface that had never been seen before.

The latest images represent the first spacecraft views of the northern portion of Mercury, encompassing another 30 percent of the surface missed during previous missions.

"It's been a wonderful day," said MESSENGER principal investigator Sean Solomon at the Carnegie Institution of Washington. "I'm excited that we have a nearly global perspective."

Mercury appears to harbor "extremely deformed regions" and "extraordinarily bright craters, with rays that extend halfway across the planet," Solomon added.

The craft will continue to beam data and images to Earth through Wednesday morning.

Craters and Rays

Based on studies of the lunar surface, Solomon said, the freshest craters on Mercury are probably bright because their minerals haven't yet been exposed to weathering. (Watch a crash-course video about lunar science.)

But more detailed studies are needed to fully explain the features, as Mercury and the moon have some key differences.

The biggest contrast is that iron, which is on the moon, hasn't yet been found on Mercury.

Fresh craters on the moon appear bright because material is newly exposed. But as the craters age, weathering of iron makes the lunar features look darker.

Without iron, some other material must be responsible for the color differences between old and new craters on Mercury, Solomon said.

"There is some difference in the detail, but it looks to be a space weathering process," he said.

Mission scientists say the most striking new observation so far is a large pattern of rays that appears to extend from a young crater in the north to regions south of a bright crater called Kuiper.

The Mariner 10 spacecraft first saw Kuiper when it flew by Mercury in 1974 and 1975, capturing images of about 45 percent of the planet's cratered surface.

The young, extensively rayed crater north of Kuiper, along with a prominent rayed crater to the southeast of Kuiper, were both seen in Earth-based radar images of Mercury but had not been imaged by spacecraft until now.

The rays are formed when material is ejected from the planet's surface during meteor impacts, Solomon said. On the moon, such features can last for hundreds of millions of years.

Although astronomers haven't teased out the geologic time scales for Mercury, "a comparable fraction of its craters have rays," he said, which suggests the planet developed along similar time scales as the moon.

Working Journey

MESSENGER, which is now about 61 million miles (99 million kilometers) from Earth, is expected to send back 1,200 pictures from its latest flyby.

Scientists are hoping that the data will continue to reveal more about geologic features on the largely unstudied planet.

Images from the first flyby, for example, have shown that tiny Mercury has a history of violent volcanism and is experiencing constant meteor bombardment.

The craft's next swing past Mercury is slated for September 2009, and it is expected to enter into orbit in March 2011.

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