Mysteries of Mercury: MESSENGER Arrives on Target to Explore the Shrinking Planet

Uesc_07_img0361 The MESSENGER spacecraft, launched on its 5 billion-mile mission in 2004, begins its science operations above the surface of Mercury this week. NASA instruments are gearing up to help unveil the planet's mysteries.

Mercury has always been a bit of an oddball as the only planet in the Solar System that does not follow a basically circular orbit. Earth and Mercury are the only two planets in the Solar System to have a magnetosphere.

MAG will collect magnetic field samples at a rate of up to 20 times per second. These measurements will be used to discover the nature and origins of the planet’s global magnetic field, determine the magnetic properties of its outer layer or crust, and explore new types of space weather that are thought to be unique to Mercury.

The interaction between the very strong flux of charged particle radiation from the Sun at 0.31 Astronomical Units (AUs) and Mercury's magnetic field, for example,  is believed to be responsible for the huge day-to-day variations in this planet's atmosphere. Mercury’s atmosphere is very tenuous and is believed to exist in part due to the sputtering of neutral atoms off the surface by solar wind ions that enter through "holes" in the shield created by this planet’s magnetic field.

MESSENGER captured images revealed that the planet is shrinking, covered in volcanic scars and sporting an unusual “birthmark”. A huge spider shape is fascinating scientists and causing speculation and some debate.

The spider shape captured is "unlike anything we've seen anywhere in the solar system," said mission chief scientist Sean Solomon of the Carnegie Institution of Washington. The image reveals what appears to be some kind of large crater with “legs” radiating out from it.

It is as if "something is pushed up," said MIT planetary scientist Maria Zuber.

The new photos also reveal that Mercury is more colorful then we knew. NASA high-tech enhancement techniques revealed delicate colors. Mercury also had a firey past in the form of heavy volcanic activity. We used to think that Mercury was just a bigger version of our Moon, until these photos revealed more intimate details of the planet’s past.

"It has very subtle red and blue areas," said instrument scientist Louise Prockter of Johns Hopkins University, which runs the Messenger mission for NASA. "Mercury doesn't look like the moon."

Planetary scientist Robert Strom, who was part of both the Mariner 10 and Messenger teams, said, "This is a whole new planet we're looking at."

And Prockter noted "there are some features we haven't been able to explain yet."

As the planet contracts, bits of crust are pushed over another, forming what Prockter calls "wrinkle ridges." As the core of Mercury cools, it contracts and the whole planet becomes smaller. It was once believed that this could also be why Earth has mountains, but the idea was later proven to be wrong in regards to Earth. However, the theory does appear to hold true for Mercury, Solomon said.

Remnants of past volcanoes are scattered across the landscape, and at least one crater seems to be filled with Mercury own version of lava, Prockter said.

The Mercury Laser Altimeter (MLA), led by Goddard Space Center, will map the planet's landforms and other surface characteristics. At the heart of the instrument are a laser that sends light to the planet's surface and a sensor that gathers the light that has been reflected back from the surface. By measuring the amount of time it takes for light to travel to the surface and back, the distance to the surface can be calculated. These distance measurements are taken eight times per second, roughly 500 yards apart on the surface, yielding very accurate descriptions of the topography, or contours, of Mercury's landscape.

MLA data combined with the Radio Science investigation will help map the planet's gravitational field and provide information about the size and characteristics of Mercury's core.

MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a NASA-sponsored scientific investigation of the planet Mercury and the first space mission designed to orbit the planet closest to the Sun. The MESSENGER spacecraft launched on August 3, 2004, and after flybys of Earth, Venus and Mercury, will start a yearlong study of its target planet in March 2011.

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