MESSENGER's images tell the tale of mysterious Mercury
BY STEPHEN CLARK
Posted: 31 March 2011
Fresh images of Mercury show previously unseen regions of the scorching, metallic world as scientists on Earth brace for an onslaught of data shedding light on the planet's internal structure and mineral make-up.
"That's just the barest hint of what we'll have on a regular basis when the mapping phase begins on the 4th of April," said Sean Solomon, MESSENGER's chief scientist from the Carnegie Institute of Washington.
Designed for a one-year reconnaissance mission at Mercury, the MESSENGER probe slipped into orbit around the solar system's innermost planet March 17. Having already circled Mercury 24 times as of Wednesday afternoon, the craft will be ready for science operations this weekend.
It is the first robotic probe to ever orbit Mercury, the last of the solar system's inner planets to be thoroughly explored.
"This extremely dynamic planet, which changes its magnetosphere and atmosphere on timescales of minutes and hours, will be on continuous display for the first time," Solomon said.
Mercury's cratered and dark surface resembles the moon at first glance, but the planet is much more dense than Earth's satellite and has a vastly different history.
"Mercury is actually fairly dark on average," Solomon said. "It is darker than the front side of Earth's moon."
MESSENGER, which stands for the Mercury Surface, Space Environment, Geochemistry and Ranging mission, is funded by NASA and managed by the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.
NASA released the first set of images of Mercury this week since MESSENGER arrived at the planet. The first picture, snapped at about 0920 GMT (5:20 a.m. EDT) Tuesday, shows ejecta from the bright Debussy crater contrasted against Mercury's dark surface.
Debussy is a relatively youthful crater less than a billion years old, and it is surrounded by ray-like patterns of ejecta and smaller craters from material carved out during the catastrophic impact of a comet or asteroid.
MESSENGER flew by Mercury three times in 2008 and 2009, taking pictures and data to build on three flybys during the Mariner 10 mission in 1974 and 1975. But enigmatic craters near Mercury's poles were left unexplored due to the orbital geometry of those fleeting visits.
The first set of images from MESSENGER's new vantage point in orbit begin to uncover some of the mysteries of those regions.
"It shows well some of the very southern craters that may be host for water ice," Solomon said. "That's a hypothesis that we've been aching to test now for 20 years, and we're finally starting imaging and other observations that will be able to peer, with chemical sensors and other instruments, into those crater floors."
All of the spacecraft's seven science instruments are now turned on and operating normally, according to Eric Finnegan, the MESSENGER mission systems engineer at the Johns Hopkins University Applied Physics Laboratory.
In addition to the tens of thousands of images MESSENGER will take of Mercury, the probe will record 10 gigabytes of other data on the planet's topography, chemical composition, tenuous atmosphere and magnetic field, Finnegan said.
All of that data will help researchers investigate Mercury's internal structure, a longtime topic of interest for planetary scientists.
Mercury's robust magnetic field must be driven by activity inside the planet's molten core, according to researchers. But scientists aren't sure why Mercury's magnetic field persisted as convective internal activity waned in larger planets like Mars and Venus.
"The first order expectation is the smaller an object is, the easier it is to lose internal heating," Solomon said. "It requires heat to generate a magnetic field."
"In the Earth, the magnetic field arises from a process known as a dynamo, involving the convective stirring of the fluid metallic iron-rich core of the Earth, and that stirring requires some kind of energy," Solomon said.
MESSENGER is tasked with resolving unanswered questions about Mercury's interrelated interior and magnetic field.
"Mercury is half the size of Mars, measured in terms of mass, yet it has a magnetic field that has lasted 4.5 billion years, we think," Solomon said. "It is surprising that the conditions on such a small planet have allowed a fluid outer core to persist, and have retained enough energy deep in the planet to drive the types of motions in the fluid core we think are required to generate a (magnetic) field with the same type of method, with some differences, we suspect, as that of Earth."
The mission will also map Mercury's tortured surface, along with studying why the barren world is so dense, the planet's geologic history, and the volatile molecules of hydrogen, helium, oxygen and other elements that make up its ultra-thin atmosphere.
"Answering those big questions is really going to take that year-long campaign," Solomon said. "That said, we're already beginning that campaign, so addressing the composition of the surface requires the dedicated geochemical remote sensing that is already ongoing."
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