"MESSENGER" stands for Mercury Surface, Space Environment, Geochemistry and Ranging mission. The science package includes:

• Mercury Dual Imaging System: Pivoting narrow- and wide-angle CCD cameras that will map Mercury's surface in black-and-white, color and stereo. Maximum resolution of the narrow-angle camera is 60 feet.
  
  
• Gamma-Ray and Neutron Spectrometer: The Gamma-Ray Spectrometer will measure gamma rays given off by atoms in Mercury's crust that have been struck by cosmic rays to identify elements in the crust. The Neutron Spectrometer will look for hydrogen as part of a search for water ice.
  
  
• X-ray Spectrometer: Designed to map elements in the uppermost millimeter of Mercury's soil.
  
  
• Magnetometer: Mounted on a 12-foot-long boom, this instrument will chart Mercury's magnetic field.
  
  
• Mercury Laser Altimeter: Measures the time taken by a laser beam to bounce off the surface to compute the relative height of landforms on the surface with an accuracy of one foot at a distance of 620 miles.
  
  
• Mercury Atmospheric and Surface Composition Spectrometer: Measures atmospheric compounds and surface minerals.
  
  
• Energetic Particle and Plasma Spectrometer: Maps Mercury's magnetosphere by studying electrically charged ions and electrons trapped by the planet's magnetic field.

Artist's impression of the MESSENGER spacecraft in orbit at Mercury. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

"One of the prime reasons (to go there) is that Mercury holds some secrets to how all the planets in the inner solar system got put together," Solomon said in an earlier interview for the book "Space Odyssey: Voyaging Through the Cosmos." "We know it's density is exceptionally high for a planet of its size and we know, therefore, that it's made mostly out of iron, the most abundant heavy element.

"So the current thinking is it's probably mostly a core of metal surrounded by a thin shell, maybe (370 miles) thick, of rocky silicates. The questions is, how did Mercury end up mostly metal? There are some competing ideas and what is nice about the ideas is they make some different predictions for what the rocky part of the planet should look like chemically.

"So we can take chemical remote sensing instruments and answer very fundamental questions about how Mercury got put together and by inference, what were the important processes that affected all of the inner planets."

There are three competing ideas to explain Mercury's oversize core. It is possible, some scientists believe, that the solar nebula that gave birth to the solar system, through the action of gases on lighter particles, naturally concentrated metals close to the sun before the planets coalesced.

Another theory holds that after Mercury formed, a sudden increase in the sun's energy output boiled off the planet's rocky outer layers. If so, Mercury's crust should be relatively devoid of silicates. Yet another theory holds that a body one sixth the size of the original planet crashed into Mercury near the dawn of the solar system and blew off its outer crust.

"These competing ideas for how Mercury ended up so metal rich make very specific predictions for the chemistry of the crustal material we can see on Mercury's surface," Solomon said. "So we hope to distinguish among those competing ideas by chemical remote sensing of exactly what's there at Mercury's surface."

Another major question is whether or not Mercury's core is solid or whether the outer region is liquid like Earth's.

"We also don't know whether the magnetic field of Mercury detected by Mariner 10 arises from motions in a fluid outer core as the Earth's magnetic field arises on our planet," Solomon said. "MESSENGER will make key measurements that will address the question of the size and the state of the core and of the origin of the magnetic field.

"By measuring Mercury's gravitational field and characteristics of its rotational rate, we can determine the size of the core and whether the outer core is fluid. By mapping the magnetic field, we can distinguish among competing ideas for what is generating that field today."

Radar observations from Earth show deposits of reflective material inside craters near Mercury's north pole that are permanently shadowed. Those deposits could be ice, left over from comet impacts.

"Because the spin axis of Mercury has almost no tilt, a crater near a pole, the south or north pole of Mercury, does not see the sun, it's in permanent shadow," Solomon said. "Mercury has such a thin atmosphere that the atmosphere does not transport heat from the equator to the poles as our atmosphere dos on Earth.

"And so, the floor of a shadowed crater, the temperature is extremely cold - minus 300 degrees F or colder - cold enough to not only freeze out any volatiles like (water) but to keep it in a solid state for the lifetime of the planet and the solar system."

But there are competing theories and elemental sulfur or even supercold silicates could produce similar radar reflections. MESSENGER will try to find out one way or the other.

MESSENGER also will study Mercury's atmosphere or, more properly, its exosphere. The density of the exosphere is so low particles do not collide with each other. Questions include how the solar wind affects the exosphere and how it interacts with the magnetosphere.

"We're looking at some very general processes and Mercury stands out as a model in composition, it's magnetic field, its ability to retain ice or other volatiles despite having a high surface temperature over most of the surface," Solomon said in the earlier interview. "So there are a host of very general processes that are at their extreme on Mercury. That's where we're going."

MISSION PREVIEW
   PART 1: MESSENGER MISSION READY TO LAUNCH
   PART 2: THE LONG AND WINDING ROAD
   PART 3: THE 'HEAT' IS ON
   PART 4: A COMPLEX SCIENCE AGENDA