The space probe, Messenger (MErcury Surface, Space ENvironment, GEochemistry, and Ranging ) is outfitted to learn just what Mercury is made of. With only one chance during its seven years in space which began in 2004, Messenger swung itself towards Mercury early this month. Two weeks after successfully maneuvering into orbit around the planet, and verifying that everything about the ship can work in the harsh thermal (840 degrees F to minus 350 degrees) environment, the spacecraft is ready to rock and roll.

Mercury Space Probe Messenger (MErcury Surface, Space ENvironment, GEochemistry and Ranging)
Along with Earth, Mercury is one of only four terrestrial (rocky) planets. Venus and Mars round out the group. These four planets are made up of a dense iron-rich core surrounded by a rocky mantle, composed largely of magnesium and iron silicates, which are neither green, nor any kind of milk product.

Exploring 4.9 billion miles (7.9 billion kilometers) away from its handlers on Earth, Messenger has six objectives, one of which is to provide major-element maps and determine local composition and mineralogy. Completion of the tasks must be accomplished within two Mercury solar days (equivalent to one Earth year), while keeping the spacecraft sunshade facing the Sun all the time. The trick is to take a quick peek since the spacecraft passes over a given spot on the surface only two times, six months apart, during its mission.
Mercury wins hands down as the smallest, most dense planet, with the oldest surface, and the largest daily fluctuations in surface temperature. Being the closest to the sun, it is the least explored. As Stuart Atkinson, author of children?s astronomy books, indicates in his poem, Secrets, we hope to find clues as to the origin and history of our solar system, "hidden in the Sun?s blinding glare."
Data garnered from Mariner’s many journeys past the planet indicates that Mercury has a huge iron-rich core and a global magnetic field. Its core is expected to be twice that of the other three terrestrial planets, with an estimate that the core makes up 75 percent of the radius of the entire planet. Thus, deduction tells us that the mantle is relatively thin. Much of its core probably is still liquid. We can liken Mercury to a large hot, melted cheese fondue pot that you?d dip chunks of French bread into. 

Space Probe Messenger carries a lot of instruments that should help scientists to decypher the secret Mercury hides...

Space Probe Messenger carries a lot of instruments that should help scientists to decypher the secret Mercury hides…

Messenger is scheduled to switch its investigative instruments into action on March 23. Come April 4, we should be getting the first data and learn what Mercury is made of. Three major theories try to explain why Mercury beats Earth, Venus and Mars in density and metal content. None of the three theories points to a composition of green cheese, however.

One theory is that tremendous heat of an early nebula vaporized part of the outer layer of rock while Mercury was forming, eliminating volatile elements, such as sodium and potassium.
Others think the primordial crust and upper mantle of the planet were stripped away by a giant impact, resulting in a loss of aluminum and calcium. In fact, the largest feature discovered on the surface so far is called the Caloris basin, 960 miles (1,550 kilometers) in diameter, probably created by an ancient asteroid impact. It is one of the targets Messenger will use to study the composition of the planet?s surface.
We won?t know which theory pans out until the composition of the rocky surface can be measured. Messenger?s science instruments should be preparing to work as you read this. The Mercury Dual Imaging System (MDIS), the Gamma-Ray and Neutron Spectrometer (GRNS), the X-Ray Spectrometer (XRS), the Magnetometer (MAG), the Mercury Laser Altimeter (MLA), the Mercury Atmospheric and Surface Composition Spectrometer (MASCS), and the Energetic Particle and Plasma Spectrometer (EPPS) are set to be very busy. The instruments communicate to the spacecraft through fully redundant Data Processing Units (DPUs). 
1954 Mercury "Merc" - the car one of our contributing editors used to learn how to drive.On March 29, the MDIS will be powered on and will take its first image from over the planet?s south pole. Hovering as close as 124 miles from the surface, Messenger will capture images of areas not previously exposed to spacecraft. The closest amateur astronomers may get to Mercury is the 1954 automobile that bears its name, outfitted not with scientific instruments, but with optional power steering and power brakes, a four-way front bench seat, electric window lifts, and Merc-O-Matic Drive.
For the rest of us, during its year long science observation which begins April 4, Messenger [Space Probe Messenger PDF Sheet at NASA] will be presenting us with previously unknown, and exciting, facts.