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Mercury: The Key to Terrestrial Planet Evolution

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Question 1: Why is Mercury so dense?

picture of solar nebula

This artist's depiction of the early solar nebula shows the time when the terrestrial planets were forming. Processes such as nebular gas drag, vaporization in the hot early nebula, and giant impacting collisions have all been suggested as possible processes that significantly affected the bulk composition of Mercury. (Painting copyright by William K. Hartmann)

Each of the terrestrial planets consists of a dense iron-rich core surrounded by a rocky mantle, composed largely of magnesium and iron silicates. The topmost layer of rock, the crust, formed from minerals with lower melting points than those in the underlying mantle, either during differentiation early in the planet's history or by later volcanic or magmatic activity. The density of each planet provides information about the relative sizes of the iron-rich core and the rocky mantle and crust, since the metallic core is much denser than the rocky components. Mercury's uncompressed density (what its density would be without compaction of its interior by the planet's own gravity) is about 5.3 grams per cubic centimeter, by far the highest of all the terrestrial planets. In fact, Mercury's density implies that at least 60% of the planet is a metal-rich core, a figure twice as great as for Earth, Venus, or Mars! To account for about 60% of the planet's mass, the radius of Mercury's core must be approximately 75% of the radius of the entire planet!

There are three major theories to explain why Mercury is so much denser and more metal-rich than Earth, Venus, and Mars. Each theory predicts a different composition for the rocks on Mercury's surface. According to one idea, before Mercury formed, drag by solar nebular gas near the Sun mechanically sorted silicate and metal grains, with the lighter silicate particles preferentially slowed and lost to the Sun; Mercury later formed from material in this region and is consequently enriched in metal. This process doesn't predict any change in the composition of the silicate minerals making up the rocky portion of the planet, just the relative amounts of metal and rock. In another theory, tremendous heat in the early nebula vaporized part of the outer rock layer of proto-Mercury and left the planet strongly depleted in volatile elements. This idea predicts a rock composition poor in easily evaporated elements such as sodium and potassium. The third idea is that a giant impact, after proto-Mercury had formed and differentiated, stripped off the primordial crust and upper mantle. This idea predicts that the present-day surface is made of rocks highly depleted in those elements that would have been concentrated in the crust, such as aluminum and calcium.

MESSENGER will determine which of these ideas is correct by measuring the composition of the rocky surface. X-ray, gamma-ray, and neutron spectrometers will measure the elements present in the surface rocks and determine if volatile elements are depleted or if elements that tend to be concentrated in planetary crusts are deficient. A visible-infrared spectrograph will determine which minerals are present and will permit the construction of mineralogical maps of the surface. Analysis of gravity and topography measurements will provide estimates of the thickness of Mercury's crust. To make these challenging, measurements of Mercury's surface composition and crustal characteristics, these instruments will need to accumulate many observations of the surface. MESSENGER's three Mercury flybys provided opportunities to make preliminary observations, but numerous measurements from an orbit around Mercury are needed to determine accurately the surface composition. Once in orbit, these measurements will enable MESSENGER to distinguish among the different proposed origins for Mercury's high density and, by doing so, gain insight into how the planet formed and evolved.

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