A  Mercury has been tidally despun since it's time of formation.

 1. Assume an initial rotation rate of 20 Earth hours.  Current rotation rate is 58.65 Earth days, and an orbital period of 87.97 Earth days.  (Ratio of centrifugal potential and rotational potential is now quite low.)

 2.  Ellipticity of Mercury's gravational potential (J2) is larger than the value expected for a planet that formed at a rotation rate of 58.65 Earth days.  This implies formation at a much higher rotation rate and tidal despinning.

B. Evidence for iron core.

 1.  The bulk density of Mercury (5.420 gm/cm³)

 2.  Mercury's mass is only 1/18 th of Earth, implying very little increase in density due to compression.

 3.  Hence, these observations suggest an iron core, comprising 60% of the planet's mass.  Rock (silicate) mantle would be 600 km thick. Radius of planet is 2439 km.  Predicted moment of inertia would be 0.325 Ma² (not yet verified).

C. Surface features.

 1.  Heavily crated like Earth's moon.

 2. Fault scarps are found over a significant fraction of its surface.

 a  Faults are thought to be caused by cooling contraction, with the radius decreasing by 1-2 km since its formation.

 b.  Hyptothesis 1 for faulting: A thin lithosphere formed over a liquid core during the time of tidal of tidal despinning (first billion years). Faulting occured during this period. A pattern of normal faulting near the poles and strike slip faulting near the equator occurs.

 c.  Hypothesis 2 for faulting: Faulting occured more recently than first billion years.  This agrees with global thrust faulting currently seen, which may have erased an earlier pattern of faulting.
 

D. Thermal Structure

 A.  A magnetic field is observed, implying a convecting liquid iron core.  This is used to constrain the temperature structure -- the core-mantle boundary must be at least as high as the temperature of the Earth's core-mantle boundary (4000-6000o K).

 B.  Problems with thermal model:  how can such a small planet still have enough heat left to stir a liquid core by convection.  Most radiogenic elements would have segregated upward into the mantle early in its formation.  Thus there must be some kind of energy source making the liquid core convect.  Possible sources might be:

 (a) Tidal strain -- too small to account for substantial heating.

 (b) Radioactivity in core -- too small if segregation of principal radiogenic material concentrates them in the mantle.

 (c) Gravitative differentiation and solification of a solid iron core -- similar to Earth but not yet tested for Mercury.

Mercury images and recent review of Mariner 10 data: see the article Mercury unveiled by G.J. Taylor on-line.
 

New NASA Missions to Mercury:

       As a result of renewed interest in Mercury, there are two related proposals being developed as potential
       Discovery class missions. Discovery is NASA's new "cheaper, better, faster" line of solar system exploration
       spacecraft. These missions are capped at $150 million total mission costs. The two Mercury proposals are the
       Mercury Polar Flyby (MPF) and Hermes (Mercury orbiter). MPF's instruments include a neutron spectrometer
       (water detection), dual polarization radar (subsurface ice mapping), camera (imaging polar region and
       hemisphere not imaged by Mariner 10). We believe a flyby is cheaper and more technically feasible. MPF is
       designed to have multiple Mercury encounters at aphelion only. At aphelion a spacecraft only has to endure the
       equivalent of four times the Earth solar flux. The orbit of Mercury is eccentric such that at perihelion there is
       eleven times Earth solar flux. An orbiter would have to endure such conditions requiring elaborate (and
       expensive) cooling and thermal shielding systems.