The huge bounty of new data about the physical and chemical makeup of Mercury and Titan that has been acquired by the NASA MESSENGER and Cassini-Huygens spacecraft since 2008 and 2004, respectively, coupled with the wealth of new data about Ganymede that was obtained through the Galileo Project between 1996 and 2003, has greatly sharpened our understanding of these 3 remarkably different planet-sized bodies. The only similarity between the three is that they all have roughly the same physical diameter, namely ~ 5000 km. Mercury has an unusually large core of metal, whose mass makes up ~71% of this planet’s mass. It also has an active magnetic dynamo, indicating that the outer portion of the core is molten. Titan is distinct because of its massive atmosphere of N2 and CH4. Since CH4 is being continuously destroyed by solar photolysis, there must be some geophysical mechanism acting within its icy mantle, most likely tidal heating, which serves to bring fresh CH4 to the surface. The mean density of 1.881 g/cm3 indicates an internal H2O ice mass fraction of ~0.490. Titan has no magnetic field and its shape is slightly oblate, possibly indicating that Titan first condensed in a solar obit before being captured by the Saturn system (Prentice 1984 Earth, Moon & Planets 30 209; 2006 PASA 23 1). Ganymede does have a large native magnetic field. It is not known if this is dynamo-driven or a frozen-in thermoremanent field. The estimated H2O ice mass fraction is ~0.475.
I present a unified set of thermal and structural models for Mercury, Titan and Ganymede which attempt to account for much of the new observational data. These models are based on the author’s modern Laplacian theory of solar system formation, according to which both the planetary system and the regular satellite systems of the gas giant planets condensed from concentric families of orbiting gas rings (Prentice 2006; 2008 LPSC abstract #1945 – see URL).