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The physical mechanism by which the Solar system was formed some 4.6 billion years ago is still poorly understood despite the vast wealth of new data about the structure of the planetary system that has been acquired. The modern Laplacian theory (MLT) of solar system formation (Prentice 1978, Prentice 2008) is able to readily account for many of the observed properties of the planetary system and the Galilean satellite system of Jupiter, such as the well-defined orbital gradations of physical density and chemistry. This uniform gradation stems from the fact that the temperatures of the gas rings scale closely as the inverse of their mean orbital radii. The Cassini-Huygens mission to Saturn and Titan has thrown a serious challenge to the MLT since the physical densities of the mid-size satellites show no obvious correlation with their orbital distances from the planet. In addition, Titan is $\sim$60 times more massive than the value expected by the theory for a native moon of Saturn. In this talk I show that if one allows for the possibility that Titan may be a captured moon of Saturn, which disrupted Saturn's pre-existing family of outer moons, and if one allows for the dissolution and expulsion of water from Saturn's central rock/ice core during contraction of the proto-Saturnian cloud, then much of the present structure of the Saturnian satellite system can be explained. The MESSENGER mission to Mercury has also challenged the MLT since the theory predicts that Mercury formed at such a high temperature (1630 K) that all iron exists in pure elemental form. But if Mercury's core consisted of pure iron, it should also presently be solid, even if it was wholly molten in the beginning. MESSENGER has, however, confirmed that Mercury has a dipole magnetic field, attesting to an active dynamo and hence to the existence of an outer molten core. Such a melt layer is readily achievable if Mercury contains a large store of iron sulphide. This has a much lower melting temperature than pure iron. This suggests that Mercury may not have accreted from a gas ring but instead drew its material from an extended disc of condensate which extended as far as the orbit of Mars. The talk will be illustrated with colourful images obtained from NASA's interplanetary space program.
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