| Abstract |
There are three primary ingredients for planets: gas (meaning predominantly hydrogen), ice (dominated by water molecules) and rock. Here, “rock” includes iron, much of which is usually in a metallic phase. This talk will be primarily about bodies that are almost entirely “rock”, though it will prove necessary to say something about the role of the other very cosmically abundant materials.
There are perhaps five primary features of Earth: (1) A mantle and well-‐ separated core. The presence of this primary layering is a consequence of immiscibility between core forming and mantle forming materials and would not happen otherwise, despite the much higher density of iron. (2) A heat engine that expresses itself through two dynamical features: mantle convection and generation of a magnetic field. In the case of Earth, mantle convection expresses itself through plate tectonics, but this is not mandatory and the precise requirements for plate tectonics are not well understood. The persistence of a core dynamo is also imperfectly understood and may depend on “details” (e.g., the nature of the light elements in the core). (3) A crust that arises from partial melting of the mantle. The most abundant crust (basalt) is recycled but some crust (primarily continental) accumulates at Earth’s surface. Water is probably important to this process. (4) A water ocean that profoundly affects the climate and landscape. Water participates in a cycle, affecting the mantle, subduction and volcanism. This water cycle is a central feature of Earth in multiple ways and is imperfectly understood. (5) An atmosphere whose properties depend on the overall chemistry of Earth, the provenance of materials delivered and interaction with the surface and interior. The atmosphere cannot be understood independently of the rest of the planet.
These five primary features form the basis for examining the properties of Earth and Earthlike planets. It is evident that in many cases we are unsure why Earth behaves as it does; this suggests caution when claiming to know what exoplanets might do. I will discuss each of these features in turn and assess the extent to which they are generic (common features of all “earthlike” planets) and the extent to which they are special (merely one of many possible outcomes for such a planet). |
