Atmosphere of the Early Earth

The Earth's atmosphere has changed a lot since our planet formed. The present atmosphere is mostly made of nitrogen (N₂) and oxygen (O₂), but we know that O₂ wasn't a major component of the atmosphere before about 2.5 billion years ago. Prior to that time, the atmosphere was probably mostly made out of N₂ and CO₂. However, there are no records of what the atmosphere was like during that time or earlier, so we have to rely on models to figure out what the atmosphere was made of in the earliest period of the planet (the time during which life first began). One way to approach this is to look at the kinds of materials that the Earth was made out of (e.g. meteorites) and try to calculate the composition of the atmospheres that they would produce when heated up, which simulates outgassing from a hot planetary interior.

Core Formation on super-Earths

Super-Earths - large rocky planets for which we have no analog in the Solar System - have much higher internal pressures than those found in the Earth. The effect of this higher pressure on the properties of mantle materials is mostly unconstrained, but may be significant. It's effect on viscosity may make mantle convection, and therefore heat transport and mantle outgassing, more sluggish. The high pressure may also effect chemistry in the deep interior which could hinder full interior differentiation into a silicate mantle and an iron core. Partitioning experiments at very high pressures indicate that the phase behavior that allows metal and silicate to separate during planetary differentiation may change behavior at pressures higher than those reached in the Earth's interior. Work on this issue is published in the Astrophysical Journal.

Ocean Formation and Planetary Thermal Evolution

The search for rocky exoplanets is essentially the search for possible habitable planets in other solar systems. However, astrobiologists believe that in order for life to originate on a planet, that planet must have liquid water (i.e., oceans). In our own Solar System, the Earth is the only example of a planet with surface oceans, but the presence of water on the surface is tied to our unique geological system of plate tectonics. On the Earth, volcanic outgassing of water from the mantle is balanced by loss of water to the mantle through subduction of water-rich oceanic seafloor. Much of this water is released immediately back to the surface through shallow, water-induced volcanism. However, a small but significant fraction of the water can be transported to deeper levels of the mantle. Mantle convection has therefore played an important role in controlling the size of Earth’s surface oceans over the planet’s lifetime.

The deep water cycle of Earth has been studied with parameterized convection models incorporating a water-dependent viscosity. The abundance of water in the mantle, which lowers the convective viscosity, evolves along with the mantle temperature. I have created a parameterized convection model that is extended to high pressures in order to study the deep water cycles of super-Earths. Assuming compositions similar to the Earth, our models indicate that ocean formation will be delayed on 5 M_Earth planets by ~1 Gyr after planet formation. Although ocean mass on these planets increases with time, the oceans remain much shallower than for smaller planets, consistent with previous studies. Intermediate mass planets (2-4 M_Earth) have immediate, but gradual outgassing and persistent oceans. Small terrestrial planets (≤ 1 M_Earth) have rapid initial outgassing, but will gradually lose a significant fraction of their surface oceans due to mantle sequestration over their lifetimes.