UCLA

The three planets of the Inner Solar System with significant atmospheres, Venus, Earth, and Mars, can be described as representing three “climate archetypes” of terrestrial planets: Venus is hot, dry, and rotates slowly; Mars is cold and dry, with fast rotation similar to Earth; Earth is the “middle ground”, warm enough to sustain liquid water on its surface but not so warm it evaporates away. These archetypes can be placed as endpoints on a spectrum of climates, where adjusting one or more planetary parameters can move a climate from one archetype to another, e.g. drying the surface can move an Earth-like planet towards the Venus and Mars archetypes. In addition to the three inner planets, there is one additional body in the Solar System that has a thick atmosphere and solid surface: Titan, a moon of Saturn. Titan presents a unique opportunity in observable planetary climates because it has a volatile liquid, or condensable, on its surface in the form of methane. This methane is able to evaporate to form clouds (Turtle et al., 2018) and likely rain (Turtle et al., 2011), but is mostly restricted to large polar lakes (Lunine and Lorenz, 2009) with the rest of the surface a vast desert (Mitchell and Lora, 2016). This means Titan’s climate archetype is between the ocean-dominated Earth and the fully-dry Venus/Mars.In this dissertation, we seek to further investigate the “in-betweens” of these climate archetypes, focusing on the transition between an Earth-like planet and a Titan-like one. To accomplish this, we recreate a Titan-like climate using an Earth-like global climate model (GCM) by varying a small set of planetary parameters. We first limit the available water by placing a continental land strip centered on the equator and varying its width. This mimics Titan’s dry tropics and wet poles, and could be similar to past continental arrangements in Earth’s history. Second, we take three of these land strip widths and vary the rotation period, starting with Earth’s rotation and moving towards Titan’s (16 Earth days). Third, for the same three land strip widths and using Earth’s rotation, we vary the volatility of the condensable via a constant multiplied to the saturation vapor pressure. Titan’s condensable, methane, is more volatile under Titan’s surface conditions than water is on Earth, resulting in high specific humidities. By artificially increasing the saturation vapor pressure, we can approximate this effect without changing the properties of the condensable. We find that simply replicating Titan’s parameters in our simulations does not fully reproduce Titan-like conditions. In addition, we find that it is possible to reproduce key Titan-like features by varying only the width of the equatorial land strips. This may indicate that there are many possible “in-between” states an Earth-like planet can have that span the gap between the Earth and Titan climate archetypes. It also suggests Titan’s current climate is primarily dependent on its surface liquid distribution, meaning an Earth-like planet with similar topography is likely to display the same features.