Dune, Waterworld, and Everything in-between: Creating a Titan-like Climate on an Earth-like Planet

We recreate a Titan-like climate using an Earth-like global climate model (GCM) by varying a small set of planetary parameters. Understanding the range of possible climate states for Earth-like planets is important for interpreting exoplanet observations and Earth’s own climate history. We find that simply reducing the available water at the surface does not fully reproduce Titan-like conditions. 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. We use three observationally motivated criteria to determine Titan-like conditions: 1) the peak in surface specific humidity is not at the equator, despite it having the warmest annual-mean temperatures (Ádámkovics et al. 2016); 2) the vertical profile of specific humidity in the equatorial column is nearly constant through the lower troposphere (Niemann et al. 2005); and 3) the relative humidity near the surface at the equator is significantly lower than saturation (lower than 60%; Niemann et al. 2005; Tokano et al. 2006). 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. Land strips alone allow some experiments to meet two Titan-like criteria, but none show the near-constant vertical profile of specific humidity. 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). Slowing the rotation results in fewer experiments meeting any of the Titan-like criteria due to increased access to oceanic moisture from the widened Hadley Circulation. For the same three land strip widths and using Earth 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. Experiments with a volatility constant of 2.5 (the maximum used in this work) meet all three Titan-like criteria, demonstrating that an Earth-like planet can display Titan-like climatology by changing only a few physical parameters.