Understanding surface temperature is important for habitability. Recent work on Mars has found that the dependence of surface temperature on elevation (surface lapse rate) converges to zero in the limit of a thin $\mathrm{CO_2}$ atmosphere. However, the mechanisms that control the surface lapse rate are still not fully understood. It remains unclear how the surface lapse rate depends on both greenhouse effect and surface pressure. Here, we use climate models to study when and why “mountaintops are cold”. We find the tropical surface lapse rate increases with the greenhouse effect and with surface pressure. The greenhouse effect dominates the surface lapse rate transition and is robust across latitudes. The pressure effect is important at low latitudes in moderately opaque ($\tau \sim 0.1$) atmospheres. A simple model provides insights into the mechanisms of the transition. Our results suggest that topographic cold-trapping may be important for the climate of arid planets.

Long term climate change on early Mars is characterized by a shift in the spatial distribution of rivers and lakes. Geological datasets suggest earlier paleo-rivers prefer higher surface elevations compared to rivers that formed later (Kite, 2019). On the other hand, modeling work also suggest a transition of surface lapse rate that comes with atmospheric escape throughout the Martian history (Wordsworth, 2016). The surface lapse rate follows the atmospheric lapse rate, which is close to dry adiabatic, when the CO2 atmosphere is thick, but decouples when the atmosphere is thin. Figuring out the surface temperature distribution on early Mars is critical, because it tells us where the water sources from ice/snowmelt would have been during warming episodes. We use the MarsWRF GCM to explore the transition of river-forming climates. We assume the atmosphere is CO2-only, but allow additional greenhouse warming by a gray gas scheme. To simplify the relation between elevation and surface temperature, we set 0 obliquity and include simulations with both idealized topography and real topography. The range of surface pressure is between 0.01 bar and 2 bar. We use a surface energy budget framework to analyze outputs (Fig. 2). Under the framework, variations in surface emission LWs correspond to surface temperature variations. We find greenhouse heating LWa is the only term that scales with surface temperature under high PCO2, in contrast to predictions from the previous literature that sensible heat SH was the cause of the regime transition (Wordsworth, 2016). This conclusion does not change with switching to realistic topography or switching CO2 radiation to a gray gas scheme. Under the low Ps but high-optical-depth κ gray gas case, the surface lapse rate still follows the atmosphere, so the regime transition can be attributed to the evolution of greenhouse gases other than CO2. In future, we will add a surface liquid water potential algorithm to link the surface energy balance to paleo-river observations. Assuming surface liquid water is formed during transient ice-melting period, surface liquid water potential can be calculated from the Ts and Ps distributions. The output will be compared with different historical epochs to find the best-fit scenario with both CO2 and non-CO2 greenhouse forcing.

The Kepler mission revealed that sub-Neptunes are about as common as stars, which defied our pre-existing notion of planet demographics. The prevailing view for sub-Neptunes was that they are mostly core by mass and atmosphere by volume (Lopez & Fortney, 2014). However, current formation models do not consider dissolution at the atmosphere-core interface. The temperature and pressure at the magma-atmosphere interface can rise to >3000 K and ~5-30 GPa (Lee et al., 2014; Piso et al., 2015), high enough for dissolution of hydrogen gas into the magma (Chachan & Stevenson, 2018). The dissolution of atmosphere into the magma may explain the drop-off in exoplanet abundance at 3 times Earth radius (Kite et al., 2019), but the puff-up of the magma due to gas dissolution has not previously been included. We propose a simple model to calculate sub-Neptune mass-radius relation, including, for the first time, the puff-up effect. Key assumptions include: (1) nonlinear solubility of gas in magma is constrained by limited laboratory data (Hirschmann et al, 2012); (2) the Fe/core mass fraction is Earth-like, and He/gas mass fraction is Solar-like; (3) ideal mixing between the dissolved gas and magma; (4) the dissolved gas is well mixed within the magma-layer. The EoS used are an Mg2SiO4 for the magma (Stewart et al., 2020); the H/He EoS (Chabrier et al., 2019); and a simple model for Fe (Seager et al., 2007). The model is integrated from the radiative-convective boundary and iterated until atmosphere-magma solubility equilibrium. We have varied the core mass, atmospheric mass and equilibrium temperature in the atmosphere. Our preliminary results are shown in Figures. The critical point for the puff-up of the core due to the dissolved gas corresponds to ~1% solubility at the magma-atmosphere boundary (Fig. 1). The puff-up effect can be important up to 0.3 Earth radius (Fig. 2), much larger than the radius error bars for a single planet in the CKS survey with Gaia DR2 data (Fulton & Petigura, 2018). In future, we will add additional constraints on gas/core mass fraction (Lee, 2019), forward-model the relationship between mass and photospheric radius, and generate predictions for exoplanet masses and radii that can be used to help interpret data from ESA’s PLATO and NASA’s TESS.

\justifying The wet-to-dry transition of Mars is recorded by a changing distribution of rivers, which was interpreted as the result of a decorrelation between surface temperature and elevation with a thinner atmosphere. Here we use a climate model to re-examine the interpretation. We find that the weakening of the greenhouse effect, rather than atmospheric pressure, accounts for the decorrelation. The decorrelation happens near surface pressure $ \sim 0.1$ bar for a pure $\mathrm{CO_2}$ atmosphere, or longwave optical depth $\sim$ 1 for a gray gas. Under the weak greenhouse limit, the surface is mainly heated by insolation and thus surface temperature decorrelates with elevation. Under the strong greenhouse limit, the surface is mainly heated by the atmosphere, whose temperature is controlled by advection and convection over the lowlands. Our results suggest that the correlation between river distribution and elevation should be re-examined with different greenhouse forcings and a low atmospheric pressure.

The modern “wet” tropics are dominated by the Intertropical Convergence Zone, however “dry” tropics likely occurred in Earth’s history. It is unclear how the tropics change between wet and dry climates because recent progress has focused on modern and warmer climates. We show the tropical hydrological cycle undergoes a wet-to-dry regime transition when surface wetness is decreased in a general circulation model. The dry regime occurs when precipitation is suppressed by negative evaporation. The regime transition is dominated by near-surface relative humidity, in contrast to our traditional understanding which assumes changes in relative humidity are small. We show near-surface relative humidity changes are controlled by re-evaporation of stratiform precipitation. The moistening effect of re-evaporation is non-local: re-evaporation happens near the lifting condensation level and moisture diffuses downward to the near-surface. Our results provide a first step toward understanding tropical hydrological cycle changes between wet and dry climates.

Fluvio-lacustrine features on the martian surface attest to a climate that was radically different in the past. Since climate models have difficulty sustaining a liquid hydrosphere at the surface, multiple cycles of runoff episodes may have characterized the ancient Mars climate. A fundamental question thus remains: what was the duration of these runoff-producing episodes? Here we use morphometric measurements from newly identified coupled lake systems (containing both an open- and a closed-basin lake). We combined hydrological balances with precipitation outputs from climate models, and found that breaching runoff episodes likely lasted 10^2–10^5 yr; other episodes may have been shorter but could not be longer. Runoff episode durations are model-dependent and spatially variable, and no climate model scenario can satisfy a unique duration for all coupled systems. In the near future, these quantitative constraints on early Mars lake persistence may be tested through in situ observations from Perseverance rover.