Faint Young Sun Paradoxes and Habitable Worlds: Comparative Climatology of Early Earth and Mars

DESCRIPTION: We will combine research tools that include observation, theory and 3-D climate modeling to more fully characterize and compare the habitable states of Earth and Mars at simultaneous points in early Solar System history. Through this systematic comparison, we will be able to more clearly delineate answers to questions such as: 1) When did at least regionally habitable conditions become established on each world (e.g., did Earth have a significant head start in providing habitat space)? 2) Over what time span could Earth and Mars have been contemporaneously habitable? 3) What is the feasibility of one world acting as refuge for life after catastrophic destruction (e.g., meteorite bombardment) of the habitable space of the other?

OUTCOMES: We have created a total of 13 configurations that incorporate a mix of land and ocean area, ranging from 6.25% to 75% land cover, with configurations that include longitudinally-oriented continents, polar continents, "ring" continents (centered on the equator), and various microcontinent scenarios. Modern Earth “control” (baseline) experiments, which used modern solar insolation and a pre-industrial (year 1850) greenhouse gas composition combined with each continent configuration, revealed trends of importance for habitability assessments:

• In the absence of surface vegetation, the global mean temperatures are equivalent or greater than modern Earth only for land fractions ≤25%.
• For meridional and ring continent configurations, increasing land area corresponds to colder and drier worlds. For land fractions ≤12.5%, dividing land into four dispersed microcontinents has an effect similar to increasing the land area of a large continent.
• The polar continent configurations are generally colder than the other simulations because poleward ocean heat transports are blocked by land, resulting in a tropical ocean that is quite warm and poles that are made colder by their isolation. The 50% polar land cap scenario deviates from the increased land/decreased temperature trend of the meridional and ring continents: there is an abundance of warm water in the tropics to provide a source of moisture to the cold mid-latitude land, where it precipitates as highly reflective snow that introduces a cooling feedback. As the edges of the polar continents at 50% coverage extend to ~30 degrees latitude, the effect is superficially similar to our ROCKE-3D Neoproterozoic snowball Earth simulations, where ocean ice cover does not encroach equatorward beyond ~30 degrees latitude.