Exploring the Inner Edge of the Habitable Zone with Fully Coupled Oceans

Abstract:

Rotation in planetary atmospheres plays an important role in
regulating atmospheric and oceanic heat flow, cloud formation and precipitation.
Using the Goddard Institute for Space Studies (GISS) three dimensional General
Circulation Model (3D-GCM) we demonstrate how varying rotation rate and
increasing the incident solar flux on a planet are related to each other and may
allow the inner edge of the habitable zone to be much closer than many previous
habitable zone studies have indicated. This is shown in particular for fully
coupled ocean runs over a large range of insolation and rotation rates.
Results with a 100m mixed layer depth and our fully coupled ocean runs are
compared with those of Yang et al. 2014, which demonstrates consistency
across models. However, there are clear differences for rotations rates of 1-16x
present earth day lengths between the mixed layer and fully coupled ocean models,
which points to the necessity of using fully coupled oceans whenever possible.
The latter was recently demonstrated quite clearly by Hu & Yang 2014 in their
aquaplanet study with a fully coupled ocean when compared with similar mixed
layer ocean studies and by Cullum et al. 2014.

Atmospheric constituent amounts were also varied alongside adjustments to cloud
parameterizations. While the latter have an effect on what a planet's global mean
temperature is once the oceans reach equilibrium they do
not qualitatively change the overall relationship between the globally averaged
surface temperature and incident solar flux for rotation rates ranging from 1
to 256 times the present Earth day length. At the same time this study
demonstrates that given the lack of knowledge about the atmospheric constituents
and clouds on exoplanets there is still a large uncertainty as to where a planet
will sit in a given star's habitable zone.

We also explore options for understanding the possibility for regional habitability
via an aridity index and a separate moisture index. The former is related to the competition
between precipitation and the potential evapotranpiration of the soil (via a modified
Penman-Monteith equation) while the latter is a measure of the amount of liquid water found
on land.

Cullum, Stevens & Joshi 2014, Astrobiology, vol 14, No. 8, pg 645

Hu & Yang 2014, PNAS, 111, 629

Yang, Cowan & Abbot 2013, ApJL, 771, 45

Yang et al. 2014, ApJL, 787, 2