Thermal Phases of Earth-Like Planets: Estimating Thermal Inertia from Eccentricity, Obliquity, and Diurnal Forcing

The Astrophysical Journal (Impact Factor: 5.99). 05/2012; 757(1). DOI: 10.1088/0004-637X/757/1/80
Source: arXiv


In order to understand the climate on terrestrial planets orbiting nearby
Sun-like stars, one would like to know their thermal inertia. We use a global
climate model to simulate the thermal phase variations of Earth-analogs and
test whether these data could distinguish between planets with different heat
storage and heat transport characteristics. In particular, we consider a
temperate climate with polar ice caps (like modern Earth), and a snowball state
where the oceans are globally covered in ice. We first quantitatively study the
periodic radiative forcing from, and climatic response to, rotation, obliquity,
and eccentricity. Orbital eccentricity and seasonal changes in albedo cause
variations in the global-mean absorbed flux. The responses of the two climates
to these global seasons indicate that the temperate planet has 3 times the bulk
heat capacity of the snowball planet due to the presence of liquid water
oceans. The temperate obliquity seasons are weaker than one would expect based
on thermal inertia alone; this is due to cross-equatorial oceanic and
atmospheric energy transport. Thermal inertia and cross-equatorial heat
transport have qualitatively different effects on obliquity seasons, insofar as
heat transport tends to reduce seasonal amplitude without inducing a phase lag.
For an Earth-like planet, however, this effect is masked by the mixing of
signals from low thermal inertia regions (sea ice and land) with that from high
thermal inertia regions (oceans), which also produces a damped response with
small phase lag. We then simulate thermal lightcurves as they would appear to a
high-contrast imaging mission (TPF-I/Darwin) and consider the inverse problem
of estimating thermal inertia based solely on time-resolved photometry.

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