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Atmospheric super-rotation in solar system and extra-solar planetary atmospheres
Super-rotation is a common phenomenon in solar system planetary atmospheres. Out of the four substantial atmospheres possessed by solid bodies in the solar system, the slowly rotating planet, Venus, and moon, Titan, are both well-known to have atmospheres that rotate on average substantially more quickly than does the solid surface underneath. The more rapidly rotating planets, Mars and Earth, have much weaker global super-rotation, but both can exhibit time-varying prograde jets near the equator which rotate more rapidly than the local surface. Atmospheric super-rotation is not restricted to planets with solid surfaces and shallow atmospheres. Cloud-tracking observations of the gas giants Jupiter and Saturn show that they both possess rapid prograde equatorial jets and hence exhibit local super-rotation.
Simplified global circulation models of extra-solar planets, including representations of ‘hot Jupiters’ and Earth-like planets rotating at different rates, can also show sustained super-rotating equatorial jets in different dynamical regimes. In the extra-solar planet cases in particular, the quantitative results are highly sensitive to model parameters.
In each case the detailed mechanism, or combination of mechanisms, which produces the super-rotating jets might vary, but all require longitudinally asymmetric motions, waves or eddies, to transport angular momentum up-gradient into the jets. The mechanism is not always easy to diagnose from observations and requires careful modelling. We review both observations of solar system planets and recent global circulation model results, combined in the case of Mars and Earth in the form of atmospheric reanalyses by data assimilation, together with simplified extra-solar planet simulations
Three-dimensional atmospheric circulation of hot Jupiters on highly eccentric orbits
Of the over 800 exoplanets detected to date, over half are on non-circular
orbits, with eccentricities as high as 0.93. Such orbits lead to time-variable
stellar heating, which has implications for the planet's atmospheric dynamical
regime. However, little is known about this dynamical regime, and how it may
influence observations. Therefore, we present a systematic study of hot
Jupiters on highly eccentric orbits using the SPARC/MITgcm, a model which
couples a three-dimensional general circulation model with a plane-parallel,
two-stream, non-grey radiative transfer model. In our study, we vary the
eccentricity and orbit-average stellar flux over a wide range. We demonstrate
that the eccentric hot Jupiter regime is qualitatively similar to that of
planets on circular orbits; the planets possess a superrotating equatorial jet
and exhibit large day-night temperature variations. We show that these
day-night heating variations induce momentum fluxes equatorward to maintain the
superrotating jet throughout its orbit. As the eccentricity and/or stellar flux
is increased, the superrotating jet strengthens and narrows, due to a smaller
Rossby deformation radius. For a select number of model integrations, we
generate full-orbit lightcurves and find that the timing of transit and
secondary eclipse viewed from Earth with respect to periapse and apoapse can
greatly affect what we see in infrared (IR) lightcurves; the peak in IR flux
can lead or lag secondary eclipse depending on the geometry. For those planets
that have large day-night temperature variations and rapid rotation rates, we
find that the lightcurves exhibit "ringing" as the planet's hottest region
rotates in and out of view from Earth. These results can be used to explain
future observations of eccentric transiting exoplanets.Comment: 20 pages, 18 figures, 2 tables; Accepted to Ap
Gliese 581g as a scaled-up version of Earth: atmospheric circulation simulations
We use three-dimensional simulations to study the atmospheric circulation on
the first Earth-sized exoplanet discovered in the habitable zone of an M star.
We treat Gliese 581g as a scaled-up version of Earth by considering increased
values for the exoplanetary radius and surface gravity, while retaining
terrestrial values for parameters which are unconstrained by current
observations. We examine the long-term, global temperature and wind maps near
the surface of the exoplanet --- the climate. The specific locations for
habitability on Gliese 581g depend on whether the exoplanet is tidally-locked
and how fast radiative cooling occurs on a global scale. Independent of whether
the existence of Gliese 581g is confirmed, our study highlights the use of
general circulation models to quantify the atmospheric circulation on
potentially habitable, Earth-sized exoplanets, which will be the prime targets
of exoplanet discovery and characterization campaigns in the next decade.Comment: Accepted by MNRAS. 15 pages, 13 figures. Sample movies of simulations
are available at http://www.phys.ethz.ch/~kheng/fms
A New 24 micron Phase Curve for upsilon Andromedae b
We report the detection of 24 micron variations from the planet-hosting
upsilon Andromedae system consistent with the orbital periodicity of the
system's innermost planet, upsilon And b. We find a peak-to-valley phase curve
amplitude of 0.00130 times the mean system flux. Using a simple model with two
hemispheres of constant surface brightness and assuming a planetary radius of
1.3 Jupiter radii gives a planetary temperature contrast of >900 K and an
orbital inclination of >28 degrees. We further report the largest phase offset
yet observed for an extrasolar planet: the flux maximum occurs ~80 degrees
before phase 0.5. Such a large phase offset is difficult to reconcile with most
current atmospheric circulation models. We improve on earlier observations of
this system in several important ways: (1) observations of a flux calibrator
star demonstrate the MIPS detector is stable to 10^-4 on long timescales, (2)
we note that the background light varies systematically due to spacecraft
operations, precluding use of this background as a flux calibrator (stellar
flux measured above the background is not similarly affected), and (3) we
calibrate for flux variability correlated with motion of the star on the MIPS
detector. A reanalysis of our earlier observations of this system is consistent
with our new result.Comment: Submitted to ApJ. 15 pages, 6 figures, 4 table