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Modelling Venus-like exoplanetary atmospheres with a GCM: planetary parameters impact on the large-scale circulation and observational prospects
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Resumo(s)
In recent years, several Earth-sized exoplanets have been detected in short-period orbits of a few Earth
days, around low-mass stars. Despite their small size compared to gas giants, their close-in orbits,
combined with the small radius of the host star compared to our Sun’s, make these worlds the best targets
for atmospheric characterisation among rocky exoplanets. These worlds have stellar irradiation levels
that can be several times that of the Earth, suggesting that a Venus-like climate is more likely.
In this work, I use a Global Circulation Model (GCM), the Generic-GCM, to model a possible Venuslike atmosphere on TRAPPIST-1 c as a benchmark of highly-irradiated rocky exoplanets orbiting Mdwarf stars. The model has been developed at the Laboratoire de Météorologie Dynamique (LMD) for
exoplanet and paleoclimate studies. It includes a 3D dynamical core common to all terrestrial planets
and a planet-specific physical part. In addition, the Generic-GCM has a generalised radiative transfer
routine for variable atmospheric compositions. The overarching goal is twofold: (1) to study the largescale atmospheric circulation of highly-irradiated rocky exoplanets; and (2) to address the observational
prospects of this kind of planet by producing phase curves (reflection and emission) and transmission
spectra to support future space missions (e.g., James Webb Space Telescope, JWST). I assumed that
TRAPPIST-1 c is a synchronous rotator with zero eccentricity and obliquity. It has a Venus-like
atmosphere, a 92 bar surface atmospheric pressure, and a radiatively active sulphuric acid prescribed
global cloud cover. I run a test to assess the Generic-GCM representation of the large-scale atmospheric
circulation on Venus, comparing the results with a Venus specific GCM: the IPSL-VGCM. First, the
Generic-GCM reproduces the superrotation pressure range and high-latitudes jets observed in the IPSLVGCM. Second, the Generic-GCM responds well to decreasing insolation by reducing the zonal wind
speeds. Third, superrotation is a robust dynamical feature present in the range of insolation explored.
The results for TRAPPIST-1 c indicate a warmer atmosphere than that of Venus, possibly a
consequence of carbon dioxide absorption of stellar radiation, which is strongest in the near-infrared.
The day-night heat redistribution in the planet is done through eastward superrotation jets (equatorial and
two high-latitudes) and meridional circulation. The latter comprises two large cells, one per hemisphere
(northern and southern), crossing the pole. Heat transport is mainly explained by its mean meridional
circulation component, with a minor poleward contribution of the stationary waves in the mid-latitudes.
The cloud top temperature field shows a distinctive chevron-like pattern and an eastward shift of the peak
thermal emission from the substellar point, suggesting an advection of warm air masses by the equatorial
zonal superrotation jet. There is evidence for an equatorial wave (c¯ ∼ 130 m s−1
, Tw = 17.5 hours).
The TRAPPIST-1 c reflection phase curves reach a maximum planet-to-star contrast on the order of
10−6
, confirming that high albedo sulphuric acid aerosols of Venus-like cloudy exoplanets may favour
their detection by JWST and other future instruments. This work also shows that thermal phase curves can
sound different atmospheric levels, depending on the spectral band: carbon dioxide absorption bands will
sound mesospheric levels, while continuum bands will sound the cloud top. The simulated transmission
spectrum of TRAPPIST-1 c is flattened by clouds, screening almost all but the strongest carbon dioxide
absorption bands (e.g., 4.3 µm, 15 µm). Detection of weaker CO2 spectral lines might be possible,
suggesting a higher abundance, Venus-like carbon dioxide atmosphere. In particular, the work shows
that detecting the 4.8 µm CO2 spectral band might be possible, indicating a high-pressure atmosphere.
The removal of Venus-like aerosols from simulations leads to a warmer deep atmosphere, including
the development of polar warming. The parametric study reveals that larger exoplanets will have more
intense zonal equatorial jets but a smaller eastward shift of the hotspot and larger planet-to-star contrast in
the phase curves. The higher-order spin-orbit resonances will modulate the amplitude and peak emission
of the thermal phase curve, suggesting that this observable can be used to constrain the rotation state.
Descrição
Tese de mestrado, Ciências Geofísicas (Meteorologia e Oceanografia) 2022, Universidade de Lisboa, Faculdade de Ciências
Palavras-chave
TRAPPIST-1 c Vénus GCM Modelação Numérica Teses de mestrado - 2022