Extrasolar Planets

The Puzzling Chemical Composition of GJ 436b's Atmosphere

By Keith Cowing
December 12, 2013
Filed under
The Puzzling Chemical Composition of GJ 436b's Atmosphere
GJ 436b

The dissipation of the tidal energy deposited on eccentric planets may induce a heating of the planet that affects its atmospheric thermal structure.

Here we study the influence of tidal heating on the atmospheric composition of the eccentric (e = 0.16) “hot Neptune” GJ 436b, for which inconclusive chemical abundances are retrieved from multiwavelength photometric observations carried out during primary transit and secondary eclipse.

We build up a one-dimensional model of GJ 436b’s atmosphere in the vertical direction and compute the pressure-temperature and molecular abundances profiles for various plausible internal temperatures of the planet (up to 560 K) and metallicities (from solar to 100 times solar), using a radiative-convective model and a chemical model which includes thermochemical kinetics, vertical mixing, and photochemistry.

We find that the CO/CH4 abundance ratio increases with metallicity and tidal heating, and ranges from 1/20 to 1000 within the ranges of metallicity and internal temperature explored. Water vapour locks most of the oxygen and reaches a very high abundance, whatever the metallicity and internal temperature of the planet. The CO2/H2O abundance ratio increases dramatically with metallicity, and takes values between 1e-5-1e-4 with solar elemental abundances and 0.1 for a metallicity 100 times solar.

None of the atmospheric models based on solid physical and chemical grounds provide a fully satisfactory agreement with available observational data, although the comparison of calculated spectra and observations seem to point to models with a high metallicity and efficient tidal heating, in which high CO/CH4 abundance ratios and warm temperatures in the dayside atmosphere are favoured.

M. Agundez, O. Venot, F. Selsis, N. Iro (Submitted on 11 Dec 2013)

Comments: Accepted for publication in ApJ

Subjects: Earth and Planetary Astrophysics (astro-ph.EP)

Cite as: arXiv:1312.3007 [astro-ph.EP]

(or arXiv:1312.3007v1 [astro-ph.EP] for this version)

Submission history From: Marcelino Agundez [v1] Wed, 11 Dec 2013 00:33:03 GMT (574kb)

SpaceRef co-founder, Explorers Club Fellow, ex-NASA, Away Teams, Journalist, Space & Astrobiology, Lapsed climber.