Title: Atmospheric Sulfur Photochemistry on Hot Jupiters
ArXiv ID: 0903.1663
Date: 2009-08-10
Authors: Researchers from original ArXiv paper
📝 Abstract
We develop a new 1D photochemical kinetics code to address stratospheric chemistry and stratospheric heating in hot Jupiters. Here we address optically active S-containing species and CO2 at 1200 < T < 2000 K. HS (mercapto) and S2 are highly reactive species that are generated photochemically and thermochemically from H2S with peak abundances between 1-10 mbar. S2 absorbs UV between 240 and 340 nm and is optically thick for metallicities [SH] > 0 at T > 1200 K. HS is probably more important than S2, as it is generally more abundant than S2 under hot Jupiter conditions and it absorbs at somewhat redder wavelengths. We use molecular theory to compute an HS absorption spectrum from sparse available data and find that HS should absorb strongly between 300 and 460 nm, with absorption at the longer wavelengths being temperature sensitive. When the two absorbers are combined, radiative heating (per kg of gas) peaks at 100 microbars, with a total stratospheric heating of about 8 x 10^4 W/m^2 for a jovian planet orbiting a solar-twin at 0.032 AU. Total heating is insensitive to metallicity. The CO2 mixing ratio is a well-behaved quadratic function of metallicity, ranging from 1.6 x 10^-8 to 1.6 x 10^-4 for -0.3 < [M/H] < 1.7. CO2 is insensitive to insolation, vertical mixing, temperature (1200 < T <2000 K), and gravity. The photochemical calculations confirm that CO2 should prove a useful probe of planetary metallicity.
💡 Deep Analysis
Deep Dive into Atmospheric Sulfur Photochemistry on Hot Jupiters.
We develop a new 1D photochemical kinetics code to address stratospheric chemistry and stratospheric heating in hot Jupiters. Here we address optically active S-containing species and CO2 at 1200 < T < 2000 K. HS (mercapto) and S2 are highly reactive species that are generated photochemically and thermochemically from H2S with peak abundances between 1-10 mbar. S2 absorbs UV between 240 and 340 nm and is optically thick for metallicities [SH] > 0 at T > 1200 K. HS is probably more important than S2, as it is generally more abundant than S2 under hot Jupiter conditions and it absorbs at somewhat redder wavelengths. We use molecular theory to compute an HS absorption spectrum from sparse available data and find that HS should absorb strongly between 300 and 460 nm, with absorption at the longer wavelengths being temperature sensitive. When the two absorbers are combined, radiative heating (per kg of gas) peaks at 100 microbars, with a total stratospheric heating of about 8 x 10^4 W/m^2 f
📄 Full Content
Stratospheric temperature inversions are ubiquitous in the Solar System, and it is beginning to look as if they are commonplace on hot Jupiters as well. Stratospheric temperature inversions form when substantial amounts of light are absorbed at low pressures (high altitudes) where radiative cooling is inefficient. Hubeny et al. (2003) pointed out that efficient absorption of visible light by gaseous TiO and VO would greatly heat the upper atmospheres of those planets already hot enough for these molecules to be present as vapor.
Thermal inversions on transiting hot Jupiters were first seen by Richardson et al. (2007) for HD 209458b and Harrington et al. (2007) for HD 149026b. The observed flux ratio at 8 µm for HD 149026b agreed only with models that included a thermal inversion (Fortney et al. 2006). Temperature inversions have since been confirmed by Spitzer observations of HD 209458b (Knutson et al. 2008a), XO-1b (Machalek et al. 2008), andTrES-4 (Knutson et al. 2009), all of which show distinctive flux ratios in IRAC bands that suggest inversions (Fortney et al. 2006;Burrows et al. 2007). More circumstantial evidence exists for HD 179949b (Barnes et al. 2008).
On the other hand TrES-1, the least irradiated planet with published Spitzer observations, does not appear to have a pronounced inversion (Burrows et al. 2008). Nor, seemingly, does HD 189733b, which is also modestly irradated (Charbonneau et al. 2008; Barman et al. 2008). One suggestion is that temperature inversions are triggered by irradiation reaching a critical level that is hot enough to evaporate TiO and VO from grains, as discussed by Burrows et al. (2007), Fortney et al. (2008), and Burrows et al. (2008). However, irradiation of XO-1b and HD 189733b is within uncertainties the same (Torres et al. 2008), which poses a challenge to the irradiation trigger.
In the Solar System, stratospheric temperature inversions are often caused by absorption of UV light by gases or aerosols produced by photochemistry. Here we ask if atmospheric chemistry might play a similar role in hot Jupiters. Speculation has tended to focus on sulfur-containing species (Tinetti 2008), as the reservoir species H 2 S is expected to be abundant (Visscher et al 2006) in these atmospheres and many of its breakdown products (S 2 , in particular) absorb violet and ultraviolet light.
Previous photochemical modeling of hot Jupiters addressed the abundance of photochemical H (Liang et al 2003) and the absence of photochemical smogs (Liang et al 2004). Liang et al (2003) focused on the high H/H 2 ratio that arises from H 2 O photolysis.
In their second paper, Liang et al (2004) argued that simple hydrocarbons would not condense to form photochemical smogs in hot solar composition atmospheres. Neither study considered sulfur.
We have developed a new general purpose 1D photochemical kinetics code applicable to hot extrasolar planets. The code is based on the sulfur photochemistry model for early Earth originally described by Kasting et al (1989) and Kasting (1990), and subsequently adapted by Zahnle et al (2006) and Claire et al (2006) to address sulfur photochemistry of Earth’s atmosphere during the Archean, and by Zahnle et al. (2008) to address martian atmospheric chemistry. Steady state solutions are found by integrating the system through time using a fully implicit backward-difference method.
Our chemical network has been upgraded from that used by Zahnle et al (1995) to address the chemistry generated when the fragments of Comet Shoemaker Levy 9 struck Jupiter. We have assembled a reasonably complete list of the reactions that can take place between the small molecules and free radicals that can be made from H, C, O, N, and S. The code solves 507 chemical reactions for 49 chemical species:
CN, HCN, N, N 2 , NO, NH, NH 2 , NH 3 , NS, H 2 S, HS, S, S 2 , S 3 , S 4 , S 8 , SO, HSO, SO 2 , OCS, CS, HCS, H 2 CS, CS 2 , and H 2 . Reaction rates, when known, are selected from the publicly available NIST database (http://kinetics.nist.gov/kinetics)
.
In order of decreasing priority, we choose between reported reaction rates according to relevant temperature range, newest review, newest experiment, and newest theory. Reverse reaction rates k r = K eq k f of two-body reactions are determined from the forward reaction rate k f and the equilibrium K eq = exp {(-∆H + T ∆S) /RT } by using H • (T ) and S • (T ) as available (R is the gas constant). Rates are not available for all reactions, especially for reactions involving elemental sulfur. We will present a full listing of the chemical reactions important to sulfur in a more general followup study.
Here we use simple descriptions of atmospheric properties. The background atmosphere is 84% H 2 and 16% He. We include Rayleigh scattering by H 2 (Dalgarno and Williams 1962). For our base case we assume an isothermal atmosphere with T=1400 K; constant vertical eddy diffusivity K zz = 1 × 10 7 cm 2 /s; a surface gravity of 20 m/s 2
