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Stabilization of CO2 Atmospheres on Exoplanets around M Dwarf Stars | | Peter Gao
; Renyu Hu
; Tyler D. Robinson
; Cheng Li
; Yuk L. Yung
; | | Date: |
27 Jan 2015 | | Abstract: | We investigate the chemical stability of CO2-dominated atmospheres of M dwarf
terrestrial exoplanets using a 1-dimensional photochemical model. On planets
orbiting Sun-like stars, the photolysis of CO2 by Far-UV (FUV) radiation is
balanced by the reaction between CO and OH, the rate of which depends on H2O
abundance. By comparison, planets orbiting M dwarf stars experience higher FUV
radiation compared to planets orbiting Sun-like stars, and they are also likely
to have low H2O abundance due to M dwarfs having a prolonged, high-luminosity
pre-main sequence (Luger & Barnes 2015). We show that, for H2O-depleted planets
around M dwarfs, a CO2-dominated atmosphere is stable to conversion to CO and
O2 by relying on a catalytic cycle involving H2O2 photolysis. However, this
cycle breaks down for planets with atmospheric hydrogen mixing ratios below ~1
ppm, resulting in ~40% of the atmospheric CO2 being converted to CO and O2 on a
time scale of 1 Myr. The increased abundance of O2 also results in high O3
concentrations, which reacts with HO2 to generate OH, forming another catalytic
cycle capable of stabilizing CO2. For atmospheres with <0.1 ppm hydrogen,
excess O atoms resulting from O3 photolysis react with CO and a third body to
directly produce CO2. This series of catalytic cycles places an upper limit of
~50% on the amount of CO2 that can be destroyed via photolysis in such a dry
atmosphere, which is enough to generate abundances of abiotic O2 and O3
rivaling that of modern Earth. Discrimination between O2 and O3 produced
biologically and those produced abiotically through photolysis can perhaps be
accomplished by noting the lack of water features in the spectra of these
H2O-depleted planets, which necessitates observations in the infrared. | | Source: | arXiv, 1501.6876 | | Services: | Forum | Review | PDF | Favorites | |
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