UV Protection of Water in the Terrestrial Planet-Forming Zone: Implications for Carbon Dioxide Emissions

Habitable zones and global climate

Spectra of OH, CO2 and H2O for UV protection and basic chemical heating and H2O models. The emitting region was constrained to the radial region in the water midplane ice line and the spectra were convolved at R=3000. LTE excitation is assumed for OH.

Carbon dioxide is an important chemical and physical tracer in the formation zone of the planet Earth.

Using a thermochemical model that has been tested against mid-infrared water emission, we reinterpret CO2 emission as observed with Spitzer. We find that the UV protection of the water and the additional chemical heating significantly reduce the total column of CO2 in the emitting layer. UV protection of water is the most effective effect, reducing the CO2 column by about 2 orders of magnitude. These low CO2 abundances lead to CO2 to H2O flux ratios closer to observed values, but CO2 emissions are still too large, especially in relative terms.

Invoking elemental oxygen depletion outside the mid-water plane ice line has a greater impact on CO2 emissions than on H2O emissions, which aligns CO2 to H2O emissions on the observed values. We conclude that the CO2 emission observed with Spitzer-IRS comes from a thin layer in the disc photosphere, similar to strong water lines. Below this layer, we expect CO2 not to be present except when replenished by a physical process. This would be visible in the spectrum of 13CO2 as well as some characteristics of 12CO2 that can be observed by JWST-MIRI.

Arthur D. Bosman, Edwin A. Bergin, Jenny K. Calahan

Comments: 8 pages, 4 figures, accepted for publication in ApJL
Subjects: Terrestrial and planetary astrophysics (astro-ph.EP)
Cite as: arXiv:2207.02236 [astro-ph.EP] (or arXiv:2207.02236v1 [astro-ph.EP] for this version)
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Submission History
From: Arthur Bosman
[v1] Tue Jul 5 2022 6:01:07 PM UTC (2089 KB)

Astrobiology, Astrochemistry

Co-founder of SpaceRef, member of the Explorers Club, ex-NASA, outside teams, journalist, space and astrobiology, deceased climber.

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