Abiotic formation of O₂ and O₃ in high-CO₂ terrestrial atmospheres

¹ California Institute of Technology, 1200 East California Blvd. Mail stop 220-6, Pasadena CA, 91125 USA e-mail: antigona@nucleares.unam.mx
² Members of the Virtual Planetary Laboratory, a project of the NASA Astrobiology Institute
³ Pennsylvania State University, 443 Deike Bldg. State College, PA 16802, USA
⁴ NASA Jet Propulsion Laboratory, 4800 Oak Grove Dr. Pasadena, CA 91109, USA
⁵ University of California, Radio Astronomy Laboratory, 601 Campbell Hall, Berkeley, CA 94720 USA

Received: 30 October 2006
Accepted: 28 June 2007

Abstract

Context.Previous research has indicated that high amounts of ozone (O₃) and oxygen (O₂) may be produced abiotically in atmospheres with high concentrations of CO₂. The abiotic production of these two gases, which are also characteristic of photosynthetic life processes, could pose a potential “false-positive” for remote-sensing detection of life on planets around other stars. We show here that such false positives are unlikely on any planet that possesses abundant liquid water, as rainout of oxidized species onto a reduced planetary surface should ensure that atmospheric H₂ concentrations remain relatively high, and that O₂ and O₃ remain low.

Aims.Our gool is to determine the amount of O₃ and O₂ formed in a high CO₂ atmosphere for a habitable planet without life.

Methods.We use a photochemical model that considers hydrogen (H₂) escape and a detailed hydrogen balance to calculate the O₂ and O₃ formed on planets with 0.2 of CO₂ around the Sun, and 0.02, 0.2 and 2 bars of CO₂ around a young Sun-like star with higher UV radiation. The concentrations obtained by the photochemical model were used as input in a radiative transfer model that calculated the spectra of the modeled planets.

Results.The O₃ and O₂ concentrations in the simulated planets are extremely small, and unlikely to produce a detectable signature in the spectra of those planets.

Conclusions.With a balanced hydrogen budget, and for planets with an active hydrological cycle, abiotic formation of O₂ and O₃ is unlikely to create a possible false positive for life detection in either the visible/near-infrared or mid-infrared wavelength regimes.