Variability due to climate and chemistry in observations of oxygenated Earth-analogue exoplanets

Abstract

The Great Oxidation Event was a period during which Earth’s atmospheric oxygen (O2) concentrations increased from ∼10−5 times its present atmospheric level (PAL) to near modern levels, marking the start of the Proterozoic geological eon 2.4 billion years ago. Using WACCM6, an Earth System Model, we simulate the atmosphere of Earth-analogue exoplanets with O2 mixing ratios between 0.1 per cent and 150 per cent PAL. Using these simulations, we calculate the reflection/emission spectra over multiple orbits using the Planetary Spectrum Generator. We highlight how observer angle, albedo, chemistry, and clouds affect the simulated observations. We show that inter-annual climate variations, as well short-term variations due to clouds, can be observed in our simulated atmospheres with a telescope concept such as LUVOIR or HabEx. Annual variability and seasonal variability can change the planet’s reflected flux (including the reflected flux of key spectral features such as O2 and H2O) by up to factors of 5 and 20, respectively, for the same planetary phase. This variability is best observed with a high-throughput coronagraph. For example, HabEx (4 m) with a starshade performs up to a factor of two times better than a LUVOIR B (6 m) style telescope. The variability and signal-to-noise ratio of some spectral features depends non-linearly on atmospheric O2 concentration. This is caused by temperature and chemical column depth variations, as well as generally increased liquid and ice cloud content for atmospheres with O2 concentrations of <1 per cent PAL.

Contemporary atmospheric oxygen levels maximize global protection by ozone

“…Two other recent studies 22,23 also exploited 3-D chemistry-climate models like in our case, however the main focuses of these studies have been different to ours and the linearity of the O3/O2 relationship has only briefly been touched. One of these studies showed a continuously linear O3/O2 relationship up to the present atmospheric level and to the one experiment that they performed with higher-than-present O2 (31.5%) 22 , whereas the other one showed a clear saturation of the O3/O2 relationship around the present level 23 , which is consistent with us, however with no experiments beyond that. Unfortunately, both studies haven't presented and discussed the details of their ozone and temperature changes in the stratosphere, which complicates the identification of potential reasons for this disagreement.…”

“…1a, however, with a maximum O3 concentration reached at ~10.5% O2 20 or ~5.25-11.55% O2 21 . Other 3-D modeling studies showed a similar shape to us up to the present O2 level but were either not performed for higher than present O2 cases 23 or showed a different, continuously increasing pattern 22 . Previous studies discussed several mechanisms responsible for the obtained shapes, however none of them interpreted their results from the point of view of feedbacks of chemistry with the middle atmospheric dynamics.…”

“…Other studies reported a monotonically negative correlation 19 , a maximum in O3 column around 50% 20 or between 25 and 55% of the present atmospheric O2 21 level and subsequent decrease with increasing O2. Recently, 3-D modeling studies have shed new light on this topic 22,23 . However, the existing contradictions regarding the linearity of the relationship between O3/O2 mixing ratios remained largely unresolved.…”

Sporadic Spin-orbit Variations in Compact Multiplanet Systems and Their Influence on Exoplanet Climate

“…For truly terrestrial worlds such as TRAPPIST-1 e and f, simulated detections suggest spectral features of CO 2 , CH 4 , and N 2 O features at the 4.3, 3.3, and 8.5 μm bands will be feasible within 30 transits for near-term observations by the JWST (e.g., Fauchez et al 2019;Lustig-Yaeger et al 2019;Wunderlich et al 2020;Kaltenegger & Lin 2021;Lustig-Yaeger et al 2022;Mikal-Evans 2022). In the next few years, more potentially habitable planets will be discovered, with the catalog of these systems to date in the order of hundreds and growing; models of various complexities and heritages are just on the rise and will quantitatively evaluate their inhabitance or habitability (Méndez et al 2020;Fauchez et al 2021;Wordsworth & Kreidberg 2022;Cooke et al 2023).…”

Understanding the Capability of Future Direct-imaging Observations to Quantify Atmospheric Chemical Effects of Stellar Proton Events

“…Photochemical modeling of rocky exoplanets was first performed in one-dimensional (1D) models (e.g., Segura et al 2005;Kaltenegger & Sasselov 2009;Segura et al 2010;Hu et al 2012;Grenfell et al 2013;Gao et al 2015;Kozakis et al 2018) that provided key insights into the effects of stellar spectral energy distribution, stellar variability, and stellar flares. More recently, such modeling efforts have been expanded to the 2D and 3D regimes, usually with atmospheric chemistry and photochemistry as additional subcomponents in global Earth system or general circulation models (Chen et al 2018(Chen et al , 2019(Chen et al , 2021Braam et al 2022;Cooke et al 2022;Ridgway et al 2023;Tsai et al 2022). While the efficiency of single-column models allows one to explore a large parameter space of planetary characteristics, higher dimensional (2D/3D) models are able to simulate the complex interplay among atmospheric dynamics, clouds, radiation, and chemistry and hence provide results with enhanced realism.…”