Abstract
Context. Characterizing temperate (200–1000 K) super-Earth atmospheres is one of the future challenges in exoplanetary science. One of the major difficulties comes from the ubiquity of aerosols in these objects, which complicates the spectroscopic analyses. The knowledge gained on the Solar System is then crucial to better understand the chemical processes of exoplanet atmospheres. Aims. This work focuses on the impact of ion chemistry on molecular diversity in a specific Titan-like exoplanet atmosphere that would be dominated by molecular nitrogen. On the largest satellite of Saturn, Titan, ion chemistry is a major component of molecular growth that forms precursors for the observed photochemical organic hazes. Methods. Based on an experimental approach, we irradiated a gaseous mixture representative of a Titan-like atmosphere (N2-dominated with CH4) using an extreme-uv photon source (16.8 eV). Trace amounts of water vapor were added to the composition of the Titan-type gas mixture to simulate an exoplanet in the habitable zone. Results. A wide variety of molecules and ions have been detected and they cannot all be identified based on our current knowledge of the organic chemistry of planetary atmospheres (mostly N- and C-based chemistry). The presence of even trace amounts of H2O significantly broadens the product distribution, and H3O+ is found to be the most abundant ion. Conclusions. This work demonstrates the complexity of the chemistry within exoplanet atmospheres. Numerical models must consider oxygen chemistry and ion-molecule reactions in order to probe the habitability of a certain type of super-Earths. The abundance of H3O+ makes it a good candidate for future observations.
Highlights
Together with mini-Neptunes, super-Earths are part of a class of planets called Kepler planets, whose size is between that of the Earth and Neptune (1– 4 R⊕, 1–30 M⊕)
About 50% of the super-Earths would be located in the habitable zone (HZ) of their host star (Bonfils et al 2013; Kopparapu et al 2013), but they do not necessarily have the conditions to support liquid water oceans or Earth-like biology because their bulk composition can be very diverse according to their origin (Scora et al 2020)
The atmospheres of super-Earths cover a wide range of chemical compositions as well as temperature and pressure conditions. They can harbor a wide variety of molecules and ions
Summary
Together with mini-Neptunes, super-Earths are part of a class of planets called Kepler planets, whose size is between that of the Earth and Neptune (1– 4 R⊕, 1–30 M⊕). The heating or cooling potential of the particles is determined by their abundance, composition, size, and shape, which define the optical properties (Zhang et al 2017; Arney et al 2016; Lavvas & Arfaux 2021). With their own broad spectral signatures, aerosols obliterate the absorption features of gases and cause the planetary spectrum to be featureless. This hampers detecting the abundant heavy volatiles (Guo et al 2020). The composition of the aerosols of cold rocky exoplanets and their major parent molecules remains largely unknown, the chemistry of their atmosphere is known to be diverse (Hu et al 2012; De Wit et al 2018; Leconte et al 2015; Owen & Mohanty 2016; Luger & Barnes 2015; Jin et al 2014; Owen & Wu 2013)
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