Atmospheric compositions and observability of nitrogen-dominated ultra-short-period super-Earths

Abstract

We explore the chemistry and observability of nitrogen dominated atmospheres for ultra-short-period super-Earths. We base the assumption, that super-Earths could have nitrogen filled atmospheres, on observations of 55 Cnc e that favour a scenario with a high-mean-molecular-weight atmosphere. We take Titan's elemental budget as our starting point and using chemical kinetics compute a large range of possible compositions for a hot super-Earth. We use analytical temperature profiles and explore a parameter space spanning orders of magnitude in C/O & N/O ratios, while always keeping nitrogen the dominant component. We generate synthetic transmission and emission spectra and assess their potential observability with the future James Webb Space Telescope and ARIEL. Our results suggest that HCN is a strong indicator of a high C/O ratio, which is similar to what is found for H-dominated atmospheres. We find that these worlds are likely to possess C/O > 1.0, and that HCN, CN, CO should be the primary molecules to be searched for in thermal emission. For lower temperatures (T < 1500 K), we additionally find NH 3 in high N/O ratio cases, and C 2 H 4 , CH 4 in low N/O ratio cases to be strong absorbers. Depletion of hydrogen in such atmospheres would make CN, CO and NO exceptionally prominent molecules to look for in the 0.6 -5.0 µm range. Our models show that the upcoming JWST and ARIEL missions will be able to distinguish atmospheric compositions of ultra-short period super-Earths with unprecedented confidence.

Magma Ocean Evolution of the TRAPPIST-1 Planets

“…The atmospheric composition of this planet is currently still debated based on tentative atmospheric HCN observations with the Hubble Space Telescope. HCN would imply a reducing, N 2 -dominated atmosphere instead of an oxidizing atmosphere as assumed in this study (Tsiaras et al, 2016;Hammond and Pierrehumbert, 2017;Zilinskas et al, 2020). Future observations with JWST or the ELT will target 55 Cancri e as an example to study the evolution of magma ocean planets and the influence of these magma oceans on the atmosphere.…”

No Escaping Helium from 55 Cnc e*

“…This point was discussed in Angelo & Hu (2017), who suggested that an N 2 -dominated atmosphere would be consistent with the Spitzer data. The possible composition of a nitrogen-dominated atmosphere and the observability of spectral features are explored in Miguel (2019) and Zilinskas et al (2020). However, such a thick atmosphere would be unlikely to change substantially on yearlong timescales.…”

Ariel planetary interiors White Paper

“…On the other hand, if HRSEs have remaining volatile elements such as H, C, N, S and Cl, they likely have atmospheres composed mainly of H 2 O and/or CO 2 with rocky vapours such as Na and SiO [10,129]. Or by HCN in case of N-dominated atmospheres [95,171] Generally, we call the former case a mineral atmosphere and the latter case a steam atmosphere. Thus, detection of rocky vapour would provide a definitive piece of evidence for HRSEs and their surface composition, including indications of their deeper structure.…”