The Detectability of Rocky Planet Surface and Atmosphere Composition with the JWST: The Case of LHS 3844b

Abstract

The spectroscopic characterization of terrestrial exoplanets over a wide spectral range from the near- to the mid-infrared will be made possible for the first time with the JWST. One challenge is that it is not known a priori whether such planets possess optically thick atmospheres or even any atmospheres altogether. However, this challenge also presents an opportunity, the potential to detect the surface of an extrasolar world. This study explores the feasibility of characterizing with the JWST the atmosphere and surface of LHS 3844b, the highest signal-to-noise rocky thermal emission target among planets that are cool enough to have nonmolten surfaces. We model the planetary emission, including the spectral signal of both the atmosphere and surface, and we explore all scenarios that are consistent with the existing Spitzer 4.5 μm measurement of LHS 3844b from Kreidberg et al. In summary, we find a range of plausible surfaces and atmospheres that are within 3σ of the observationless reflective metal-rich, iron-oxidized, and basaltic compositions are allowed, and atmospheres are restricted to a maximum thickness of 1 bar, if near-infrared absorbers at ≳100 ppm are included. We further make predictions on the observability of surfaces and atmospheres and find that a small number (∼3) of eclipse observations should suffice to differentiate between surface and atmospheric features. We also perform a Bayesian retrieval analysis on simulated JWST data and find that the surface signal may make it harder to precisely constrain the abundance of atmospheric species and may falsely induce a weak H2O detection.

Constraining the Thickness of TRAPPIST-1 b’s Atmosphere from Its JWST Secondary Eclipse Observation at 15 μm

“…To calculate the eclipse spectrum of different surfaces and atmospheres, we use HELIOS, an open-source 1D radiative transfer code that computes the thermal profile of a planetary atmosphere in radiative-convective equilibrium (Malik et al 2017(Malik et al , 2019a(Malik et al , 2019b. Most of our approach closely follows Whittaker et al (2022), which performed a similar analysis for the Spitzer observation of LHS 3844 b, and we refer the readers to that work for more details of the modeling.…”

“…For the composition of the atmospheres, in addition to a 100% CO 2 atmosphere, we choose to vary the abundance of trace CO 2 , at 1 ppm, 100 ppm, and 1%, against background gases of N 2 , O 2 , and H 2 O. Moreover, we also consider atmospheres containing a range of other trace gases plausible in secondary atmospheres (Turbet et al 2020;Krissansen-Totton & Fortney 2022;Whittaker et al 2022), which may not necessarily absorb at 15 μm but may be detected via observations at other wavelengths. For this purpose, we adopt the same trace abundance grids (i.e., 1 ppm, 100 ppm, 1%) for CO, CH 4 , H 2 O, and SO 2 , against a background gas of N 2 for the former two and O 2 for the latter.…”

Observability of silicates in volatile atmospheres of super-Earths and sub-Neptunes

“…We allowed convective adjustment to take place using an adiabatic coefficient of κ = 2/7, applicable to diatomic atmospheres. The profiles were treated with a rocky surface boundary, whose implementation is described in detail in Malik et al (2019a); Whittaker et al (2022). All of our models were reiterated until convergence such that the attained surface temperature is in good agreement with the atmospheric chemistry.…”

Narrow Loophole for H2-Dominated Atmospheres on Habitable Rocky Planets around M Dwarfs

“…Meanwhile, N 2 -CO 2 or other high mean molecular weight atmospheres should probably be considered as the default assumption when planning for future spectroscopic observations of rocky planets around M dwarfs. This would require planning more repeated visits of preferred targets for transmission spectroscopy (e.g., Batalha et al 2018) or turning to thermal emission spectroscopy and phase-curve mapping (e.g., Angelo & Hu 2017;Kreidberg et al 2019;Mansfield et al 2019;Whittaker et al 2022).…”