No Umbrella Needed: Confronting the Hypothesis of Iron Rain on WASP-76b with Post-processed General Circulation Models

Abstract

High-resolution spectra are unique indicators of three-dimensional (3D) processes in exoplanetary atmospheres. For instance, in 2020, Ehrenreich et al. reported transmission spectra from the ESPRESSO spectrograph yielding an anomalously large Doppler blueshift from the ultrahot Jupiter WASP-76b. Interpretations of these observations invoke toy model depictions of gas-phase iron condensation in lower-temperature regions of the planet’s atmosphere. In this work, we forward model the atmosphere of WASP-76b with double-gray general circulation models (GCMs) and ray-striking radiative transfer to diagnose the planet’s high-resolution transmission spectrum. We confirm that a physical mechanism driving strong east–west asymmetries across the terminator must exist to reproduce large Doppler blueshifts in WASP-76b’s transmission spectrum. We identify low atmospheric drag and a deep radiative-convective boundary as necessary components of our GCM to produce this asymmetry (the latter is consistent with existing Spitzer phase curves). However, we cannot reproduce either the magnitude or the time-dependence of the WASP-76b Doppler signature with gas-phase iron condensation alone. Instead, we find that high-altitude, optically thick clouds composed of Al 2 O 3 , Fe, or Mg 2 SiO 4 provide reasonable fits to the Ehrenreich et al. observations—with marginal contributions from condensation. This fit is further improved by allowing a small orbital eccentricity ( e ≈ 0.017), consistent with prior WASP-76b orbital constraints. We additionally validate our forward-modeled spectra by reproducing lines of nearly all species detected in WASP-76b by Tabernero et al. Our procedure’s success in diagnosing phase-resolved Doppler shifts demonstrates the benefits of physical, self-consistent, 3D simulations in modeling high-resolution spectra of exoplanet atmospheres.