Exoplanet atmospheres

Abstract:

Spectroscopic phase curves of transiting hot Jupiters are spectral measurements at multiple orbital phases, giving a set of disc-averaged spectra that probe multiple hemispheres. By fitting model phase curves to observations, we can constrain the atmospheric properties of hot Jupiters, such as molecular abundance, aerosol distribution, and thermal structure, which offer insights into their atmospheric dynamics, chemistry, and formation. We propose a novel 2D temperature parametrization consisting of a dayside and a nightside to retrieve information from near-infrared phase curves and apply the method to phase curves of WASP-43b observed by HST/Wide Field Camera 3 and Spitzer/Infra-Red Array Camera. In our scheme, the temperature is constant on isobars on the nightside and varies with cosⁿ(longitude/ϵ) on isobars on the dayside, where n and ϵ are free parameters. We fit all orbital phases simultaneously using the radiative transfer package NEMESISPY coupled to a Bayesian inference code. We first validate the performance of our retrieval scheme with synthetic phase curves generated from a Global Circulation Model and find that our 2D scheme can accurately retrieve the latitudinally averaged thermal structure and constrain the abundance of H₂O and CH₄. We then apply our 2D scheme to the observed phase curves of WASP-43b and find: (1) The dayside temperature–pressure profiles do not vary strongly with longitude and are non-inverted. (2) The retrieved nightside temperatures are extremely low, suggesting significant nightside cloud coverage. (3) The H₂O volume mixing ratio is constrained to 5.6 × 10⁻⁵–4.0 × 10⁻⁴, and we retrieve an upper bound for CH₄ mixing ratio at ∼10⁻⁶.

Abstract:

Previous observations of dark vortices in Neptune’s atmosphere, such as Voyager 2’s Great Dark Spot (1989), have been made in only a few broad-wavelength channels, hampering efforts to determine these vortices’ pressure levels and darkening processes. We analyse spectroscopic observations of a dark spot on Neptune identified by the Hubble Space Telescope as NDS-2018; the spectral observations were made in 2019 by the Multi Unit Spectroscopic Explorer (MUSE) of the Very Large Telescope (Chile). The MUSE medium-resolution 475–933 nm reflection spectra allow us to show that dark spots are caused by darkening at short wavelengths (<700 nm) of a deep ~5 bar aerosol layer, which we suggest is the H2S condensation layer. A deep bright spot, named DBS-2019, is also visible on the edge of NDS-2018, with a spectral signature consistent with a brightening of the same 5 bar layer at longer wavelengths (>700 nm). This bright feature is much deeper than previously studied dark-spot companion clouds and may be connected with the circulation that generates and sustains such spots.