Exoplanet atmospheres

Abstract:

Whether or not there is a particulate layer in the lowest 10 km of the Venusian atmosphere is still an open question. Some of the past in situ experiments showed the presence of a detached particulate layer, and a few suggested the existence of finely dispersed aerosols, while other instruments supported the idea of no particulate matter in the deep atmosphere. In this work, we investigate the presence of a near‐surface particulate layer (NSPL) using in situ data from the Venera 13 mission. While the original spectrophotometric data from Venera 13 were lost, we have reconstructed a part of this data by digitizing the old graphic material and selected the eight most reliable Venera 13 downward radiance profiles from 0.48 to 0.8 μ ${\upmu }$ m for our retrievals. The retrievals suggest the existence of the particulate layer with a peak in the altitude range of 3.5–5 km. They further indicate a log‐normal particle size distribution with a mean radius between 0.6 and 0.85 μ ${\upmu }$ m. The retrievals constrain the real refractive index of the particles to lie around the range of 1.4–1.6, with the imaginary refractive index of a magnitude of 10 − 3 ${10}^{-3}$ . Based on refractive index retrievals, uplifted basalt particles or volcanic ash could be responsible for near‐surface particulates. In comparison, volatile condensates appear less likely to be behind the formation of NSPL.

Abstract:

The full range of conditions under which rocky planets can host atmospheres remains poorly understood, especially in the regime of close-in orbits around late-type stars. One way to assess the presence of atmospheres on rocky exoplanets is to measure their dayside emission as they are eclipsed by their host stars. Here, we present Spitzer observations of the 4.5 μm secondary eclipses of the rocky super-Earth TOI-431 b, whose mass and radius indicate an Earth-like bulk composition (3.07 ± 0.35 M⊕, 1.28 ± 0.04 R⊕). Exposed to more than 2000 times the irradiation of Earth, dayside temperatures of up to 2400 K are expected if the planet is a dark bare rock without a significant atmosphere. Intriguingly, despite the strong stellar insolation, we measure a secondary-eclipse depth of only 33 ± 22 ppm, which corresponds to a dayside brightness temperature of 1520−390+360 K. This notably low eclipse depth disagrees with the dark bare-rock scenario at the 2.5σ level, and suggests either that the planet is surrounded by an atmosphere or that it is a bare rock with a highly reflective surface. In the atmosphere scenario, the low dayside emission implies the efficient redistribution of heat to the nightside, or by molecular absorption in the 4–5 μm bandpass. In the bare-rock scenario, a surface composition made of a high-albedo mineral species such as ultramafic rock can lead to reduced thermal emission consistent with low eclipse depth measurement. Follow-up spectroscopic observations with the James Webb Space Telescope hold the key to constraining the nature of the planet.

Abstract:

Over the past two decades, a coherent picture has emerged of the atmospheric dynamics of hot Jupiters from a combination of three-dimensional general circulation models and astronomical observations. This paradigm consists of hot Jupiters being spin-synchronized due to their close-in orbit, with a resulting large day-to-night irradiation gradient driving a day-to-night temperature contrast. This day-to-night temperature contrast in turn raises day-to-night pressure gradients that are balanced by a circulation with wind speeds on the order of km s−1. The dominant feature of this circulation is a super-rotating equatorial jet, maintained by eddy-mean flow interactions that pump momentum into the jet. In this work, I explore the dependence of this circulation paradigm on the initial thermal and dynamical conditions in atmospheric circulation models of hot Jupiters. To do so, I conduct MITgcm simulations of the atmospheric circulation of hot Jupiters with both varying initial wind directions and initial temperature profiles. I find that the results are insensitive to the initial conditions, implying that the current paradigm of hot-Jupiter circulation exhibits at most limited hysteresis. I demonstrate that there is a single characteristic wind speed of hot Jupiters for given planetary and atmospheric parameters using an idealized scaling theory, and discuss implications for the interpretation of hot Jupiter observations.