Diurnal variations in the stratosphere of an ultrahot exoplanet

The temperature profile of a planetary atmosphere is a key diagnostic of radiative and dynamical processes governing the absorption, redistribution, and emission of energy. Observations have revealed dayside stratospheres that either cool [1,2] or warm [3,4] with altitude for a small number of gas giant exoplanets, while others are consistent with constant temperatures [5,6,7,8]. Here we report spectroscopic phase curve measurements for the gas giant WASP-121b,[9] which constrain stratospheric temperatures throughout the diurnal cycle. Variations measured for a water vapor spectral feature reveal a temperature profile that transitions from warming with altitude on the dayside hemisphere to cooling with altitude on the nightside hemisphere. The data are well explained by models assuming chemical equilibrium, with water molecules thermally dissociating at low pressures on the dayside and recombining on the nightside [10,11]. Nightside temperatures are low enough for perovskite (CaTiO3) to condense, which could deplete titanium from the gas phase [12,13] and explain recent non-detections at the day-night terminator [14,15,16,17]. Nightside temperatures are also low enough for refractory species, such as magnesium, iron, and vanadium, to condense. Detections [16,17,18,19] of these metals at the day-night terminator suggest, however, that if they do form nightside clouds, cold trapping is not as effective at removing them from the upper atmosphere. Note: Numbered references have been entered into the "Manuscript Comment" box.

Brief communication: Lower-bound estimates for residence time of energy in the atmospheres of Venus, Mars and Titan

The residence time of energy in a planetary atmosphere, τ, which was recently introduced and computed for the Earth's atmosphere (Osácar et al., 2020), is here extended to the atmospheres of Venus, Mars and Titan. τ is the timescale for the energy transport across the atmosphere. In the cases of Venus, Mars and Titan, these computations are lower bounds due to a lack of some energy data. If the analogy between τ and the solar Kelvin–Helmholtz scale is assumed, then τ would also be the time the atmosphere needs to return to equilibrium after a global thermal perturbation.

Ariel planetary interiors White Paper

The recently adopted Ariel ESA mission will measure the atmospheric composition of a large number of exoplanets. This information will then be used to better constrain planetary bulk compositions. While the connection between the composition of a planetary atmosphere and the bulk interior is still being investigated, the combination of the atmospheric composition with the measured mass and radius of exoplanets will push the field of exoplanet characterisation to the next level, and provide new insights of the nature of planets in our galaxy. In this white paper, we outline the ongoing activities of the interior working group of the Ariel mission, and list the desirable theoretical developments as well as the challenges in linking planetary atmospheres, bulk composition and interior structure.

Perspective on Energetic and Thermal Atmospheric Photoelectrons

Atmospheric photoelectrons are central to the production of planetary ionospheres. They are created by photoionization of the neutral planetary atmosphere by solar EUV and soft X-ray irradiance. They provide the energy to heat the thermosphere. Thermalized photoelectrons permeate magnetospheres creating polarization electric fields and plasma waves as they interact with ions to maintain charge neutrality. Energetic photoelectrons (>1 eV) have a distinctive energy spectral shape as first revealed in data from the Atmosphere Explorer satellites. Energetic photoelectrons escaping the ionosphere follow local magnetic fields illuminating the planet's magnetic topology. Current models using state-of-the-art EUV observations accurately capture their production and transport. However, in spite of 60 years of space research the electron thermalization processes occurring below 1 eV at low altitudes in planetary thermospheres are not understood quantitatively. Results from event analysis of data from the Mars Atmosphere and Volatile Evolution (MAVEN) mission are not consistent with current models of photoelectron thermalization. The lack of quantitative understanding reflects the complexity of the physics and the lack of a large data base of simultaneous neutral, ion, and electron densities and temperatures in lower planetary thermospheres.

Automatic Scheduling Tool for Balloon-Borne Planetary Optical Remote Sensing

The balloon-borne Planetary Atmosphere Spectroscopic Telescope (PAST), China’s first planetary optical remote-sensing project, will be launched for testing and conducting scientific flights during 2021 and 2022. Images of the planetary atmosphere and plasma in ultraviolet and visible wavelengths will be used to investigate the diversity of the planetary space environment in the solar system and their different drivers. Because simultaneous observation of multiple target planets in the solar system is possible, effective observation scheduling is critical to acquire high scientific merit spectroscopic imaging data. Herein, we demonstrate an automatic scheduling tool (AST) to aid the planning of observation schedules. The AST is primarily based on a planetary ephemeris and is realized on the basis of the geometrical information and optical requirements of the telescope. The temporal variations of the planetary reference frames can also be obtained to assist in the positioning and data processing of the telescope. As a part of the Chinese deep-space exploration plan, several ground-based planetary optical telescopes will be constructed in China in the future. With the use of the proposed AST, such telescopes can achieve maximum efficiency.

Direct observation of long-lived cyanide anions in superexcited states

The cyanide anion (CN−) has been identified in cometary coma, interstellar medium, planetary atmosphere and circumstellar envelopes, but its origin and abundance are still disputed. An isolated CN− is stabilized in the vibrational states up to ν = 17 of the electronic ground-state 1Σ+, but it is not thought to survive in the electronic or vibrational states above the electron autodetachment threshold, namely, in superexcited states. Here we report the direct observation of long-lived CN− yields of the dissociative electron attachment to cyanogen bromide (BrCN), and confirm that some of the CN− yields are distributed in the superexcited vibrational states ν ≥ 18 (1Σ+) or the superexcited electronic states 3Σ+ and 3Π. The triplet state can be accessed directly in the impulsive dissociation of BrCN− or by an intersystem transition from the superexcited vibrational states of CN−. The exceptional stability of CN− in the superexcited states profoundly influences its abundance and is potentially related to the production of other compounds in interstellar space.

Characterising regimes of atmospheric circulation in terms of their global super-rotation

The global super-rotation index S compares the integrated axial angular momentum of the atmosphere to that of a state of solid-body co-rotation with the underlying planet. S is similar to a zonal Rossby number, which suggests it may be a useful indicator of the circulation regime occupied by a planetary atmosphere. We investigate the utility of S for characterising regimes of atmospheric circulation by running idealised Earth-like general circulation model experiments over a wide range of rotation rates Ω, 8ΩE to ΩE/512, where ΩE is the Earth’s rotation rate, in both an axisymmetric and three-dimensional configuration. We compute S for each simulated circulation, and study the dependence of S on Ω. For all rotation rates considered, S is of the same order of magnitude in the 3D and axisymmetric experiments. For high rotation rates, S ≪ 1 and (S ∝ Ω−2, while at low rotation rates S ≈ 1/2 = constant. By considering the limiting behaviour of theoretical models for S, we show how the value of S and its local dependence on Ω can be related to the circulation regime occupied by a planetary atmosphere. S ≪ 1 and S ∝ Ω−2 defines a regime dominated by geostrophic thermal wind balance, and S ≈ 1/2 = constant defines a regime where the dynamics are characterised by conservation of angular momentum within a planetary-scale Hadley circulation. S ≫ 1 and S ∝ Ω−1 defines an additional regime dominated by cyclostrophic balance and strong equatorial super-rotation that is not realised in our simulations.

Brief Communication: Residence Time of Energy in the Atmospheres of Venus, Earth, Mars and Titan

The concept of residence time of energy in a planetary atmosphere τR, recently introduced and computed for the Earth's atmosphere (Osácar et al., 2020), is here extended to the atmospheres of Venus, Mars and Titan. After a global thermal perturbation, τR is the time scale the atmosphere needs to return to equilibrium. The residence times of energy in the atmospheres of Venus, Earth, Mars and Titan have been computed. In the cases of Venus, Mars and Titan, these are mere lower bounds due to a lack of some energy data.

GJ 436b and the stellar wind interaction: simulations constraints using Lyα and Hα transits

The GJ 436 planetary system is an extraordinary system. The Neptune-size planet that orbits the M3 dwarf revealed in the Lyα line an extended neutral hydrogen atmosphere. This material fills a comet-like tail that obscures the stellar disc for more than 10 hours after the planetary transit. Here, we carry out a series of 3D radiation hydrodynamic simulations to model the interaction of the stellar wind with the escaping planetary atmosphere. With these models, we seek to reproduce the $\sim 56\%$ absorption found in Lyα transits, simultaneously with the lack of absorption in Hα transit. Varying the stellar wind strength and the EUV stellar luminosity, we search for a set of parameters that best fit the observational data. Based on Lyα observations, we found a stellar wind velocity at the position of the planet to be around [250-460] km s−1 with a temperature of [3 − 4] × 105 K. The stellar and planetary mass loss rates are found to be 2 × 10−15 M⊙ yr−1 and ∼[6 − 10] × 109 g s−1, respectively, for a stellar EUV luminosity of [0.8 − 1.6] × 1027 erg s−1. For the parameters explored in our simulations, none of our models present any significant absorption in the Hα line in agreement with the observations.