Photochemically produced SO2 in the atmosphere of WASP-39b.

Photochemistry is a fundamental process of planetary atmospheres that regulates the atmospheric composition and stability1. However, no unambiguous photochemical products have been detected in exoplanet atmospheres so far. Recent observations from the JWST Transiting Exoplanet Community Early Release Science Program2,3 found a spectral absorption feature at 4.05 µm arising from sulfur dioxide (SO2) in the atmosphere of WASP-39b. WASP-39b is a 1.27-Jupiter-radii, Saturn-mass (0.28 MJ) gas giant exoplanet orbiting a Sun-like star with an equilibrium temperature of around 1,100 K (ref. 4). The most plausible way of generating SO2 in such an atmosphere is through photochemical processes5,6. Here we show that the SO2 distribution computed by a suite of photochemical models robustly explains the 4.05-µm spectral feature identified by JWST transmission observations7 with NIRSpec PRISM (2.7σ)8 and G395H (4.5σ)9. SO2 is produced by successive oxidation of sulfur radicals freed when hydrogen sulfide (H2S) is destroyed. The sensitivity of the SO2 feature to the enrichment of the atmosphere by heavy elements (metallicity) suggests that it can be used as a tracer of atmospheric properties, with WASP-39b exhibiting an inferred metallicity of about 10× solar. We further point out that SO2 also shows observable features at ultraviolet and thermal infrared wavelengths not available from the existing observations.

Slow evolution of Europa's interior: metamorphic ocean origin, delayed metallic core formation, and limited seafloor volcanism.

Europa's ocean lies atop an interior made of metal and silicates. On the basis of gravity data from the Galileo mission, many argued that Europa's interior, like Earth, is differentiated into a metallic core and a mantle composed of anhydrous silicates. Some studies further assumed that Europa differentiated while (or soon after) it accreted, also like Earth. However, Europa probably formed at much colder temperatures, meaning that Europa plausibly ended accretion as a mixture containing water-ice and/or hydrated silicates. Here, we use numerical models to describe the thermal evolution of Europa's interior assuming low initial temperatures (~200 to 300 kelvin). We find that silicate dehydration can produce Europa's current ocean and icy shell. Rocks below the seafloor may remain cool and hydrated today. Europa's metallic core, if it exists, may have formed billions of years after accretion. Ultimately, we expect the chemistry of Europa's ocean to reflect protracted heating of the interior.

A bimodal distribution of haze in Pluto's atmosphere.

Pluto, Titan, and Triton make up a unique class of solar system bodies, with icy surfaces and chemically reducing atmospheres rich in organic photochemistry and haze formation. Hazes play important roles in these atmospheres, with physical and chemical processes highly dependent on particle sizes, but the haze size distribution in reducing atmospheres is currently poorly understood. Here we report observational evidence that Pluto's haze particles are bimodally distributed, which successfully reproduces the full phase scattering observations from New Horizons. Combined with previous simulations of Titan's haze, this result suggests that haze particles in reducing atmospheres undergo rapid shape change near pressure levels ~0.5 Pa and favors a photochemical rather than a dynamical origin for the formation of Titan's detached haze. It also demonstrates that both oxidizing and reducing atmospheres can produce multi-modal hazes, and encourages reanalysis of observations of hazes on Titan and Triton.

Distinguishing the Origin of Asteroid (16) Psyche.

The asteroid (16) Psyche may be the metal-rich remnant of a differentiated planetesimal, or it may be a highly reduced, metal-rich asteroidal material that never differentiated. The NASA Psyche mission aims to determine Psyche's provenance. Here we describe the possible solar system regions of origin for Psyche, prior to its likely implantation into the asteroid belt, the physical and chemical processes that can enrich metal in an asteroid, and possible meteoritic analogs. The spacecraft payload is designed to be able to discriminate among possible formation theories. The project will determine Psyche's origin and formation by measuring any strong remanent magnetic fields, which would imply it was the core of a differentiated body; the scale of metal to silicate mixing will be determined by both the neutron spectrometers and the filtered images; the degree of disruption between metal and rock may be determined by the correlation of gravity with composition; some mineralogy (e.g., modeled silicate/metal ratio, and inferred existence of low-calcium pyroxene or olivine, for example) will be detected using filtered images; and the nickel content of Psyche's metal phase will be measured using the GRNS.

Explaining the Galilean Satellites' Density Gradient by Hydrodynamic Escape.

The Galilean satellites exhibit a monotonic decrease in density (and increase in ice mass fraction) with distance from Jupiter (Pollack & Fanale 1982). Whether this is because of the background conditions when they formed (Lunine & Stevenson 1982; Canup & Ward 2002; Mosqueira & Estrada 2003a; Ronnet et al. 2017), the process of accretion itself (Dwyer et al. 2013), or later loss due to tidal heating (Canup & Ward 2009), has been in dispute for forty years. We find that a hitherto largely neglected process - vapor loss driven by accretional heating (Kuramoto & Matsui 1994) - can reproduce the observed density trend for accretion timescales ≳300 kyr, consistent with gas-starved satellite formation models (Canup & Ward 2002, 2006). In this model both Io and Europa develop an early surface liquid water ocean. Vapor escape from this ocean causes the water inventories of Io and Europa to be completely and mostly lost, respectively. Isotopic fractionation arising from vapor loss means that Europa will develop a higher D/H ratio compared with Ganymede and Callisto. We make predictions that can be tested with in situ measurements of D/H of potential Europa plumes (Roth et al. 2014) by the Europa Clipper spacecraft, or infrared spectroscopic determinations (Clark et al. 2019) of D/H at all three bodies.