Exoplanet atmospheres

Abstract:

In this paper, the improved design of an Ice Giants Net Flux Radiometer (IG-NFR), for inclusion as a payload on a future Uranus probe mission, is given. IG-NFR will measure the net radiation flux, in seven spectral bands, each with a 10° Field-Of-View (FOV) and in five viewing angles as a function of altitude. Net flux measurements within spectral filter bands, ranging from solar to far-infrared, will help derive radiative heating and cooling profiles, and will significantly contribute to our understanding of the planet’s atmospheric heat balance and structure, tropospheric 3-D flow, and compositions and opacities of the cloud layers. The IG-NFR uses an array of non-imaging Winston cones integrated to a matched thermopile detector Focal Plane Assembly (FPA), with individual bandpass filters and windows, housed in a vacuum micro-vessel. The FPA thermopile detector signals are read out in parallel mode, amplified and processed by a multi-channel digitizer application specific integrated circuit (MCD ASIC) under field programmable gate array (FPGA) control. The vacuum micro-vessel rotates providing chopping between FOV’s of upward and downward radiation fluxes. This unique design allows for small net flux measurements in the presence of large ambient fluxes and rapidly changing temperatures during the probe descent to ≥10 bar pressure.

Abstract:

The recent inference of sulfur dioxide (SO₂) in the atmosphere of the hot (approximately 1,100 K), Saturn-mass exoplanet WASP-39b from near-infrared JWST observations^1-3 suggests that photochemistry is a key process in high-temperature exoplanet atmospheres⁴. This is because of the low (<1 ppb) abundance of SO₂ under thermochemical equilibrium compared with that produced from the photochemistry of H₂O and H₂S (1-10 ppm)^4-9. However, the SO₂ inference was made from a single, small molecular feature in the transmission spectrum of WASP-39b at 4.05 μm and, therefore, the detection of other SO₂ absorption bands at different wavelengths is needed to better constrain the SO₂ abundance. Here we report the detection of SO₂ spectral features at 7.7 and 8.5 μm in the 5-12-μm transmission spectrum of WASP-39b measured by the JWST Mid-Infrared Instrument (MIRI) Low Resolution Spectrometer (LRS)¹⁰. Our observations suggest an abundance of SO₂ of 0.5-25 ppm (1σ range), consistent with previous findings⁴. As well as SO₂, we find broad water-vapour absorption features, as well as an unexplained decrease in the transit depth at wavelengths longer than 10 μm. Fitting the spectrum with a grid of atmospheric forward models, we derive an atmospheric heavy-element content (metallicity) for WASP-39b of approximately 7.1-8.0 times solar and demonstrate that photochemistry shapes the spectra of WASP-39b across a broad wavelength range.