Effect of the 3D distribution on water observations made with the SWI

Abstract

Context. Characterising and understanding the atmospheres of Jovian icy moons is one of the key exploration goals of the Submillimetre Wave Instrument (SWI), which is to be flown on ESA’s Jupiter Icy Moons Explorer (JUICE) mission. Aims. The aim of this paper is to investigate how and under which conditions a 3D asymmetric distribution of the atmosphere may affect the SWI observations. In this work we target the role of phase angle for both nadir and limb geometries for unresolved and partially resolved disc observations from large distances. Methods. We adapted the LIME software package, a 3D non-local thermodynamical equilibrium radiative transfer model, to evaluate ortho-H2O populations and synthesise the simulated SWI beam spectra for different study cases of Ganymede’s atmosphere. The temperature and density vertical distributions were adopted from a previous work. The study cases presented here were selected according to the distance and operational scenarios of moon monitoring anticipated for SWI during the Jupiter phase of the JUICE mission. Results. We demonstrate that nadir and limb observations at different phase angles will modify the line amplitude and width. Unresolved observations where both absorption against surface continuum and limb emission contributes within the beam will lead to characteristic line wing emission, which may also appear in pure nadir geometry for specific phase angles. We also find that for Ganymede, the 3D non-local thermodynamical equilibrium populations are more highly excited in the upper atmosphere near the sub-solar region than they are in 1D spherically symmetric models. Finally, the 3D radiative transfer is better suited to properly simulate spectral lines for cases where density or population gradients exist along the line of sight.

A sublimated water atmosphere on Ganymede detected from Hubble Space Telescope observations

“…The Jupiter Icy Moons Explorer (JUICE) mission of the European Space Agency will investigate Ganymede in great detail with its eleven science instruments during several flybys in the first mission phase (expected for the period 2031-2034) and finally from orbit around the moon (from 2034). Several science instruments are equipped to measure Ganymede's neutral gas environment and particularly the H 2 O abundance by remote sensing of UV, optical, infrared and submillimetre emissions 49,50 , as well as in situ sensing with the neutral particle detector.…”

A sublimated water atmosphere on Ganymede detected from Hubble Space Telescope observations

“…Several science instruments are equipped to measure Ganymede's neutral gas environment and in particular the H 2 O abundance by remote sensing of UV, optical, infrared, and sub-millimeter emissions [47,48], as well as in-situ with the neutral particle detector.…”

Rate coefficients for rotational state-to-state transitions in H2O + H2O collisions for cometary and planetary applications, as predicted by mixed quantum-classical theory

“…Water is rather special among the many molecules in space. It is found in large variety of astrophysical environments, ranging from cold interstellar molecular clouds (van Dishoeck et al (2021); Tielens (2005)) to stellar photospheres and circumstellar envelopes (Caselli et al (2010); Chen & Neufeld (1995); Codella et al (2016); van Dishoeck et al (2021); van Dishoeck et al (2011)), atmospheres of icy planets (Hartogh et al (2011); Vorburger et al (2022); Wirström et al (2020)), and cometary comas (Bockelée-Morvan et al (2010); Cochran et al (2015); Dones et al (2015)). In these media, water molecules represent the main reservoir of oxygen and control the chemistry of many species, both in the gas phase and on grain surfaces (van Dishoeck et al (2021); Henning & Semenov (2013); Roueff & Lique (2013); Sakai et al (2014); Tielens (2005); Tielens & Allamandola (1987)).…”