Assessment of the feasibility of space-based stellar occultation observations of Uranus and Neptune

Abstract

UV occultation measurements made by Voyager 2 of the upper atmospheres of Uranus and Neptune in the late 1980s found both planets contain warm stratospheres and extremely hot thermospheres. Sunlight reaching the ice giants is far too weak to account for these reported temperatures. This is sometimes called the Giant Planet Energy Crisis. Theorized mechanisms to account for the heating observed at Uranus and Neptune suffer from a dearth of reliable observations. A similar energy crisis was reported from Voyager observations of Jupiter and Saturn, but observations made by the Galileo, Juno, and Cassini-Huygens missions as well as from ground-based facilities have vastly improved efforts to understand upper atmospheric heating. For Uranus and Neptune, Earth-based stellar occultation observations can fill in this significant gap in observations and produce reliable atmospheric measurements. These measurements can also assist in motivating and planning for a flagship mission to either ice giant.A stellar occultation occurs when a solar system body passes across the line of sight from an observer to a star. Refraction of starlight by the upper atmosphere of the occulting body is observed as a light curve and can be used to sample the temperature structure with high vertical resolution. In this paper, we demonstrate that observing stellar occultations by Uranus and Neptune from satellites in low-Earth orbit can make high-quality upper atmospheric measurements of the Ice Giants. Such observations would not be subjected to atmospheric scintillation or disruption due to weather and could be observed at any time of day. We predict that from 2025 through 2035, as many as 56 Uranus and 14 Neptune occultations could be observed from orbit, compared to 20 Uranus and 6 Neptune events from the ground. We find many of these events can be observed with comparable or higher signal-to-noise via a small, Earth-orbiting satellite than a large ground-based observatory and encourage others to explore the launch feasibility of one or more such missions.