Enceladus and Jupiter as exoplanets: The opposition surge effect

¹ Center for Space and Habitability, University of Bern, Gesellschaftsstrasse 6, 3012 Bern, Switzerland
² Space Telescope Science Institute, 3700 San Martin Dr, Baltimore, MD 21218, USA
³ Ludwig Maximilian University, University Observatory Munich, Scheinerstrasse 1, Munich 81679, Germany
⁴ University of Warwick, Department of Physics, Astronomy & Astrophysics Group, Coventry CV4 7AL, UK
⁵ University of Bern, ARTORG Center for Biomedical Engineering Research, Murtenstrasse 50, 3008, Bern, Switzerland

^★ Corresponding author; kathryn.jones@unibe.ch

Received: 14 June 2024
Accepted: 22 January 2024

Abstract

Planets and moons in our Solar System have strongly peaked reflected light phase curves at opposition. In this work, we produce a modified reflected light phase curve model and use it to fit the Cassini phase curves of Jupiter and Enceladus. This ‘opposition effect’ is caused by shadow hiding (SH; particles or rough terrain cast shadows which are not seen at zero phase) and coherent backscattering (CB; incoming light constructively interferes with outgoing light). We find tentative evidence for CB preference in Jupiter compared to SH, and no evidence of preference in Enceladus. We show that the full-width half-maximum (FWHM) of Jupiter’s opposition peak is an order of magnitude larger than that of Enceladus and conclude that this could be used as a solid-surface indicator for exoplanets. We investigate this and show that modelling the opposition peak FWHM in solid-surface exoplanets would be unfeasible with JWST or the Future Habitable Worlds Observatory due to the very large signal-to-noise required over a small phase range.