Ganymede's UV Reflectance From Juno‐UVS Data. Issue 23 (12th December 2022)

• Record Type: Journal Article
• Authors: Molyneux, P. M.; Greathouse, T. K.; Gladstone, G. R.; Versteeg, M. H.; Hue, V.; Kammer, J.; Davis, M. W.; Bolton, S. J.; Giles, R.; Connerney, J. E. P.; Gérard, J. C.; Grodent, D. C.
• Is Part Of: Geophysical research letters. Volume 49:Issue 23(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-12-12
• Subjects: ganymede -- UV reflectance -- ice; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2022GL099532
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: During the Juno orbit 34 Ganymede encounter, the ultraviolet spectrograph mapped UV sunlight reflected by Ganymede from a closest approach altitude of 1, 044 km, allowing us to study spatial variations in Ganymede's far ultraviolet reflectance at higher resolution than has previously been possible. We find that a characteristic signature of water ice seen around 165 nm in laboratory spectra is absent over much of the observed area, but is detectable in the north high latitude region. We suggest that the spectral difference between the high latitudes and other icy regions, such as Tros crater, may be explained by the presence of additional UV‐absorbing contaminants such as NH3 at lower latitudes. We also note a decrease in the relative reflectance of the high latitude regions at wavelengths >190 nm, which may be the start of a previously observed ozone absorption feature. Plain Language Summary: The ultraviolet spectrograph (UVS) instrument on the Juno spacecraft observed ultraviolet sunlight reflected by the surface of Jupiter's largest moon, Ganymede. Although we know from previous observations that Ganymede's surface contains a large amount of water ice, we found that Ganymede does not reflect ultraviolet light in the way we expected: while water ice is very reflective at ultraviolet wavelengths longer than 165 nm, Ganymede's surface does not reflect well at these wavelengths. The northern high latitudes were found to be more reflective than the low latitudes, suggesting that the polar regions are more water‐ice rich than parts of the surface closer to the equator. The bright Tros crater, which is known to contain a large amount of water ice, had a low UV reflectance, which we suggest may be due to UV‐absorbing material trapped within the ice there. These results will be useful for planning future investigations of Ganymede's surface composition by the UVS instrument on the upcoming JUICE mission, which will tour all three of Jupiter's icy moons before entering orbit around Ganymede. Key Points: Juno ultraviolet spectrograph mapped UV sunlight reflected by Ganymede's surface during the Juno orbit 34 Ganymede flyby The UV signature of pure water ice was detected in the north polar regions but not within other ice‐rich regions including Tros crater Additional absorption features observed may be related to ammonia and ozone