Observations of Loki Patera by Juno during Close Flybys

Abstract We used data from the Juno spacecraft to investigate both the spatial and temporal properties of Loki Patera on Io, acquired in two infrared bands between 2022 December and 2024 April, at pixel sizes ranging from 400 m to 15 km. Loki shows a thermal structure unlike other active lava lakes previously reported, with some brightening near the lake’s perimeter but lacking the continuous “hot ring” seen at other paterae. Modeling the slow rate of cooling suggests there is a significant volume of magma beneath the crust to provide the latent heat necessary to decelerate the cooling. A thermal propagation that may represent the signature of a resurfacing wave, going from the southwest of the lake to the north, was observed with a velocity of ∼2–3 km day −1 . Data collected in 2024 may indicate the onset of a new resurfacing wave originating from a point source, rather than the foundering of a linear section of the crust. We also observed many small (∼3 km wide), closely spaced (∼10 km apart) islands that have persisted in the same locations for at least 45 years, since first being imaged by Voyager 1. The persistence of these islands challenges resurfacing models of Loki, as they have remained fixed—likely anchored to the lava lake floor—and have not noticeably changed in size, arguing against large-scale thermal erosion. The central island of Loki shows a few thermal structures associated with the fractures that cross the island, indicating that the fractures most likely contain molten lava.