The History of Eruptions at Acala Fluctus, Io: Source of Multiple Outbursts

Recent ground-based Infrared Telescope Facility observations showed that a hot spot observed at the location of the surface feature Acala Fluctus was volcanically active for ∼18 months in 2019–2020 and exhibited two outbursts with a temperature of ∼1200 K. A high-temperature hot spot at Acala was also observed by Galileo SSI in the late 1990s over multiple flybys. Low-temperature hot spots in this area were detected in 2000 by the Galileo Photopolarimeter Radiometer and in 1979 by Voyager IRIS. However, neither the Galileo NIMS instrument nor any instrument on the New Horizons spacecraft, which flew by Io in 2007, saw any evidence of an Acala hot spot. It is also possible that earlier ground-based disk-integrated observations of hot spots are due to Acala, even though they were originally attributed to other volcanoes, such as Loki. These include outbursts in 1978 and 1990 and a persistent low-temperature source in the 1980 and 1990s. From these observations, we propose that Acala consists of highly variable high-temperature fire fountains and a large area of low-temperature, older flows. Due to these recent outbursts, we expect that any images of Acala obtained by JunoCam will show surface changes from Galileo images.