A 2020 Observational Perspective of Io

Abstract

Jupiter's Galilean satellite Io is one of the most remarkable objects in our Solar System. The tidal heating Io undergoes through its orbital resonance with Europa and Ganymede has resulted in a body rich in active silicate volcanism. Over the past decades, Io has been observed from ground-based and Earth-orbiting telescopes and by several spacecraft. In this review we summarize the progress made toward our understanding of the physical and chemical processes related to Io and its environment since the Galileo era. Io science has been revolutionized by the use of adaptive optics techniques on large, 8- to 10-m telescopes. The resultant ever-increasing database, mapping the size, style, and spatial distribution of Io's diverse volcanoes, has improved our understanding of Io's interior structure, its likely composition, and the tidal heating process. Additionally, new observations of Io's atmosphere obtained with these large optical/infrared telescopes and the Atacama Large Millimeter/submillimeter Array reveal the presence of volcanic plumes, the (at times) near-collapse of Io's atmosphere during eclipse, and the interactions of plumes with the sublimation atmosphere. ▪ Extensive new data sets of Io at ultraviolet, mid- to near-infrared, and radio wavelengths have been gathered since the Galileo era. ▪ New data and models inform us about tidal heating, surface properties, and magma composition across Io—although key questions remain. ▪ Atmospheric observations indicate a dominant sublimation-supported component and reinforce the presence of stealth volcanism. ▪ Observations of volcanic plumes show high gas velocities (up to ∼1 km/s) and their effect on Io's atmosphere. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 49 is May 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

NaCl and KCl in Io’s Atmosphere

“…If this were the case, SO 2 should be observed at all locations where NaCl and KCl are observed, while NaCl and KCl would be limited to only high-temperature eruptions. Interestingly, NaCl and KCl are often observed at high latitudes, indicative of being sourced by deep mantle heating (Segatz et al 1988;de Kleer & de Pater 2016;de Pater et al 2021). Perhaps at these latitudes, the temperature of the atmosphere is cold enough that SO 2 condenses out very quickly and is not detectable in our observations, although SO 2 emissions have been detected over energetic high-latitude eruptions (e.g., Tvashtar et al 2012).…”

“…Their emission appears to be directly located over Isum, though it should be noted that Zamama is also nearby. Isum has no known plume, and SO 2 was not observed over Isum at the time of observation (de Pater et al 2021).…”

“…Io's atmosphere is primarily sulfur dioxide (SO 2 ), thin and tenuous (Lellouch et al 1990), and is replenished by plume outgassing from its many active volcanoes, as well as sublimation of surface frost deposits (Lellouch et al 2007;Tsang et al 2016;de Pater et al 2020). As such, Ioʼs atmosphere is not uniform across its surface but rather highly spatially localized around active volcanoes and over surface frost deposits at latitudes <| | - 30 40 (e.g., Feaga et al 2009;de Pater et al 2021).…”

“…Plume outgassing on Io is primarily SO 2 , with trace gases SO, NaCl, and KCl (Lellouch et al 2007;de Pater et al 2021). The NaCl was first observed in Ioʼs atmosphere by Lellouch et al (2003), while KCl was first detected by Moullet et al (2013).…”

“…The origin of NaCl and KCl in Ioʼs atmosphere is likely volcanic outgassing, though sputtering of materials on Ioʼs surface may also be a source, which is the case for the atmospheres of the Moon and Mercury (Hunten et al 1988;Killen & Ip 1999). Sputtering as a primary source of these materials, however, would be limited to areas where the atmospheric column density is 10 16 cm −2 , such as at high latitudes or when Io is in Jupiterʼs shadow and its SO 2 atmosphere has collapsed (Lanzerotti et al 1982;de Pater et al 2021). In the case where these gases are indeed volcanic in origin, their ratio of abundances can be used to indirectly infer the temperature and composition of the magma chambers from which they originate (Fegley & Zolotov 2000), something that is at present poorly constrained.…”