Abstract
Io’s atmosphere is predominately SO2 that is sustained by a combination of volcanic outgassing and sublimation. The loss from the atmosphere is the main mass source for Jupiter’s large magnetosphere. Numerous previous studies attributed various transient phenomena in Io’s environment and Jupiter’s magnetosphere to a sudden change in the mass loss from the atmosphere supposedly triggered by a change in volcanic activity. Since the gas in volcanic plumes does not escape directly, such causal correlation would require a transient volcano-induced change in atmospheric abundance, which has never been observed so far.Here we report four observations of atmospheric SO2 and NaCl from the same hemisphere of Io, obtained with the IRAM NOEMA interferometer on 11 December 2016, 14 March, 6 and 29 April 2017. These observations are compared to measurements of volcanic hot spots and Io’s neutral and plasma environment. We find a stable NaCl column density in Io’s atmosphere on the four dates. The SO2 column density derived for December 2016 is about 30% lower compared to the SO2 column density found in the period of March to April 2017. This increase in SO2 from December 2016 to March 2017 might be related to increasing volcanic activity observed at several sites in spring 2017, but the stability of the volcanic trace gas NaCl and resulting decrease in NaCl/SO2 ratio do not support this interpretation. Observed dimmings in both the sulfur ion torus and Na neutral cloud suggest rather a decrease in mass loading in the period of increasing SO2 abundance. The dimming Na brightness and stable atmospheric NaCl furthermore dispute an earlier suggested positive correlation of the sodium cloud and the hot spot activity at Loki Patara, which considerably increased in this period. The environment of Io overall appears to be in a rather quiescent state, preventing further conclusions. Only Jupiter’s aurora morphology underwent several short-term changes, which are apparently unrelated to Io’s quiescent environment or the relatively stable atmosphere.
Highlights
The atmosphere of Jupiter’s volcanic moon Io consists primarily of SO2 and is generated through a combination of direct volcanic outgassing and sublimation of volcanic frost deposits
During the three observations taken in March and April 2017 the SO2 column density appears to be stable, i.e. the results are consistent with a constant abundance with the 1-σ uncertainties (Fig. 3c and Table 4)
Comparing the combined fluxes directly derived from the four SO2 spectra (Table 3), the Dec 2016 value is lower than the average from the March and April 2017 tracks (by 15(±7)% as compared to 30(±14)% from the model), suggesting that a change in the SO2 atmosphere did happen in this period
Summary
The atmosphere of Jupiter’s volcanic moon Io consists primarily of SO2 and is generated through a combination of direct volcanic outgassing and sublimation of volcanic frost deposits (see e.g. review by Lellouch et al, 2007). Infrared (IR) observations of an SO2 absorption band obtained over a Jupiter season revealed a stable SO2 atmosphere, with moderate but clear dependence on heliocentric distance suggesting that both sublimation and (constant) volcanic outgassing are viable sources (Tsang et al, 2012). The most compelling evidence for sublimation being the dominating source was provided by IR measurements: Tsang et al (2016) detected an SO2 collapse by a factor of 5 ± 2 after Io entered the shadow of Jupiter, which they explained with a decrease of surface temperature and of sublimation. Besides direct outgassing of gaseous NaCl, vaporization of NaCl condensates might become a possible source if volcanic eruptions lead to surface temperatures above 1000 K (Von Vartenberg and Albreeht, 1921) as occasionally observed at Io (McEwen et al, 1998; Keszthelyi et al, 2007)
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