High Spatial and Spectral Resolution Observations of the Forbidden 1.707 μm Rovibronic SO Emissions on Io: Evidence for Widespread Stealth Volcanism*

Abstract

We present observations obtained with the 10 m Keck telescopes of the forbidden SO a1Δ → X3Σ− rovibronic transition at 1.707 μm on Io while in eclipse. We show its spatial distribution at a resolution of ∼0.″12 and a spectral resolution of R ∼ 2500, as well as disk-integrated spectra at a high spectral resolution (R ∼ 15,000). Both the spatial distribution and the spectral shape of the SO emission band vary considerably across Io and over time. In some cases the SO emissions either in the core or the wings of the emission band can be identified with volcanoes, but the largest areas of SO emissions usually do not coincide with known volcanoes. We suggest that the emissions are caused by a large number of stealth plumes, produced through the interaction of silicate melts with superheated SO2 vapor at depth. The spectra, in particular the elevated wing of the emission band near 1.69 μm, and their spatial distribution strongly suggest the presence of nonlocal thermodynamic equilibrium processes in addition to the direct ejection of excited SO from the (stealth and other) volcanic vents.

An Energetic Eruption With Associated SO 1.707 Micron Emissions at Io's Kanehekili Fluctus and a Brightening Event at Loki Patera Observed by JWST

“…Low emission levels were also seen over large fractions of Io's surface, in particular at latitudes between 0° and 40°S. In the present paper we rule out potential excitation by the Lyman-α flux from the interplanetary medium, hence strengthening conclusions from 2010 to 2,015 observations that the SO 1.707 μm emissions may originate from stealth volcanism (de Pater et al, 2020).…”

“…In contrast, Loki Patera shows a quasi‐periodic behavior, where the intensity increases every 440–540 days (de Kleer & de Pater, 2017; Rathbun & Spencer, 2006). The increase and subsequent decrease typically happen over a period of order 1–2 months each, separated by a ∼3‐month period of high intensity (∼120 GW/ μ m/sr at 3.78 μm) (de Kleer & de Pater, 2017; de Pater et al., 2017; Howell et al., 2001). The most up‐to‐date Loki Patera graph of intensity versus time (de Kleer & Rathbun, 2023) shows that the volcano's last brightening event stretched over April–September 2021.…”

“…Panels (c) and (d) in Figure 3 show the spatial distribution of the SO emission integrated over the center of the emission band (1.705-1.709 μm), as observed in July 2010 and December 2015 (de Pater et al, 2020). The distribution in those years was very different, and the authors at that time concluded that there was no obvious correlation with hot spots.…”

“…The composition and temperature of Io's magma, however, which are indicative of the state of Io's highly processed mantle, are poorly constrained. Blackbody radiation from volcanic hot spots is observable in the near and mid‐infrared, but reflected sunlight from Io's bright surface overwhelms the thermal emission at wavelengths shortwards of ∼3 μm (de Pater et al., 2004, 2017). The shorter wavelengths are most sensitive to high‐temperature eruptions (1,000–2,000 K blackbody curves peak at 3 and 1.5 μm, resp.…”

Spectroscopic Mapping of Io’s Surface with HST/STIS: SO2 Frost, Sulfur Allotropes, and Large-scale Compositional Patterns

“…Figure 5 shows the SO 2 distribution we obtain based on the 280 nm mid-UV feature. In agreement with distributions derived from Voyager and HST imaging (McEwen et al 1988;Sartoretti et al 1996) and Galileo and ground-based mapping of Io's infrared SO 2 bands (Carlson et al 1997;Laver & de Pater 2008de Pater et al 2020), we observe strong SO 2 absorption at equatorial latitudes within Bosphorus Regio and Colchis Regio between ∼100 and 250°W. In fact, our SO 2 map shows a distribution qualitatively identical at our resolution to that produced from the Voyager imagery (McEwen et al 1988) and to the visibly bright regions of the USGS Galileo/Voyager color mosaic of Io, 6 with all three showing additional deposits near the sub-Jovian point, at localized high northern latitudes, and within Tarsus Regio (∼40°S, 55°W), which are less apparent in the infrared maps.…”