ALMA Observations of Io Going into and Coming out of Eclipse

We present 1 mm observations constructed from Atacama Large (sub)Millimeter Array (ALMA) data of SO 2 , SO, and KCl when Io went from sunlight into eclipse (2018 March 20) and vice versa (2018 September 2 and 11). There is clear evidence of volcanic plumes on March 20 and September 2. The plumes distort the line profiles, causing high-velocity (≳500 m s −1 ) wings and red-/blueshifted shoulders in the line profiles. During eclipse ingress, the SO 2 flux density dropped exponentially, and the atmosphere re-formed in a linear fashion when reemerging in sunlight, with a “post-eclipse brightening” after ∼10 minutes. While both the in-eclipse decrease and in-sunlight increase in SO was more gradual than for SO 2 , the fact that SO decreased at all is evidence that self-reactions at the surface are important and fast, and that in-sunlight photolysis of SO 2 is the dominant source of SO. Disk-integrated SO 2 in-sunlight flux densities are ∼2–3 times higher than in eclipse, indicative of a roughly 30%–50% contribution from volcanic sources to the atmosphere. Typical column densities and temperatures are N ≈ (1.5 ± 0.3) × 10 16 cm −2 and T ≈ 220–320 K both in sunlight and in eclipse, while the fractional coverage of the gas is two to three times lower in eclipse than in sunlight. The low-level SO 2 emissions present during eclipse may be sourced by stealth volcanism or be evidence of a layer of noncondensible gases preventing complete collapse of the SO 2 atmosphere. The melt in magma chambers at different volcanoes must differ in composition to explain the absence of SO and SO 2 , but simultaneous presence of KCl over Ulgen Patera.