ALMA Observations of Io Going into and Coming out of Eclipse

We present 1-mm observations constructed from ALMA [Atacama Large (sub)Millimeter Array] data of SO$_2$, SO and KCl when Io went from sunlight into eclipse (20 March 2018), and vice versa (2 and 11 September 2018). There is clear evidence of volcanic plumes on 20 March and 2 September. The plumes distort the line profiles, causing high-velocity ($\gtrsim$500 m/s) wings, and red/blue-shifted shoulders in the line profiles. During eclipse ingress, the SO$_2$ flux density dropped exponentially, and the atmosphere reformed in a linear fashion when re-emerging in sunlight, with a "post-eclipse brightening" after $\sim$10 minutes. While both the in-eclipse decrease and in-sunlight increase in SO was more gradual than for SO$_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 $\sim$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 $N \approx (1.5 \pm 0.3) \times 10^{16}$ cm$^{-2}$ and $T \approx 220-320$ K both in-sunlight and in-eclipse, while the fractional coverage of the gas is 2--3 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 non-condensible 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.