  We present the first comprehensive study of NaCl and KCl gases in Io's
atmosphere in order to investigate their characteristics, and to infer
properties of Io's volcanoes and subsurface magma chambers. In this work, we
compile all past spectral line observations of NaCl and KCl in Io's atmosphere
from the Atacama Large Millimeter/submillimeter Array (ALMA) and use
atmospheric models to constrain the physical properties of the gases on several
dates between 2012 and 2018. NaCl and KCl appear to be largely spatially
confined and for observations with high spectral resolution, the temperatures
are high (~500-1000 K), implying a volcanic origin. The ratio of NaCl:KCl was
found to be ~5-6 in June 2015 and ~3.5-10 in June 2016, which is consistent
with predictions based on observations of Io's extended atmosphere, and less
than half the Na:K ratio in chondrites. Assuming these gases are volcanic in
origin, these ratios imply a magma temperature of ~1300 K, such that the magma
will preferentially outgas KCl over NaCl.
