  Tidal heating is the major source of heat in the outer Solar System. Because
of its strong tidal interaction with Jupiter and the other Galilean Satellites,
Io is incredibly volcanically active. We use the directly measured volcanic
activity level of Io's volcanoes as a proxy for surface heat flow and compare
to tidal heating model predictions. Volcanic activity is a better proxy for
heat flow than simply the locations of volcanic constructs. We determine the
volcanic activity level using three data sets: Galileo PPR, Galileo NIMS, and
New Horizons LEISA. We also present a systematic reanalysis of the Galileo NIMS
observations to determine the 3.5 micron brightness of 51 active volcanoes. We
find that potential differences in volcanic style between high and low
latitudes make high latitude observations unreliable in distinguishing between
tidal heating models. Observations of Io's polar areas, such as those by JUNO,
are necessary to unambiguously understand Io's heat flow. However, all three of
the data sets examined show a relative dearth of volcanic brightness near 180 W
(anti-Jovian point) and the equator and the only data set with good
observations of the sub-Jovian point (LEISA) also shows a lack of volcanic
brightness in that region. These observations are more consistent with the
mantle-heating model than the asthenospheric-heating model. Furthermore, all
three of the data sets are consistent with four-fold symmetry in longitude and
peak heat flow at mid-latitudes, which best matches with the combined heating
case of Tackley et al. (2001).
