Ground-based observations are providing new information about the volcanic phenomena at Io's surface. Thermal emission from lava can be seen routinely at infrared wavelengths. One result of recent work is the reinvigoration of a familiar theme---silicate volcanism. In the following paragraphs we will focus on such advances which have resulted from a better understanding of Io's thermal emission. This emission tells us about ongoing volcanic processes and heat flow. Io's total heat flow is especially important because of the tidal interactions among the bodies in the jovian system. The value of this heat flow not only constrains models for Io's interior but also those for Jupiter and for the long-term orbital evolution of the whole system.
It has been some fifteen years since the unambiguous
detection of active volcanoes on Jupiter's innermost satellite Io
by the Voyager spacecraft. The discovery of large geyser-like
eruptive plumes and extensive lava flows on Io's bright,
yellow-brown surface initiated a long running debate on the
nature of the processes and the composition of the volcanic
fluids. Remote sensing data suggested sulfur or sulfur compounds
on the surface, and the Voyager infrared spectrometer (IRIS)
detected SO
gas above a volcanic vent. Sulfur and SO
were proposed as the working fluids for the phase change
volcanism driving the plumes. Sulfur liquid phases were also
suggested as compositional candidates for the lava comprising the
many lakes and flows evident in the images [Sagan, 1979]. On the
other hand, strength and structural arguments and Io's relatively
high density (3500 kg m
) were advanced as arguments for a
major role for silicate volcanism [Carr et al., 1979].
Volcanic eruptions also provided an explanation for Io's
unusual infrared spectrum and earlier, pre-Voyager, observations
of short-lived, outbursts of infrared radiation. These are now
interpreted as the result of varying levels of eruptive activity.
Since the Voyager flybys, astronomical observers continued to
measure Io's thermal emission at infrared wavelengths. They find
that a large amount of heat is being radiated continually from a
small number of volcanically heated regions on Io. Outbursts, or
short-lived enhancements of flux from high temperature sources,
while not common, have been observed since Witteborn et al.'s
[1979] initial (pre-Voyager) measurement of a 
600 K source.
The second sighting was made by Sinton et al. [1980] who observed
a large 4.8 
m flux on one night between the two Voyager
flybys. It was suggested that this outburst was correlated with
a change in the albedo and surroundings of the feature called
Surt. Over the following years numerous ``outbursts'' were
reported. They were all characterized by short periods (i.e.,
hours to days) of large increases in flux at 4.8 
m [Sinton
et al., 1983; Howell and Sinton, 1989; McEwen et al., 1989;
Johnson et al., 1988; Veeder et al., 1994b].
While these outbursts continued to create excitement as they popped on and off, it was quickly realized that even with their relatively high temperatures, they were not contributing significantly to Io's heat flow because of the short durations of their activity. Accordingly, their global geophysical significance was not clear and for a while they were treated as a separate phenomenon, apart from the cooler, long-lived sources which dominated Io's heat flow.
Io's globally averaged heat flow is about 2 W m
. This
value is high! It is thirty times the corresponding value for
the Earth and one hundred times that for the Moon, a body of the
same size as Io [Matson et al., 1980, 1981]. Tidal dissipation
theory is the only mechanism that has yielded levels of power
which approach the observed heat flow. As a result of tidal
interactions with Jupiter, energy is extracted from Io's orbit
and deposited as heat in Io's interior [Peale et al., 1979;
Yoder, 1979]. The amount of power available from this process is
determined by the tidal dissipation properties of Io, Jupiter and
the complicated orbital mechanics of the Jovian system.
In the past few years analysis of new data has provided
strong evidence for the presence of silicate lavas on the surface
and suggests that silicate volcanism may be the primary
resurfacing mechanism as opposed to plume deposition or sulfur
flows. The data consist of two outbursts which were observed at
several wavelengths sumultaneously in 1986 and 1990. In the
first case, a source temperature of about 1550 K is required. In
the second, the source started out hot, 
1200 K, with a
circular equivalent radius of 
5.6 km, and then cooled and
grew in size for three hours, reaching a temperature of 
700
K and a radius of 13 km---clearly suggesting an ongoing lava
flow. In both of these cases the temperatures were well above
the boiling point of sulfur (i.e., 715 K at Standard Temperature
and Pressure) and thus clearly established the presence of
silicate lava at Io's surface [Johnson et al., 1988; Veeder et
al., 1994b].