426 Encyclopedia of the Solar System
are interpreted to be composed of coarse- to moderate-
grained sulfur dioxide that condensed from plumes and
later recrystallized. Black areas (<2% of surface) mostly
correlate with active hot spots and occur as patera floors,
lava flow fields, or as dark diffuse materials near or sur-
rounding active vents. These materials are most consistent
with magnesium-rich orthopyroxene, indicative of silicate
lava flows or lava lakes (within paterae) or diffuse silicate
pyroclastic deposits near paterae. Perhaps the most intrigu-
ing materials on Io’s surface are the small greenish yellow
deposits seen in a few isolated patches in or near active
vents, which are thought to be composed of either sulfur
compounds contaminated by iron or silicates such as olivine
or pyroxene with or without sulfur-bearing contaminants.
Detection of other substances on Io’s surface has been
difficult because sulfur dioxide condensed from volcanic
plumes blankets most of the surface and hinders detection
of other species.GalileoNIMS detected a broad absorp-
tion at about 1μm, which had been seen from telescopic
observations. However, it is still not known what substance
this spectral absorption is due to, though NIMS observa-
tions showed that it is anticorrelated with recently emplaced
lavas. NIMS also detected local patches of almost pure SO 2 ,
in one case, in Balder Patera, topographically confined, rais-
ing the possibility that it was emplaced as a fluid.
4. Io’s Volcanic Eruptions
Shortly after theVoyagermission, the major controversy
about Io’s volcanic activity concerned the nature of the
volcanism: sulfur or silicates? Io’s surface colors were in-
terpreted as sulfur deposits and this, among other factors,
made the sulfur volcanism hypothesis attractive. One way
to distinguish between sulfur and silicate volcanism is to
measure the temperature of the molten material because
sulfur has a lower melting temperature than silicate lavas.
Sulfur volcanism would not produce temperatures exceed-
ing∼700 K (427◦C), whereas basaltic lavas on Earth range
from 1300 to 1450 K (1027–1177◦C). The temperatures
of the hot spots measured by theVoyagerIRIS instrument
were relatively low (below∼650 K) and could be consistent
with either molten sulfur or silicates. However,Voyagerin-
struments lacked the sensitivity and wavelength coverage
needed to detect small areas at higher temperatures; hence,
Voyagerwas “seeing” only the cooler areas, perhaps cool-
ing silicate lava flows. Between theVoyagerobservations
in 1979 and theGalileoobservations that started in 1996,
several of Io’s hot spots were detected by ground-based
telescopes. Temperature measurements using infrared de-
tectors mounted on telescopes showed higher temperatures
than had been measured by IRIS—such as 900 K reported
by T. Johnson and colleagues in 1988, and 1225 and 1500 K
reported by G. Veeder and colleagues in 1991. These mea-
surements are consistent with silicate magmas but not with
sulfur volcanism.
Galileoincluded much more sensitive instruments than
Voyager, such as the SSI system sensitive from∼400 to
1000 nm wavelengths and the NIMS sensitive from 700
to 5200 nm, but both had limitations. SSI was able to de-
tect only spots hotter than∼700 K and only when Io was
in eclipse (in Jupiter’s shadow) to eliminate reflected and
scattered light. NIMS had the ideal spectral coverage for
detecting both the temperatures and spectral reflectances
expected from silicate lavas, but it had limited spatial reso-
lution (120 km or more) except during the close Io flybys.
However,Galileo’s instruments soon showed the hot spot
temperatures to be indeed consistent with silicate rather
than sulfur volcanism. The greatest surprise was the de-
tection of very high temperature volcanism on Io when
Galileo’s NIMS and SSI instruments observed a vigorous
eruption at Pillan in 1997 (Fig. 6). The results, reported by
A. McEwen and colleagues in 1998, provided evidence of
temperatures exceeding 1500 K at several hot spots and,
in the case of the Pillan eruption, temperatures of about
1800 K. The Pillan eruption temperatures are higher than
any seen on lavas erupting on Earth now and in recent times.
It is possible that very high temperature lavas (>1500 K) are
typical for Io, although more rigorous measurements are re-
quired. The question remains open whether the Pillan erup-
tion, because it was so unusually vigorous, allowed the de-
tection of large areas at very high temperatures, or whether
the eruption was unusual in its composition. A third possi-
bility that cannot be ignored is that errors were underesti-
mated in the Pillan temperature calculations.
Nevertheless, it is clear that several Ionian eruptions de-
tected byGalileohad minimum eruption temperatures hot-
ter than current terrestrial basaltic eruptions; what types
of lavas were erupted on Io? The most popular expla-
nation is that the lavas are ultramafic (komatiite-like) in
composition. Komatiites and komatiitic basalts are ultra-
mafic volcanic rocks on Earth that are rich in magne-
sium and dominated by olivine or pyroxene. Color data
on Io’s dark volcanic materials obtained fromGalileoin-
dicate the presence of orthopyroxene. Komatiitic lavas are
perhaps the closest analogs to the lavas erupted at Pil-
lan. These lavas have very rarely been erupted on Earth
since the Proterozoic, about 1.8 billion years ago. There-
fore, studying Io’s current volcanism may lead to a better
understanding of the emplacement of lavas on the ancient
Earth.
Another hypothesis to explain Io’s hottest eruption is
superheating. Magma can be superheated by rapid ascent
from a deep, high-pressure source. Melting temperatures
of dry silicate rocks increase with pressure; therefore, the
erupted lava can be significantly hotter than its melting tem-
perature at surface pressure. Rapid ascent of basaltic mag-
mas resulting in ∼ 100 ◦Celsius of superheating should be
possible. However, no record of such an eruption is known
on Earth.
It is important to note that at present there are no direct
measurements of the composition of Io’s lavas. The most