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Fig. 15.2 The margin of the
lava flow field associated with the
Prometheus volcano on Jupiter's
moon Io is seen in this
12 m/pixel image, acquired by
the Galileo spacecraft on
February 22, 2000. The dark lava
has margins similar to those
formed by fluid lava flows on
Earth. This entire area is under
the active plume of Prometheus,
which is constantly raining bright
material. The older plains (upper
right) are covered by ridges with
an east-west trend. These ridges
may have formed by the folding
of a surface layer or by
deposition or erosion. Bright
streaks across the ridged plains
emanate from the lava flow
margins, perhaps where the hot
lava vaporizes sulphur dioxide.
The bright material must be
ejected at a low angle because it
only coats the lava-facing sides
of the ridges. North is slightly to
the right of straight up. Image
credit NASA/JPL/University of
Arizona, Photojournal image
PIA02557
around Jupiter. This eccentricity leads to strong tidal forces
which deform the world slightly and this tidal kneading
generates appreciable heating of Io's interior, with the result
that Io is volcanically very active. This activity was dis-
covered by Voyager 1 in 1979, when a large fountain-like
plume was observed on Io's limb. Further Voyager and
Galileo observations have found many active volcanic
centers, some with exposed silicate lavas with temperatures
in excess of 1400 K, others merely sulphur dioxide plumes
jetting into space before falling back down (as sulphur
dioxide frost).
Io's atmosphere is predominantly of sulphur dioxide, but
is very tenuous indeed. Spectroscopic measurements from
Earth show that the pressure on the sub-Jovian hemisphere
is only about a tenth of a nanobar, although the same data
show that the antiJovian hemisphere has a pressure of
 
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