Page 230 - Fundamentals of Physical Volcanology

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VOLCANISM ON OTHER PLANETS 207

Fig. 13.17 This comparison image shows changes in lava flow field being formed from the volcano Prometheus on Io.
The right hand frame is the ratio of two images taken by the Galileo spacecraft on October 11, 1999 and February 22, 2000.
Dark areas in the ratio image indicate fresh lava eruptions during the ∼14 week interval. Each Galileo spacecraft image frame is
∼65 km wide. (Image courtesy of NASA/JPL/University of Arizona.)

1996 and 2002 by the Galileo probe, which
obtained visible and infrared images and took
temperature data. More than 300 vents have been
identified on the surface, and at least ten of these
are likely to be active at any one time. Some appear
to continue to erupt for at least several years
whereas other eruptions are over in at most a few
days. Some of the long-lived eruptions feed com-
plex lava flow sheets (Fig. 13.17), the great lengths,

small thicknesses, and high eruption temperatures
of which suggest that they are unusually hot basalts.
Other long-lived eruptions are explosive and give
rise to what are usually called eruption plumes
(Fig. 13.18), although more accurately they are giant
lava fountains since there is no significant atmo-
sphere on Io to support a convecting eruption cloud.
A major feature of Io is the brightly colored sur-
face. The colors (red, orange, yellow, white, even
a tinge of green) are due to deposits of sulfur Fig. 13.18 Near the north pole of Io a 290 km high eruption
and sulfur compounds, especially sulfur dioxide, plume rises from one of the vents of the Tvashtar volcanic
together with small amounts of metal salts. What system. A smaller plume from the volcano Prometheus is
visible near the equator on the left side of the image, and
are not seen are any clear indications of water
the bright spot on the night-side of Io not far from the south
or carbon dioxide, the two commonest volcanic
pole is the top of a plume from the volcano Masubi, just
volatiles. Io does not have any significant atmo-
catching the sunlight. This image was taken by the Long
sphere other than the gases emitted from the volca- Range Reconnaissance Imager on the New Horizons
noes, and so is prone to losing those gases into spacecraft as it traveled through the Jupiter satellite
space. Indeed, an annular ring of atoms of sulfur, system on its way to Pluto. (NASA image.)