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Click on image for Animation
Io is the most volcanically active body in the Solar System. Volcanoes
erupt massive volumes of silicate lava, sulphur and sulphur dioxide,
constantly changing Io's appearance. This new basemap of Jupiter's moon Io
was produced by combining the best images from both the Voyager 1 and
Galileo Missions. Although the subjovian hemisphere of Io was poorly seen
by Galileo, superbly detailed Voyager 1 images cover longitudes from 240 W
to 40 W and the nearby southern latitudes. A monochrome mosaic of the
highest resolution images from both Galileo and Voyager 1 was assembled
that includes 51 Voyager 1 images with spatial resolutions sometimes
exceeding the 1 km/pixel scale of the final mosaic. Because this mosaic is
made up of images taken at various local times of day, care must be taken
to note the solar illumination direction when deciding whether topographic
features display positive or negative relief. In general, the illumination
is from the west over longitudes 40 to 270 W, and from the east over
longitudes 270 W to 40 W. Color information was later superimposed from
Galileo low phase angle violet, green, and near-infrared (756 nanometer
wavelength) images. The Galileo SSI camera's silicon CCD was sensitive to
longer wavelengths than the vidicon cameras of Voyager, so that
distinctions between red and yellow hues can be more easily discerned. The
"true" colors that would be visible to the eye are similar but much more
muted than shown here. Image resolutions range from 1 to 10 km/pixel along
the equator, with the poorest coverage centered on longitude 50 W.
This mosaic is in an equal area cylindrical map projection, centered
onlongitude 180, with grid lines at 30 degree intervals. Full scale
versions of this mosaic, and the data products used to generate it, can be
obtained from the USGS Astrogeology website,
http://astrogeology.usgs.gov/Projects/JupiterSatellites/.
About the Animation
In the same way that the Moon always has the same side facing Earth, Io
always has the same side facing Jupiter. The movie shows two speeded-up
rotations of Io (a single rotation really takes 1.77 days), and begins
with a view of the Jupiter-facing hemisphere. With rotation in an
easterly direction, after two seconds the volcano Prometheus (on the
equator) comes into view. The massive red deposit around Pele (seconds
5-10) is the most distinctive expression of volcanic activity on Io, and
just to the north-west is the horse shoe-shaped Loki Patera, the most
powerful volcano on Io.
The animation was made using a computer program that wrapped the Io mosaic
around a sphere to produce a globe. In all, 360 images were used, each
differing by one degree in longitude from the previous image.