# Description of the USGS Io Basemaps

Io_SSI_color_SIMP0.cub

Galileo Color Mosaic

This *Galileo* global false color mosaic was constructed from low phase
angle violet, green, and near-infrared (756 nanometer) images from
orbits G2, E6, C9, and C21. The images were calibrated using the best
end-of-mission calibration information, corrected empirically for
limb-darkening, and map projected using the camera-pointing corrections
of *Archinal et al*. (2001). The coregistered color images were next
hand-edited to remove topographic shadows and pixels too near the limb,
and mosaicked using a numerical procedure (*Soderblom et al*., 1978)
that reduces the mismatch at the seams. This mosaic represents our best
understanding of Io\'s color as pictured during the *Galileo* Mission.
The true colors that would be visible to the eye are similar but much
more muted than shown here. The spatial resolution of the mosaic ranges
from 1.3 to 21 km/pixel at the equator, with the poorest resolution on
the Jupiter-facing hemisphere of Io.

Io_SSI-only_bw_SIMP0.cub

Galileo High Resolution Mosaic

The best quality global monitoring images taken by the *Galileo* imaging
system at spatial resolutions up to 1 km/pixel have been assembled here
to depict the global and regional morphology of Io. This mosaic is made
up of 32 monochrome images taken at various phase angles and local times
of day, so 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 0
to 270 W, and from the east over longitudes 270 W to 360 W. The images
were empirically adjusted in brightness and contrast to match one
another in areas of overlap. Most of the images were taken in the clear
filter, but green and 756 nm filter images were substituted when they
were more detailed than other available images. Image resolutions range
from 1.3 to 10 km/pixel along the equator, with the poorest coverage on
the Jupiter-facing side of Io.

Io_SSI_VGR_bw_SIMP0.cub

Galileo/Voyager High Resolution Mosaic

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 global mosaic of the best
resolution images from both *Galileo* and *Voyager 1* is shown here that
includes 51 *Voyager 1* images with spatial resolutions sometimes
exceeding the 1 km/pixel scale of the final mosaic. While most of these
images neatly overlie the *Galileo* coverage, the *Voyager 1* images of
regions west of 0˚ longitude appear displaced from the positions
predicted from the control net derived from the solution of the combined
*Galileo* and *Voyager* data set. We suspect that the discrepancy arises
from regional topography west of the sub-Jupiter point, and have
adjusted the positions of the *Voyager 1* images (all acquired near the
limb of Io) to match the best-fit geometry of *Archinal et al*. (2001).
Solar illumination of these longitudes was from the east during the
*Voyager 1* flyby. Image resolutions range from 1.0 to 10 km/pixel along
the equator, with the poorest coverage centered on longitude 50 W.

Io_SSI-only_color_merge_SIMP0.cub

Io_SSI_VGR_color_merge_SIMP0.cub

Merged Products

To present more information-rich views of Io, the global color derived
from the *Galileo* color images was superimposed on the more detailed,
higher resolution monochrome mosaics. The procedure adopted was to
calculate color ratio images from the *Galileo* data and apply them to
the monochrome mosaics, requiring that the color ratios of the composite
images match the color ratios of the *Galileo* data. 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. However, this approach suffers
from an obvious disadvantage in areas where the surface appearance
changed dramatically during the 20+ year interval between the *Voyager*
and *Galileo* eras, for example near Kanehekili.

Contacts:

Tammy Becker (<tbecker@usgs.gov>) and Paul Geissler
(pgeissler@usgs.gov).

General Reference:

Becker, T., Geissler, P.E. 2005.Galileo Global Color Mosaics of Io. 36th
Annual

Lunar and Planetary Science Conference 36, 1862.

References Cited:

Archinal, B. A., Davies, M. E., Colvin, T. R., Becker, T. L., Kirk, R.
L., and Gitlin, A. R. 2001. An Improved RAND-USGS Control Network and
Size Determination for Io. *Lunar and Planetary Institute Conference
Abstracts* 32, 1746.

Soderblom, L. A., Edwards, K., Eliason, E. M., Sanchez, E. M., and
Charette, M. P. 1978. Global color variations on the Martian surface.
*Icarus* 34, 446-464.