This New Horizons image of Jupiter's volcanic moon Io was taken at 13:05
Universal Time during the spacecraft's Jupiter flyby on February 28, 2007.
It shows the reddish color of the deposits from the giant volcanic
eruption at the volcano Tvashtar, near the top of the sunlit crescent, as
well as the bluish plume itself and the orange glow of the hot lava at its
source. The relatively unprocessed image on the left provides the best
view of the volcanic glow and the plume deposits, while the version on the
right has been brightened to show the much fainter plume, and the
Jupiter-lit night side of Io.
New Horizons' color imaging of Io's sunlit side was generally overexposed
because the spacecraft's color camera, the super-sensitive Multispectral
Visible Imaging Camera (MVIC), was designed for the much dimmer
illumination at Pluto. However, two of MVIC's four color filters, the blue
and "methane" filter (a special filter designed to map methane frost on
the surface of Pluto at an infrared wavelength of 0.89 microns), are less
sensitive than the others, and thus obtained some well-exposed views of
the surface when illumination conditions were favorable. Because only two
color filters are used, rather than the usual three, and because one
filter uses infrared light, the color is only a rough approximation to
what the human eye would see.
The red color of the Tvashtar plume fallout is typical of Io's largest
volcanic plumes, including the previous eruption of Tvashtar seen by the
Galileo and Cassini spacecraft in 2000, and the long-lived Pele plume on
the opposite side of Io. The color likely results from the creation of
reddish three-atom and four-atom sulfur molecules (S3 and S4) from plume
gases rich in two-atom sulfur molecules (S2 After a few months or years,
the S3 and S4 molecules recombine into the more stable and familiar
yellowish form of sulfur consisting of eight-atom molecules (S8), so these
red deposits are only seen around recently-active Io volcanos. Though the
plume deposits are red, the plume itself is blue, because it is composed
of very tiny particles that preferentially scatter blue light, like smoke.
Also faintly visible in the left image is the pale-colored Prometheus
plume, almost on the edge of the disk on the equator at the 9 o'clock
position.
Io was 2.4 million kilometers from the spacecraft when the picture was
taken, and the center of Io's disk is at 77 degrees West longitude, 5
degrees South latitude. The solar phase angle was 107 degrees.
