The first images returned to Earth by New Horizons during its close
encounter with Jupiter feature the Galilean moon Io, snapped with the Long
Range Reconnaissance Imager (LORRI) at 0840 UTC on February 26, while the
moon was 2.5 million miles (4 million kilometers) from the spacecraft.
Io is intensely heated by its tidal interaction with Jupiter and is thus
extremely volcanically active. That activity is evident in these images,
which reveal an enormous dust plume, more than 150 miles high, erupting
from the volcano Tvashtar. The plume appears as an umbrella-shaped feature
of the edge of Io's disk in the 11 o'clock position in the right image,
which is a long-exposure (20-millisecond) frame designed specifically to
look for plumes like this. The bright spots at 2 o'clock are high
mountains catching the setting sun; beyond them the night side of Io can
be seen, faintly illuminated by light reflected from Jupiter itself.
The left image is a shorter exposure -- 3 milliseconds -- designed to look
at surface features. In this frame, the Tvashtar volcano shows as a dark
spot, also at 11 o'clock, surrounded by a large dark ring, where an area
larger than Texas has been covered by fallout from the giant eruption.
This is the clearest view yet of a plume from Tvashtar, one of Io's most
active volcanoes. Ground-based telescopes and the Galileo Jupiter orbiter
first spotted volcanic heat radiation from Tvashtar in November 1999, and
the Cassini spacecraft saw a large plume when it flew past Jupiter in
December 2000. The Keck telescope in Hawaii picked up renewed heat
radiation from Tvashtar in spring 2006, and just two weeks ago the Hubble
Space Telescope saw the Tvashtar plume in ultraviolet images designed to
support the New Horizons flyby.
Most of those images will be stored onboard the spacecraft for downlink to
Earth in March and April.
