This dramatic image of Io was taken by the Long Range Reconnaissance
Imager (LORRI) on New Horizons at 11:04 Universal Time on February 28,
2007, just about 5 hours after the spacecraft's closest approach to
Jupiter. The distance to Io was 2.5 million kilometers (1.5 million miles)
and the image is centered at 85 degrees west longitude. At this distance,
one LORRI pixel subtends 12 kilometers (7.4 miles) on Io.
This processed image provides the best view yet of the enormous
290-kilometer (180-mile) high plume from the volcano Tvashtar, in the 11
o'clock direction near Io's north pole. The plume was first seen by the
Hubble Space Telescope two weeks ago and then by New Horizons on February
26; this image is clearer than the February 26 image because Io was closer
to the spacecraft, the plume was more backlit by the Sun, and a longer
exposure time (75 milliseconds versus 20 milliseconds) was used. Io's
dayside was deliberately overexposed in this picture to image the faint
plumes, and the long exposure also provided an excellent view of Io's
night side, illuminated by Jupiter. The remarkable filamentary structure
in the Tvashtar plume is similar to details glimpsed faintly in 1979
Voyager images of a similar plume produced by Io's volcano Pele. However,
no previous image by any spacecraft has shown these mysterious structures
so clearly.
The image also shows the much smaller symmetrical fountain of the plume,
about 60 kilometers (or 40 miles) high, from the Prometheus volcano in the
9 o'clock direction. The top of a third volcanic plume, from the volcano
Masubi, erupts high enough to catch the setting Sun on the night side near
the bottom of the image, appearing as an irregular bright patch against
Io's Jupiter-lit surface. Several Everest-sized mountains are highlighted
by the setting Sun along the terminator, the line between day and night.
This is the last of a handful of LORRI images that New Horizons is sending
"home" during its busy close encounter with Jupiter -- hundreds of images
and other data are being taken and stored onboard. The rest of the images
will be returned to Earth over the coming weeks and months as the
spacecraft speeds along to Pluto.
