The New Horizons Long Range Reconnaissance Imager (LORRI) captured these
two images of Jupiter's outermost large moon, Callisto, as the spacecraft
flew past Jupiter in late February. New Horizons' closest approach
distance to Jupiter was 2.3 million kilometers (1.4 million miles), not
far outside Callisto's orbit, which has a radius of 1.9 million kilometers
(1.2 million miles). However, Callisto happened to be on the opposite side
of Jupiter during the spacecraft's pass through the Jupiter system, so
these images, taken from 4.7 million kilometers (3.0 million miles) and
4.2 million kilometers (2.6 million miles) away, are the closest of
Callisto that New Horizons obtained.
Callisto's ancient, crater-scarred surface makes it very different from
its three more active sibling satellites, Io, Europa and Ganymede.
Callisto, 4,800 kilometers (3000 miles) in diameter, displays no
large-scale geological features other than impact craters, and every
bright spot in these images is a crater. The largest impact feature on
Callisto, the huge basin Valhalla, is visible as a bright patch at the 10
o'clock position. The craters are bright because they have excavated
material relatively rich in water ice from beneath the dark, dusty
material that coats most of the surface.
The two images show essentially the same side of Callisto -- the side that
faces Jupiter -- under different illumination conditions. The images
accompanied scans of Callisto's infrared spectrum with New Horizons'
Linear Etalon Imaging Spectral Array (LEISA). The New Horizons science
team designed these scans to study how the infrared spectrum of Callisto's
water ice changes as lighting and viewing conditions change, and as the
ice cools through Callisto's late afternoon. The infrared spectrum of
water ice depends slightly on its temperature, and a goal of New Horizons
when it reaches the Pluto system (in 2015) is to use the water ice
features in the spectrum of Pluto's moon Charon, and perhaps on Pluto
itself, to measure surface temperature. Callisto provided an ideal
opportunity to test this technique on a much better-known body.
The left image, taken at 05:03 Universal Time on February 27, 2007, is
centered at 5 degrees south, 5 degrees west, and has a solar phase angle
of 46 degrees. The right image was taken at 03:25 Universal Time on
February 28, 2007. It is centered at 4 degrees south, 356 degrees west,
and has a solar phase angle of 76 degrees.
