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June 30, 1999: This morning at 07:47 UT, NASA's Galileo
spacecraft zoomed past Jupiter's moon Callisto at a distance
of only 1,047 km. JPL reports that the encounter was a success
and that the spacecraft is operating normally.
The main purpose of today's flyby was to modify Galileo's orbit
and bring the craft closer to Io, the innermost of Jupiter's
large satellites. Io is one of the most exotic places in the
solar system. It is literally bursting with volcanoes that spew
sulfurous plumes over 300 km high. One called Prometheus may
have been active for at least 18 years! In October or November,
after a series of four orbit-changing encounters with Callisto
(today's was the second), Galileo is scheduled to make two daring
close approaches to Io, possibly flying through a volcanic plume.
Although the most dramatic flybys of Io won't take place until
later in 1999, scientists will get a closer look at the volcanic
moon later this week when the spacecraft passes it at a distance
of just 127,000 km. Galileo was within just 900 km of Io in December
1995 but the spacecraft wasn't taking pictures at that time,
so this week's encounter may provide some of the best ever pictures
of Io's volcanoes.
Left: Ron Baalke and David Seal
of NASA's Jet Propulsion Laboratory have prepared several computer-generated
animations of today's Callisto flyby. Click here
for viewing options.
"We'll also have the first real passage since 1995 through
the outer edge of the Io torus during this orbit," says
JPL's Dr. Torrence Johnson, the chief scientist for the Galileo
mission.
The Io torus is a gigantic ring of ionized gas circling Jupiter
formed by sulfurous material ejected from Io's volcanoes. With
a diameter the size of Io's orbit it spans 844 thousand km and
has an important impact on Jupiter's magnetic environment. As
Io moves along its orbit and through this magnetized plasma torus,
a huge electrical current flows between Io and Jupiter. Carrying
about 2 trillion watts of power, it's the biggest DC electrical
circuit in the solar system.
"Galileo will spend nearly a day inside the edge of the
torus in an overall region of space between Io and Europa,"
noted Johnson. "We've been there once before in 1995. We
know generally what the Io torus is and what it's made of. Now
we're going to look at its detailed structure, to investigate
what sort of physics is going on."
Although at this point in the Galileo mission Callisto is primarily
a turning point on the road to Io, it is also of considerable
interest to scientists.
With a diameter of 4,800 km, Callisto is nearly the same size
as the planet Mercury. Its icy surface is the most heavily cratered
place in the Solar System, but there are no volcanoes or even
any large mountains. It is thought that little has happened to
alter the surface for billions of years, other than occasional
impacts with asteroids, comets, and other interplanetary debris.
Right: Callisto's many craters are
apparent in this contrast-enhanced image taken by the Galileo
spacecraft in 1997. Visible near the image center is Valhalla,
one of the largest impact craters in the Solar System, measuring
about 4,000 kilometers across.
"One of the biggest mysteries about Callisto involves its
craters," says Torrence Johnson. "As on other cratered
bodies, the craters on Callisto come in all sizes. There are
a few really large craters, like Valhalla, and then as you look
at smaller and smaller impact features there are more and more
of them. On Callisto, craters smaller than about 1 km seem to
have been partially obliterated, or 'disaggregated' by some unknown
process. On planets like Mars or Earth where there's been weathering
or erosion, the obliteration of small craters is expected. But
Callisto doesn't have a substantial atmosphere or any obvious
sources of erosion, so what's happening to these craters? It's
a real puzzle.
"The disaggregation in small craters on Callisto
is a theme for all four flybys leading up to the Io encounters,"
he continued. "All of the science instruments will be online
during the flyby. The Solid State Imaging Camera will take high
resolution pictures that we can use to study cratering statistics.
We'll also be using the Near-infrared Mapping Spectrometer (NIMS)
and the UV spectrometer to get a handle on the chemical composition
of the surface. We really want to examine places on Callisto
that we haven't looked at before to see if the process that wipes
out the small craters might be related to big impact features
or to variations in the composition of the terrain."
The UV spectrometer will also peer at Callisto's limb during
the flyby in hopes of gleaning more information about the satellite's
tenuous carbon dioxide atmosphere. Galileo's Dust
Detector will be active, too, making measurements of dust
particles around Callisto that might be ejecta from ongoing impact
events.
Another interesting story lies beneath Callisto's surface. Galileo
magnetometer data released in 1998 indicate that Callisto, like
another of Jupiter's moons Europa, may harbor an underground
ocean. Callisto has a magnetic field that fluctuates in time
with Jupiter's rotation. So far, the best explanation for Callisto's
peculiar magnetism is an underground layer of melted ice. If
the liquid is salty like Earth's oceans, it can carry sufficient
electrical currents (induced by Jupiter's powerful rotating magnetic
field) to produce a fluctuating magnetic field around Callisto.
Above: Voyager and Galileo have
returned tantalizing evidence for a liquid water ocean beneath
the surface of Europa. Now researchers are reporting telltale
indications that the battered Jovian moon Callisto may also harbor
a subsurface ocean. This cutaway view of Callisto shows a whitish
200 kilometer thick band of ice just beneath the moon's surface.
The hypothetical ocean - indicated by the underlying light blue
stripe - is potentially a salty layer of liquid water up to 10
kilometers thick, while the rest of the interior is seen as a
jumble of rock and ice. More
information...
Scientists are intrigued by the prospects of an ocean on Callisto
because liquid water is a prerequisite for life as we know it
on Earth.
"The basic ingredients for life -- what
we call 'pre-biotic chemistry' -- are abundant in many solar
system objects, such as comets, asteroids and icy moons,"
explained Dr. Torrence Johnson in 1998. "Biologists believe
liquid water and energy are needed to actually support life,
so it's exciting to find another place where we might have liquid
water. However, energy is another matter, and Callisto may not
have enough. Callisto's ocean is only being heated by radioactive
elements, whereas Europa has tidal energy as well. That makes
Europa a better prospect for life."
There are many questions about Callisto, and scientists hope
that the latest series of Galileo flybys will provide some answers.
The next two flybys of Callisto are scheduled for August 14 and
September 16, 1999, when the spacecraft's orbit will be further
altered to bring it closer to Jupiter and Io. Science@NASA will
continue to cover Galileo's exploits as the spacecraft heads
for its dramatic encounters with Io's volcanoes on October 11
and Nov. 26, 1999.
Above: Scientists are curious about
the differences between Callisto and another of Jupiter's satellites,
Ganymede. The two are similar in size and mass, yet appear to
have a very different geology and internal structure. Galileo
data indicate that on Ganymede all the rock and heavy metals
that make up the satellite have separated from the ice completely,
forming a dense inner core. Callisto is different. There is a
shell of differentiated material near the surface (extending
to a depth of few hundred km), but inside it's mixed rock and
ice. Both Callisto and Ganymede are covered by huge tracts of
relatively dark, heavily cratered terrains, shown above at similar
image resolution and solar illumination. Ancient dark terrain
on Ganymede is complex featuring scarps, furrows, and domes.
Dark terrain on Callisto rarely shows these features. Callisto
appears to have had a much simpler geologic history than its
nearby neighbor Ganymede. |
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