In July,
NASA's Cassini spacecraft made its latest flyby of Saturn's moon Enceladus,
revealing an unexpected hot spot on the moon's south pole.
The finding
flipped everything scientists knew about Enceladus on its head, because what
should have been a dead moon appeared to be geologically active and what was
supposed to be the moon's coldest region turned out to be its warmest.
"This
is as astonishing as if we'd flown past Earth and found that Antarctica was
warmer than the Sahara," said John Spencer, an astronomer from the
Southwest Research Institute in Colorado and a co-investigator of the Cassini
mission.
Because of
its orientation to the sun, scientists expected the moon's south pole to be
about 75 degrees Kelvin (-324 degrees Fahrenheit), but what Cassini revealed
instead was that it was over 110 degrees Kelvin (-261 degrees Fahrenheit).
Solar heating alone can't account for the discrepancy, so scientists think internal
heating is occurring within the moon.
The finding
could explain an old mystery concerning Enceladus, but it also presents a new
puzzle of its own.
Discovered
in 1789 by a British astronomer, Enceladus is named after a mythological Greek
giant. Despite its namesake, it is a tiny moon, only about 300 miles in
diameter, and small enough to fit snugly inside the state of Arizona.
The surface
of Enceladus is coated in a thin layer of ice that reflects back nearly all of
the sunlight striking it, making it the most reflective object in the solar
system.
Cassini's
July flyby of Enceladus had it dipping within 109 miles of the moon's icy
surface, its closest approach yet. In addition to the south pole hot spot,
Cassini also revealed that the "icy veins" were actually a series of
fractures on the moon's surface.
Even more
surprising, the fractures appeared to be active, violently spewing a slushy jet
of warm water and ice into space.
Together,
the venting fractures and the hot spot provide strong evidence for geologic
activity on Enceladus. If true, the findings could explain the moon's
connection with one of Saturn's rings, a relationship that has puzzled
scientists for years.
Saturn's E
ring stretches nearly 200,000 miles from its inside edge to its outer bound and
is made up of microscopic particles of ice and dust. The ring is so faint that
scientists didn't discover it until about 30 years ago, but when they did, they
noticed a curious thing: the ring was brightest around Enceladus, which, along
with some of Saturn's other moons, wades through the E ring's plane of debris
while circling the planet.
This
observation caused some scientists to suspect that Enceladus was somehow
supplying material for the ring. It was a strange idea, but the fact that the E
ring existed at all was evidence for it.
"The
particles are so small they should not last very long," Spencer told
SPACE.com.
"You
couldn't have the E ring just sit there for the age of the solar system. It had
to be regenerated somehow."
There were
two possibilities for how Enceladus could supply material for the ring.
According to one scenario, impacts from cosmic particles were blasting tiny
bits of Enceladus off into space, providing Saturn with fodder for a ring.
In the
second scenario, geysers or water volcanoes on the surface of Enceladus spewed
out clouds of ice and dust into the moon's atmosphere, and because the moon is
so small and its gravity so weak, the ice and dust soon float off into space.
Like drifting steam from a tugboat, Enceladus would deposit a trail of
microscopic debris in its wake as it orbited Saturn—debris that Saturn's
gravity would then rope in to make a ring.
Based on
what scientists knew about Enceladus, the first scenario seemed more likely.
Enceladus was thought to be geologically dead, and without geologic activity,
there could be no spouting fountains of water and ice on the moon's surface.
That's why
the discovery of the hot spot on the moon's south pole is so important, because
it provides the moon with an engine to drive the geysers and volcanoes.
Why the
south pole is so active is still a mystery. One theory is that radioactive
material left over from the moon's formation billions of years ago is acting as
a heat source, said Linda Spilker, Cassini's deputy project scientist.
Another
theory is that a change in the moon's spin rate caused fractures to form on the
moon's surface.
Like many
moons, Enceladus takes as long to rotate on its axis as it does to make one
orbit around Saturn, thus only one of its hemispheres faces Saturn.
"If
Enceladus moved in closer, then it would have to try to spin more quickly to
keep one side facing Saturn," Spilker said. "And maybe that change in
spin might have caused the cracking seen at the south pole."
How or when
this might have occurred is still unknown.
Cassini is
scheduled to make an even closer pass of Enceladus in 2008, during which
scientists hope to gather new pieces for the puzzle.
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