By Matthew Fordahl
AP Science Writer
SAN FRANCISCO –– Ganymede, the solar system's largest
moon, appears to have a liquid, saltwater ocean deep beneath its
cratered and fractured surface of solid ice, researchers said
Saturday.
With the new
findings, Ganymede joins Europa and Callisto as yet another moon
of Jupiter suspected to harbor underground water – a key
ingredient for life.
The ocean
is the latest finding announced by scientists interpreting data
from NASA's Galileo spacecraft, which began orbiting Jupiter in
December 1995.
"Galileo
is sparking a new revolution in our understanding of the solar
system by opening our minds to the role played by that precious
ingredient to life – water," said Robert Pappalardo,
a senior research associate at Brown University.
Scientists
received the latest data as Galileo made its closest flyby of
Ganymede, coming within 503 miles on May 20. They discussed the
findings at the fall meeting of the American Geophysical Union.
An instrument
that measures magnetic fields detected a subtle change in the
moon's internal field – a signature of a global layer of
a conducting liquid, such as saltwater.
"After
months and months of wrestling with the data ... we believe there
is very strong evidence of a layer of melted water beneath Ganymede's
icy surface," said Margaret Kivelson, a space physicist at
the University of California, Los Angeles.
If the interpretation
is correct, a layer of water several miles thick lies more than
100 miles below the surface, Kivelson said. Researchers believe
the temperature of the water is about 68 degrees below zero but
doesn't freeze because of the high pressure deep inside the moon.
"It's
not the silver bullet that we would like to have but it looks
really good," she said.
Galileo scientists
also said the latest high-resolution images from the spacecraft
suggest Ganymede's surface is active like Earth's.
In Ganymede's
case, the surface of solid ice apparently floats on a layer of
more malleable ice, which is heated by the decay of uranium and
other radioactive elements found toward the icy moon's core.
On parts of
Ganymede, regions are pulling apart at the surface. That allows
the ductile material to ooze upward and form smooth bands. Scientists
previously thought the patterns were formed by volcanoes that
spewed water when Ganymede's icy surface heated up.
The Galileo
spacecraft, which is now making joint observations of Jupiter
with NASA's Saturn-bound Cassini probe, also has provided strong
evidence of liquid underground oceans on the moons Europa and
Callisto.
Europa remains
the leading candidate for life and will be the target of a future
NASA orbiter because, unlike Callisto and Ganymede, Europa's suspected
ocean lies only a few miles beneath its surface.
Scientists
said their understanding of Ganymede has improved dramatically
in the last five months thanks to the Galileo data.
"It always
had the advantage of being big," Kivelson said of Ganymede,
a moon larger than the planets Mercury and Pluto.
"Relative
to what we knew five months ago, Ganymede has become a great deal
more exciting."
|
