Saturn's
moon Iapetus has virtually no gray. Rather, its features are all stark black
and white. The appearance has long puzzled astronomers.
New
detailed images suggest sunlight is melting ice on one side of Iapetus, leaving
the moon's dark surface exposed, while the opposite half retains its reflective
ice-mixed shell.
Since the
moon's discovery by Giovanni Domenico Cassini in 1671, Iapetus' appearance has
baffled astronomers. The leading edge of Iapetus, which faces the direction of
its orbit, is black as asphalt, while its trailing
side appears bright as snow. Iapetus is 907 miles (1,460 kilometers) wide
and circles Saturn at a distance of about 2.2 million miles (3.6 million
kilometers).
High-resolution
images of Iapetus acquired last month by the Cassini-Huygens spacecraft during its
low pass over the moon have uncovered telling details on its surface that may
well yield the reason for its strange bright
and dark patterns.
"While
there are many details yet to be worked out, we think we now understand the
essence of why Iapetus looks the way it does," said Carolyn Porco, the
leader of the imaging team at the Space Science Institute in Boulder, Colo.
The new
observations add support to a two-part explanation for Iapetus' appearance. First,
as Iapetus treks around Saturn, its
leading edge scoops up a thin coating of dark material, which amplifies
sunlight absorption.
"Dusty
material spiraling in from outer moons hits Iapetus head-on and causes the
forward-facing side of Iapetus to look different than the rest of the
moon," said Tilmann Denk, Cassini imaging scientist at the Free University
in Germany.
Over time,
as the black-ish surfaces warm, the rate of evaporation increases until finally
all the surface ice in that region melts away. Infrared observations from the Cassini
flyby confirm the dark dust material is approximately -230 degrees Fahrenheit (-146
degrees Celsius)--warm enough for the release of water vapor from the ice.
The water vapor
formed then condenses on the nearest cold spot, such as along polar regions and
icy areas at lower latitudes on the trailing side of the moon. In that way, the
dark material loses the mixed-in ice and gets even darker, while the bright
material accumulates more ice and gets brighter, in what the astronomers call a
runaway process that leaves no gray area.
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