This plot shows how daytime temperatures at low latitudes on the dark
material on Saturn's moon Iapetus vary with time of day, from about 130
Kelvin (-226 Fahrenheit) at noon to about 70 Kelvin (-334 Fahrenheit) at
sunset.
The observations are compared to a "forecast" model (green line) which
predicts temperatures based on an assumed value of a parameter called
the "thermal inertia." This measures how well the surface can retain heat
as conditions change. Rock or solid ice has a high thermal inertia,
roughly 2,000,000 as measured in the obscure units used for thermal
inertia, meaning that it is good at storing heat and cools down or heats
up relatively slowly.
On Iapetus, in contrast, temperatures drop precipitously in the afternoon
as the Sun sinks towards the horizon, and a very small value of the
thermal inertia (30,000 units) is needed in the model to match the data.
This means that Iapetus's surface is extremely bad at storing heat, and
is thus extremely fluffy, probably due to the pulverizing effect of
billions of years of meteorite impacts, though the mysterious process
that has darkened this side of Iapetus may also have played a role.
The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. The Jet Propulsion
Laboratory, a division of the California Institute of Technology in
Pasadena, manages the mission for NASA's Science Mission Directorate,
Washington, D.C. The Cassini orbiter and its two onboard cameras were
designed, developed and assembled at JPL. The composite infrared
spectrometer team is based at NASA's Goddard Space Flight Center,
Greenbelt, Md.
For more information about the Cassini-Huygens mission visit,
http://saturn.jpl.nasa.gov and the instrument team's home page,
http://cirs.gsfc.nasa.gov/..
