Iapetus Temperature Variation Map
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Iapetus Temperature Variation Map

November 16, 2009
This plot shows how daytime temperatures at low latitudes on the dark material on Iapetus vary with time of day, from about 130 Kelvin at noon to about 70 Kelvin at sunset.

The observations are compared to a "forecast" model (green line) that 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 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.

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