Thermal Segregation

No, it’s not the consequence of husbands and wives having different tolerances for heat and cold. It is a theory on why Saturn’s mysterious moon Iapetus has either white or very dark surface areas, but almost no gray. A good moon for the ideologically narrow-minded. No matter; it’s still my favorite.

The Cassini web site posted a press release on it today, which included:

Using multiple instruments on Cassini, scientists are piecing together a complex story to explain the bright and dark faces of Iapetus. But yet to be fully understood is where the dark material is coming from. Is it native or from outside the moon? It has long been hypothesized that this material did not originate from within Iapetus, but instead was derived from other moons orbiting at a much greater distance from Saturn in a direction opposite to Iapetus. Scientists are now converging on the notion that the darkening process in fact began in this manner, and that thermal effects subsequently enhanced the contrast to what we see today.


(Scientist) Tilmann Denk said, “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.”


Once the leading side is even slightly dark, thermal segregation can proceed rapidly. A dark surface will absorb more sunlight and warm up, explains Spencer, so the water ice on the surface evaporates. The water vapor then condenses on the nearest cold spot, which could be Iapetus’s poles, and possibly bright, icy areas at lower latitudes on the side of the moon facing in the opposite direction of its orbit. So the dark stuff loses its surface ice and gets darker, and the bright stuff accumulates ice and gets brighter, in a runaway process.

This is cool. A process at incredibly cold temperatures can produce a geologically dynamic result. Iapetus’ white and dark sides are detectable from Earth, did you know? For centuries Earthbound astronomers have noted that Iapetus is significantly dimmer when on the left side of Saturn.

The diagram shows there’s less than a twenty degree difference on the Kelvin/Celsius scale. On Earth that’s the equivalent of a frosty early morning and a shirt sleeve day. At these temperatures, your bare arms (and the rest of your body) would flash freeze in an instant regardless of your choice of white or dark on Iapetus.

One last fun image:

This is the moon during Cassini’s approach last month. The image at the top of the post is the view after Cassini completed the successful flyby.

About catholicsensibility

Todd lives in Minnesota, serving a Catholic parish as a lay minister.
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3 Responses to Thermal Segregation

  1. Dustin says:

    What’s the theory about the equatorial ridge?

  2. Iapetus was spinning fast enough to raise an equatorial bulge during a time when it was partially liquid. The bulge then froze. Link here:

    I don’t know. It sounds off to me, but I don’t have an alternative.

  3. Pauli says:

    Cool picture. Looks like a B&W of an orange becoming moldy on one side. Moons rock.

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