Iapetus, Light and Dark

I would be remiss if I didn’t mention the recent papers on Saturn’s moon Iapetus – a namesake for this blog along with the paleo-ocean and the Greek Titan, for which both the moon and the ocean are named. Iapetus the moon has a striking surface dichotomy of light and dark regions first recognized when Giovanni Cassini discovered the moon in 1671. The leading hemisphere of the moon is dark and most of the trailing hemisphere is light. The geometry of the regions led to speculation that Iapetus was collecting material as a fine rain of dust from other Saturnian moons that coated portions of Iapetus’ otherwise bright icy surface.

 Iapetus (NASA/JPL/Space Science Institute)

High-resolution images taken by the Cassini spacecraft since 2004 provided further detail of the regions. They show that small, fresh craters in the dark regions are bright, indicating the dark mantle is only 10’s of centimeters to meters thick. They also show that the transition zones between the dark and light regions are mottled with smaller-scale regions of dark and light material, rather than having a gradational transition of grays. In the mottled transition zones, the equator-facing walls of craters, which receive the most sunlight, are dark while the poleward-facing walls, which receive less sunlight, are light-colored.

Portion of the transition zone on Iapetus. The dark material is concentrated on the equator-facing sides of the craters (in this picture, North). (NASA/JPL/Space Science Institute) 

Two papers published online in Science last week synthesize the Cassini data into a model for the dark-light dichotomy. The abstracts can be found here and here.

(1) As to the source of the dark mantle on Iapetus: it shares spectral characteristics with Saturn’s moon Phoebe (itself a fascinating body), and the recent discovery (Nature abstract here) of a large diffuse ring coincident with Phoebe’s orbit provides a likely source for the dark material. Whether Phoebe itself is feeding the ring by shedding material during impacts or if the ring is the detritus of another Phoebe-like body, now destroyed, is uncertain.

(2) As to the mottled pattern: Iapetus has a very slow rotation of 79 Earth days and it therefore has large temperature differentials between the long days and nights. Exacerbating this is the effect of albedo -- areas that have some dark material absorb more sunlight (i.e., they have a lower albedo) and are warmer while lighter areas reflect more sunlight and stay colder. This provides a feedback mechanism wherein those areas with some dark mantle are warmer, thus more ice sublimates to gas and migrates towards colder (i.e., brighter) spots. There should be some critical amount of dark material that perpetuates this feedback for any given light regime, so that areas with that much or more mantle become increasingly dark, while nearby areas with less than the critical amount of mantle may host deposition of ice and become increasingly light. The leading edge receives a lot of dust and is dark; the trailing hemisphere receives little dust and is light; the transition zones receive some dust and the local albedo depends on the sunlight budget which in turn depends on topography.

This story was reported last week in Science Daily, the New York Times, and elsewhere.