April 1, 2019 | David F. Coppedge

Saturn’s Ring Moons Collect Ring Dust

How long have these little moons been gathering ring dust on their equators?

Scientists continue rummaging through more than 635 gigabytes of data collected by the Cassini spacecraft during its 13-year mission at Saturn. Recently, they processed detailed images of the innermost “ring moons” of Saturn, some of them which orbit within gaps inside the rings. The close-up photos were taken during the “Grand Finale” part of the second extended mission, which afforded close fly-bys.

Strangely, these moons resemble little models of Saturn itself. For a long time, they have attracted ring particles to their equators, giving them a Saturn-like shape. What does this imply about the ages of the rings and moons? Here’s what the JPL press release says from March 28:

Scientists also found the moon surfaces to be highly porous, further confirming that they were formed in multiple stages as ring material settled onto denser cores that might be remnants of a larger object that broke apart. The porosity also helps explain their shape: Rather than being spherical, they are blobby and ravioli-like, with material stuck around their equators.

“We found these moons are scooping up particles of ice and dust from the rings to form the little skirts around their equators,” Buratti said. “A denser body would be more ball-shaped because gravity would pull the material in.”

“Perhaps this process is going on throughout the rings, and the largest ring particles are also accreting ring material around them. Detailed views of these tiny ring moons may tell us more about the behavior of the ring particles themselves,” said Cassini Project Scientist Linda Spilker, also at JPL.

Of the satellites studied, the surfaces of those closest to Saturn – Daphnis and Pan – are the most altered by ring materials. The surfaces of the moons Atlas, Prometheus and Pandora, farther out from Saturn, have ring material as well – but they’re also coated with the bright icy particles and water vapor from the plume spraying out of Enceladus.

The article on Phys.org contains the most convenient access to images of these odd moons with their “skirts” of accumulated ring particles, reproduced from the paper in Science. As for the ring particle collection, it could be a reversible process: material collects but also erodes in a steady state. The material from Enceladus, however, must be one way. There is no way for these tiny moons to “return to sender” any icy material they swept up from the E-ring. Space.com explains how coloration aided the conclusion:

The scientists found that the appearance of these ring moons depended on their position with respect to the rings, with Pan the reddest and closest in of these moons and Epimetheus the bluest and farthest out. This suggested that the moons’ appearance depended on two competing factors, the researchers said: contamination by a red material from the main rings, which could consist of a mix of iron and organic compounds, and showers of ice particles or water vapor from volcanic plumes originating on Saturn’s moon Enceladus.

A new paper in Icarus shows that tweaking a model with the parameters of shell thickness can keep the internal heating in a “steady state,” but the model makes the heat escape greater than that of the plumes. Even if scientists were able to keep the interior fluid, it would remain a huge challenge to explain the little Iowa-diameter moon pumping out material for billions of years – enough to coat the little ring moons and create an entire ring (the E-ring) around Saturn.

The Cassini scientists do superb work of collection and analysis of data, but there’s something strange about their writings. They seem very reluctant to consider the implications: that the Saturn system, including the rings and moons, must be young.

Diagram of Saturn's E-ring created by Enceladus

Diagram of Saturn’s E-ring created by Enceladus

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