Rhea (R-4) Flyby

It seems a long way to go for a simple answer, but the Cassini R-4 Rhea Flyby slated for Saturday morning (shortly after ten, Pacific Time) is just one of the many simple answers sought by scientists studying the Saturnian system. And that simplicity, in a way, is a striking reminder of what humanity can achieve.

Rhea (R-4) FlybyThis gravity flyby is designed to understand the internal structure of Saturn’s second largest moon. Is Rhea a homogeneous body or did it differentiate into a core, mantle, and crust like the Earth? The radio science subsystem will use radio waves beamed to Earth to perform precise measurements designed to answer this question.

True, ’tis a far more humble endeavor than a starship databank and network capability to know every little detail about any planet one happens to sail past. But the question of how the solar system and its components exist at all is a far deeper mystery than one might presume about an astronomy that chases after the most fundamental forces and processes in the Universe. Compared to the Higgs boson, or dark matter and energy, perhaps the physical structure of a roundish rock floating around a giant storm of gas and gravity seems a little mundane.

But the discussion of Rhea’s structure is fascinating:

Earlier it was assumed that Rhea had a rocky core in the center. However measurements taken during a close flyby by the Cassini orbiter in 2005 (see below) cast this into doubt, though this remains controversial. In a paper published in 2007 it was claimed that the axial dimensionless moment of inertia coefficient was 0.4. Such a value indicated that Rhea had an almost homogeneous interior (with some compression of ice in the center) while the existence of a rocky core would imply a moment of inertia of about 0.34. In the same year another paper claimed the momentum of inertia was about 0.37 implying that Rhea was partially differentiated. A year later yet another paper claimed that the moon may not be in hydrostatic equilibrium meaning that the moment of inertia can not be determined from the gravity data alone. In 2008 an author of the first paper tried to reconcile these three disparate results. He concluded that there is a systematic error in the Cassini radio Doppler data used in the analysis, but after restricting the analysis to a subset of data obtained closest to the moon, he arrived at his old result that Rhea was in hydrostatic equilibrium and had the moment inertia of about 0.4, again implying a homogeneous interior. Further measurements are necessary to resolve this problem.

I mean, come on. We are the human species, and we have achieved the ability and luxury to actually have such discussions. And the Rhea flyby (R-4) on Saturday should essentially put all that to rest. Before the year is out we can expect papers on the data. If we get to summer without reaasonably founded rumors of hypotheses and implications, that would probably be surprising.

Learn more about RheaThe Wikipedia paragraph above is going to be fundamentally recontextualized in a period of mere weeks. And it pointed me to nifty paper on the saturnian icy satellites, a fascinating article in and of itself, but absolutely no help in resolving the question. But it does frame the question well:

While the measurements may permit a shape consistent with a relaxed, differentiated structure, the uncertainties also allow for substantially different interior models.

And, well, you know. The Cassini Solstice R-4 flyby might well actually settle the point.


I mean, come on. That’s pretty cool.

P. C. Thomas et al., “Shapes of the saturnian icy catellites and their significance”. Icarus 190. 2007. (p.580)

That would be page eight (8) of the PDF, I think.


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