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The NASA News article NASA's Cassini Finds Saturn's Rings Coat Tiny Moons says:

The new research, from data gathered by six of Cassini's instruments before its mission ended in 2017, is a clear confirmation that dust and ice from the rings accretes onto the moons embedded within and near the rings.

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."

Question: How were scientists able to "(find) the moon surfaces to be highly porous"? I can understand that this can be hypothesized, but it sounds like there are measurements that show the surfaces to be porous, rather than just rationalizations.

I'm not asking why it is likely to be true, there seems to be measurements that confirm this.


From Phys.org's New close-ups of the mini-moons in Saturn's rings and the original is in the new and paywalled paper in Science Close Cassini flybys of Saturn’s ring moons Pan, Daphnis, Atlas, Pandora, and Epimetheus:

Ravioli moons of Saturn

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  • $\begingroup$ I've also asked Help understanding this unsettling image of Titan, Epimetheus, and Saturn's rings? $\endgroup$
    – uhoh
    Mar 29 '19 at 7:05
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    $\begingroup$ +1 for some of my favourite moons $\endgroup$
    – Ingolifs
    Mar 29 '19 at 7:22
  • $\begingroup$ I wonder what is the length of the white bar. This image gives a number of 6 miles or 10 kilometers, but is it the same value for all bars? $\endgroup$
    – Uwe
    Mar 29 '19 at 10:16
  • $\begingroup$ @Uwe the linked paper in Science is paywalled. I've added the link to the paper that Phys.org cites to the question. The answer will be in the figure caption. $\endgroup$
    – uhoh
    Mar 29 '19 at 10:34
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Converting my comments to provisional answer.

"Saturn's Small Inner Satellites: Clues to Their Origins" by C. Porco et al. - this article in Science is paywalled, too, but pictures from the article are accessible with captions, as well as tables.

From table there (mass and density):

             mass (× 10^19 g)      ρ (g/cm^-3)

Pan          0.495 ± 0.075        0.41 ± 0.15

Daphnis     0.0084 ± 0.0012       0.34 ± 0.21

Atlas         0.66 ± 0.06         0.46 ± 0.10

Prometheus   15.67 ± 0.20         0.47 ± 0.065

Pandora      13.56 ± 0.23         0.50 ± 0.085

Epimetheus   53.07 ± 0.14         0.69 ± 0.13

From caption of the table:

Masses are determined from orbital integrations [i.e., satellite-satellite perturbations (27)] or, for Pan (9) and Daphnis (32), from the effects of the moons on the rings.

...

Moons' estimated densities, ρ, are obtained from satellite masses and observed shapes.

So, the authors are rather precize about densities, with relatively small error bars. And the densities obtained are far below than density of water ice. So the moons should be porous.

Here is Figure 3 from the article:(link)

enter image description here

In caption to the picture the authors say:

Modeled gravitational topography for the ring-region moons Pans, Atlas, Prometheus, and Pandora, assuming core and mantle densities of 0.9 g/cm–3 and 0.15 g/cm–3, respectively. The figures are based on the appropriate spin, tidal forces, and potential energy at the surface. Left panels show magnitude of surface accelerations. Rights panels show gravitational topography, which is the potential energy (relative to the south pole) divided by an average acceleration. Views, from left to right, are of the Saturn-facing, leading, anti-Saturn, trailing, northern, and southern hemispheres.

bold is myne.

0.15 g/cm^–3 is very very porous, indeed. It resembles fresh snow. But if I read correctly, the claim of 0.15 g/cm^–3 is model-based, not direct-observation based.

The recent Cassisni observations by 2017 updated the moons' shapes and densities, but did not make much difference, I suppose.

Nevertheless, the average densities (0.4 - 0.5 g/cm^–3) are results of direct observations.

That's I could find. If somebody can write better answer than myne I would be glad to read.

PS

Another curious observation - if I read correctly Pan just can't accrete more dust from the rings because its outer parts are already beyond Roche lobe. (see Table in the article)

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