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 did scientists "(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: