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I assume that the A ring is the densest, but I might be wrong. Nevertheless, I failed to find any explicit information over the net about the minimum and maximum of densities for the different rings. As a less practical question: what would one see upon entering one of the denser rings? Would it be obvious that it is a dense but thin (few metres - 1 km) cloud of particles, continuously bombarding the spacecraft irregardless whether it orbits along within the ring, or one would not see more than just a distant haze (if lit by the Sun) that spans over the horizon?

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Elements of answer:

It is not an easy question, as we lack of data to constrain strongly the density and mass of Saturn's rings.

However, a first clue of their density is their optical depth (that is a measure of the transparency of a medium). The densest rings are the one of the main ring system (rings A, Cassini Division, B and C) plus ring F; they are characterized by optical depth larger than 0.1. Rings in the faint ring system (E and G) have very small optical depth (lower than $10^{-5}$).

With this regard, A is indeed the densest ring of Saturn's rings.

Side note:

Optical depth is a measure of transparency of one medium; it measures how radiation is neither scattered nor absorbed by the medium. It expression is given by

$$\frac{I}{I_0} = {\rm e}^{-\tau}$$

where $I_0$ is the source intensity, $I$ the observed intensity, and $\tau$ the optical depth.


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