When I consider thy heavens, the work of thy fingers,
the moon and the stars, which thou hast ordained;
What is man, that thou art mindful of him?
and the son of man, that thou visitest him?
Psalm 8:3,4
"Saturn’s B ring is much less dense than previously thought, challenging attempts to keep it billions of years old. Scientists have always been worried about the apparent youthfulness of planetary rings
Back in 2007, Cassini scientist and planetary ring specialist Larry Esposito posited that Saturn’s rings might be billions of years old. He based it on the fact that the B-ring appeared dense enough to allow for repeated recycling of the material. This, he proposed, would maintain the ring’s brightness despite continuous infalling of dark meteoritic dust. (The B-ring is the densest part of the whole ring system.)
That explanation is now challenged by new measurements that find the B-ring to be one-half to one-seventh the mass of previous estimates. “Saturn’s Ring Puffs Itself Up With Optical Illusion,” a headline on National Geographic says. With a magic trick that’s more smoke than mirrors, one of Saturn’s most mysterious rings has deceived astronomers for decades with an optical illusion. As
described by astronomers Matt Hedman and Phillip Nicholson, the B ring—Saturn’s brightest—has been deceptively opaque, making astronomers think that it contains up to seven times more mass than it actually does. The open-access paper in Icarus used stellar occultations from multiple Cassini orbits to re-estimate the mass of the B-ring. Instead of equaling twice the mass of moon Mimas, it is probably between 1/3 to 2/3 of that. If the mass is so low, it puts pressure on theorists to imagine a special event in more recent times that created the rings. The problem is that ad hoc events that cannot be observed can be criticized as special pleading. The paper in Icarus dances around the age problem in the introduction, but fails to address it in the body of the paper: The large uncertainties in the B-ring’s mass and its typical surface mass density not only hamper efforts to understand the structure and dynamics of this ring, but also complicate efforts to ascertain the age and history of Saturn’s ring system.
A JPL press release on the paper notes the connection between density and age: Research on the mass
of Saturn’s rings has important implications for their age. A less massive ring would evolve faster than a ring containing more material, becoming darkened by dust from meteorites and other cosmic sources more quickly. Thus, the less massive the B ring is, the younger it might be — perhaps a few hundred million years instead of a few billion.
That this represents a problem is clear from a quick calculation. A hundred million years is 1/45th the assumed age of the solar system (4.5 billion years). What happened 44/45ths into the lifetime of the planet to form rings? Why did it happen so recently that we can observe the rings today?
The word “perhaps” indicates that the statement lacks empirical foundation. It could be far younger than a few hundred million years old.
Besides, there are other forces at work to hasten the disappearance of rings besides infalling meteoritic dust: among them,
sputtering (erosion at the atomic scale),
collisional spreading,
and sunlight pressure (the Poynting-Robertson effect).
And if the densest ring is no more than a few hundred million years old, what of the more tenuous D, C, A, F, G, and E rings? One must recall that planetary scientists were similarly astonished to find Jupiter, Uranus and Neptune ringed by orbiting material with apparently short lifetimes. It’s getting harder to keep rings old. Further refinements of the B-ring’s mass will become possible in 2017 when the orbiter goes into highly-inclined orbits over the rings prior to its grand-finale descent into Saturn." CEH
