Slice of Shadow: This 2012 photo from NASA's Cassini probe showcases the B ring; which is so opaque that it casts a pitch-black shadown onto Saturn. It also hints at the rings' size: the Moon Tethys, upper left, is 660 miles (1,062 kilometers) wide
By Michael Greshko, National Geographic
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 study, published January 22 in the journal Icarus, marks the latest attempt to weigh the B ring, an important piece of data for understanding how Saturn’s rings formed.
The planet’s other rings haven’t proven nearly so tricky. Astronomers used density waves, ripples that form like a traffic jam as ring particles get jostled by Saturn’s moons. Change a ring’s density, and the waves within it change shape—letting scientists calculate the ring’s mass from the waves alone.
But the B ring has a complex structure that makes it hard to see and track density waves. So for years, many astronomers have guesstimated: If there’s more stuff in the ring, the thinking goes, the ring would be less transparent.
If the B ring is anything, it’s opaque—the solar system’s most opaque ring, in fact—leading some to suggest that by itself, it has up to twice as much mass as Saturn’s moon Mimas.
The new analysis from NASA’s Cassini probe pores over subtle density waves in the B ring to suggest that, depending where you look, the B ring holds just half to a seventh of the mass it appears to. It’s a convincing trick, a bit like pretending to perform incredible feats of strength with hollow plastic dumbbells.
The downward revision doesn’t dethrone the B ring, however: it’s still probably the most massive ring around Saturn. But scientists will have to wait until 2017 to definitively peer behind the B ring’s curtain. Cassini will finish its mission at Saturn with a “grand finale” next year, giving astronomers a chance to narrow down the ring’s mass, once and for all.