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Saturn’s icy rings reveal another secret: they’re young

By: Blaine Friedlander ,  Cornell Chronicle
Tue, 01/22/2019

Data from the last days of the NASA spacecraft Cassini show that Saturn’s beautiful, extensive rings are relatively young – perhaps created when dinosaurs roamed the Earth – because the ring’s mass is less than previously thought and its frozen components are surprisingly bright and free from dusty cosmic impurities, according to a study published Jan. 17 in Science.

“Based on previous research, we suspected the rings were young, but not everyone was convinced,” said Phil Nicholson, Cornell professor of astronomy and a co-author of “Measurement and Implications of Saturn’s Gravity Field and Ring Mass.”

Before Cassini’s demise when it crashed into Saturn in September 2017, the spacecraft passed repeatedly between the rings and the planet’s cloud tops to study Saturn’s gravity field and the rings’ mass.

Cassini (and two Voyager spacecraft) had studied Saturn’s rings from afar, but no craft had yet ventured into the rings to obtain up-close data. Before its final planetary plunge, Cassini dove through the rings 22 times, using six passes to measure the gravity field by tracking the radio signal from the spacecraft. (The technique is similar to a police radar, but more precise; Cassini’s velocity was measured with an accuracy of better than 0.1 millimeter per second.)

The scientists found that the rings – particularly the dense B-ring, one of the three main rings and the brightest visible in a telescope – had lower masses than many had expected, indicating a relatively young age. While Saturn is about 4.5 billion years old, the new Cassini data indicate that the rings probably formed between 10 million and 100 million years ago, according to the lead researchers from Sapienza University in Rome.

Had the rings been contaminated and darkened by interplanetary debris over a longer period, they would appear much darker, according to NASA’s Jet Propulsion Laboratory.

“The new mass measurement is firm, because Cassini was able to pass inside the rings. In our prior research, we used waves driven in the rings by Saturn’s moons to indirectly estimate their mass density at several locations, which we then extrapolated to estimate the total mass of the rings,” said Nicholson, who had conducted that earlier research at Cornell with Matt Hedman, now an assistant professor of astronomy at the University of Idaho. “Our final result was very close to the new measurement, but lower than most earlier estimates.”

“From what we know based on Cassini’s spectral and radar measurements, the rings are also less contaminated than previously thought – probably less than 1 percent,” said Nicholson. “They are close to pure water ice.”

In 2016, Zhimeng Zhang, Ph.D. ’16, led work examining the dust content of Saturn’s C ring. This research determined that the C ring, once thought to have formed in the primordial era, was less than 100 million years old. In 2017, she reported on similar measurements of the A and B rings, obtaining similarly young ages.

“Think of an unused desk in an unused room. The more it sits there, the more it collects dust,” Zhang said when she published her work. “The C ring is the same way. While it is composed mostly of water ice, it collects silicate-containing dust from the far-off Kuiper Belt. … [I]n this case, the dust – in terms of the age of the solar system – has not been here a long time.”

Among ring scientists, Nicholson and others had wagered what Cassini might find in terms of ring mass. The result was close to Nicholson’s prediction. He said: “This is quite gratifying from a scientific and personal point-of-view that we got close to the real number when Cassini finally measured it.”