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A brief astronomical history of Saturn’s amazing rings

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Many dream of what they would do had they a time machine. Some would travel 100 million years back in time, when dinosaurs roamed the Earth. Not many, though, would think of taking a telescope with them, and if, having done so, observe Saturn and its rings.

Whether our time-traveling astronomer would be able to observe Saturn’s rings is debatable. Have the rings, in some shape or form, existed since the beginnings of the solar system, 4.6 billion years ago, or are they a more recent addition? Had the rings even formed when the Chicxulub asteroid wiped out the dinosaurs?

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I am a space scientist with a passion for teaching physics and astronomy, and Saturn’s rings have always fascinated me as they tell the story of how the eyes of humanity were opened to the wonders of our solar system and the cosmos.

Our view of Saturn evolves

When Galileo first observed Saturn through his telescope in 1610, he was still basking in the fame of discovering the four moons of Jupiter. But Saturn perplexed him. Peering at the planet through his telescope, it first looked to him as a planet with two very large moons, then as a lone planet, and then again through his newer telescope, in 1616, as a planet with arms or handles.

Four decades later, Christiaan Huygens first suggested that Saturn was a ringed planet, and what Galileo had seen were different views of Saturn’s rings. Because of the 27 degrees in the tilt of Saturn’s rotation axis relative to the plane of its orbit, the rings appear to tilt toward and away from Earth with the 29-year cycle of Saturn’s revolution about the Sun, giving humanity an ever-changing view of the rings.

But what were the rings made of? Were they solid disks as some suggested? Or were they made up of smaller particles? As more structure became apparent in the rings, as more gaps were found, and as the motion of the rings about Saturn was observed, astronomers realized that the rings were not solid, and were perhaps made up of a large number of moonlets, or small moons. At the same time, estimates for the thickness of the rings went from Sir William Herschel’s 300 miles in 1789, to Audouin Dollfus’ much more precise estimate of less than two miles in 1966.

Astronomers understanding of the rings changed dramatically with the Pioneer 11 and twin Voyager missions to Saturn. Voyager’s now famous photograph of the rings, backlit by the Sun, showed for the first time that what appeared as the vast A, B and C rings in fact comprised millions of smaller ringlets.

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Voyager 2 false color image of Saturn’s B and C rings showing many ringlets.
NASA

The Cassini mission to Saturn, having spent over a decade orbiting the ringed giant, gave planetary scientists even more spectacular and surprising views. The magnificent ring system of Saturn is between 10 meters and one kilometer thick. The combined mass of its particles, which are 99.8% ice and most of which are less than one meter in size, is about 16 quadrillion tons, less than 0.02% the mass of Earth’s Moon, and less than half the mass of Saturn’s moon Mimas. This has led some scientists to speculate whether the rings are a result of the breakup of one of Saturn’s moons or the capture and breakup of a stray comet.

The dynamic rings

In the four centuries since the invention of the telescope, rings have also been discovered around Jupiter, Uranus and Neptune, the giant planets of our solar system. The reason why the giant planets are adorned with rings and Earth and the other rocky planets are not was first proposed by Eduard Roche, a French astronomer in 1849.

A moon and its planet are always in a gravitational dance. Earth’s moon, by pulling on opposite sides of the Earth, causes the ocean tides. Tidal forces also affect planetary moons. If a moon ventures too close to a planet, these forces can overcome the gravitational “glue” holding the moon together and tear it apart.
This causes the moon to break up and spread along its original orbit, forming a ring.

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The Roche limit, the minimum safe distance for a moon’s orbit, is approximately 2.5 times the planet’s radius from the planet’s center. For enormous Saturn, this is a distance of 87,000 kilometers above its cloud tops and matches the location of Saturn’s outer F ring. For Earth, this distance is less than 10,000 kilometers above its surface. An asteroid or comet would have to venture very close to the Earth to be torn apart by tidal forces and form a ring around the Earth. Our own Moon is a very safe 380,000 kilometers away.

NASA’s Cassini spacecraft about to make one of its dives between Saturn and its innermost rings as part of the mission’s grand finale.
NASA/JPL-Caltech

The thinness of planetary rings is caused by their ever-changing nature. A ring particle whose orbit is tilted with respect to the rest of the ring will eventually collide with other ring particles. In doing so, it will lose energy and settle into the plane of the ring. Over millions of years, all such errant particles either fall away or get in line, leaving only the very thin ring system people observe today.

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During the last year of its mission, the Cassini spacecraft dived repeatedly through the 7,000 kilometer gap between the clouds of Saturn and its inner rings. These unprecedented observations made one fact very clear: The rings are constantly changing. Individual particles in the rings are continually jostled by each other. Ring particles are steadily raining down onto Saturn.

The shepherd moons Pan, Daphnis, Atlas, Pandora and Prometheus, measuring between eight and 130 kilometers across, quite literally shepherd the ring particles, keeping them in their present orbits. Density waves, caused by the motion of shepherd moons within the rings, jostle and reshape the rings. Small moonlets are forming from ring particles that coalesce together. All this indicates that the rings are ephemeral. Every second up to 40 tons of ice from the rings rain down on Saturn’s atmosphere. That means the rings may last only several tens to hundreds of millions of years.

Could a time-traveling astronomer have seen the rings 100 million years ago? One indicator for the age of the rings is their dustiness. Objects exposed to the dust permeating our solar system for long periods of time grow dustier and darker.

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Saturn’s rings are extremely bright and dust-free, seeming to indicate that they formed anywhere from 10 to 100 million years ago, if astronomers’ understanding of how icy particles gather dust is correct. One thing is for certain. The rings our time-traveling astronaut would have seen would have looked very different from the way they do today.

This story has been corrected to reflect that it was Christiaan Huygens, not Giovanni Cassini, who first suggested that Saturn had rings.

[ Like what you’ve read? Want more? Sign up for The Conversation’s daily newsletter. ]The Conversation

Vahe Peroomian, Associate Professor of Physics and Astronomy, University of Southern California – Dornsife College of Letters, Arts and Sciences

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This article is republished from The Conversation under a Creative Commons license. Read the original article.


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Masks take center stage in presidential race as Biden slams Trump for ‘costing people’s lives’

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In an interview with CNN's Dana Bash on Tuesday, former Vice President Joe Biden laid into President Donald Trump for his comments belittling his decision to wear a mask at the Memorial Day events at the beginning of the week.

"He's a fool, an absolute fool to talk that way," said Biden. He added that "This macho stuff ... It's costing people's lives."

Trump has frequently refused to don a mask while speaking to the media, even when he is in public places where masks are required.

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“He’s a fool, an absolute fool to talk that way,” Biden to @DanaBashCNN about Trump belittling his wearing of a mask. “This macho stuff ... It’s costing people’s lives.”

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COVID-19

1 in 5 teachers—citing COVID-19 concerns—likely won’t return to US schools this fall: survey

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While most U.S. schools have ended in-person instruction for the rest of this academic year because of the coronavirus pandemic, polling results published Tuesday show that the majority of parents and teachers expect classrooms to reopen in the fall and worry about what that will mean for safety and education.

In mid-May, Ipsos conducted a pair of online polls for USA Today of K-12 teachers and parents of school-aged children. Pollsters found that if schools reopen in the fall—with strict new rules to limit Covid-19 infections—nearly six in 10 parents would consider not sending their kids back and one in five educators likely would not return to teaching. Among teachers 55 and older, that figure was one in four.

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Trump says he can ‘absolutely’ force governors to reopen churches if he decides to do so

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At Tuesday's coronavirus press briefing, President Donald Trump was pressed on whether he really has the authority to force governors to allow houses of worship to reopen amid the coronavirus pandemic. "Can you explain what authority you had in mind when you said that you would do that?" asked a reporter.

The president emphasized that he does have the power — but did not elaborate on how specifically he would do so, and added that he doesn't think he will have to.

"I can absolutely do it if I want to," said Trump. "I don't think I'm going to have to, because it's starting to open up. We need our churches and our synagogues and our mosques. We want them open, churches, synagogues, mosques, and other — we want them open and we want them open as soon as possible."

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