The rings are raining down onto the planet, and disappearing surprisingly fast.
4.5 billion years ago, our Solar System first formed.
Protoplanetary disks, which all solar systems are thought to form with, will coalesce into planets over time, as this illustration shows. It’s important to recognize that the central star, the individual planets, and the leftover primordial material (which will, for example, become asteroids or Kuiper belt objects) may all have variations in ages on the order of tens of millions of years. (NAOJ)
Arising from a protoplanetary disk, many young protoplanets resulted in 8 long-term survivors.
Initially, young solar systems form with large numbers of planets and protoplanets, but these systems evolve chaotically over time. There may have been substantially more than 8 planets in the early solar system and it’s even possible that some of today’s planets existed in a different order than we see them today. But in space, all we have left are the survivors. (WIKIMEDIA COMMONS USER ASTROMARK)
After billions of years of cosmic evolution, Saturn stands out from the rest.
The planets of the Solar System, shown to the scale of their physical sizes, all orbit according to certain specific rules. As the Sun loses mass as it burns through its nuclear fuel, the rules remain constant but the orbits themselves change. Over the history of the Solar System, our Sun has lost 0.03% of its original mass: approximately the mass of Saturn. Of all the planets, only Saturn has a major, large, visible ring system. (NASA)
While all gas giants possess ring systems, Saturn’s is uniquely large, prominent, and brilliant.
From its unique vantage point in the shadow of Saturn, the atmosphere, the main rings, and even the outer E-ring are all visible, along with the visible ring gaps of the Saturnian system in eclipse. Saturn’s rings, made primarily of water-ice, are unique among all the planets in the Solar System. (NASA / JPL-CALTECH / SPACE SCIENCE INSTITUTE)
Made of 99% water-ice, nobody knows where they originated from.
Visible and radio images of Saturn’s rings and their structure, as delivered by Cassini. The gaps, thickness, density, and composition of these rings can all be revealed by Cassini data, but the infall of material from the rings onto the planet itself could only be measured when Cassini passed between the rings and the main planet itself. (NASA/JPL/SPACE SCIENCE INSTITUTE)
Perhaps they’ve always existed, with gravitation creating clumps while tidal forces and collisions break them up.
Within Saturn’s rings, many small moons and moonlets, such as Daphnis, can be found. These objects are likely created by accreting particles, then destroyed by collisions and tidal forces. their uniform composition and decaying nature suggests that they were created relatively recently, with one longstanding theory contending that a larger, destroyed moon gave them their origin as little as tens but as many as hundreds of millions of years ago. (NASA / JPL-CALTECH / SPACE SCIENCE INSTITUTE)
Alternatively, perhaps they arose from a moon-destroying collision.
Saturn’s 7th largest moon, Mimas, appears to hover above the colorful rings. This image was taken by the Cassini spacecraft and, despite their enormous size differences, show two entities of comparable mass. Mimas is approximately twice the mass of the entirety of the ring system, and a collision between a fast-moving object and a Mimas-like moon could have given rise to the ring system. (UNIVERSAL IMAGES GROUP VIA GETTY IMAGES)
Others have suggested an origin from captured asteroids and comets.
This view of Comet Halley’s nucleus was obtained by the Halley Multicolour Camera (HMC) on board the Giotto spacecraft, as it passed within 600 km of the comet nucleus on 13 March 1986. The comet was clearly quite active at the time, and the composition of comets resembles the composition of Saturn’s rings. (ESA/MPAE LINDAU)
Irrespective of their creation, these rings are nearing the end of their lifespan.
This image of Saturn’s rings, with the planet itself behind them, was taken by Cassini at a distance of 725,000 km from the planet. Due to the fact that the ring system is “raining” down material onto Saturn, we can conclude that the rings will be entirely gone, based on the current rate of mass loss, in another 300 million years. (NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE)
NASA’s Cassini mission revealed Saturn, up close, as never before.
The entirety of Saturn’s main rings, from the inner D ring to the outer F ring, may be much newer than the rest of the Solar System. It’s plausible that a few hundred million years ago, before the rise of the dinosaurs, these rings may not have existed at all. In another 300 million years ago, they likely will have disappeared entirely. (NASA / JPL)
Near mission’s end, it repeatedly passed between the main rings — including through ring gaps — and the planet.
During the final year of its mission, 2017, NASA’s Cassini spacecraft switched to a highly eccentric orbit, enabling the spacecraft to pass between the main rings and the planet itself a total of 22 times, measuring with its instruments in situ this previously unexplored region of space. (NASA/JPL-CALTECH)
What it found was shocking: a massive “rainstorm” of atoms and molecules.
As Cassini passed interior to Saturn’s rings, it was able to detect a myriad of nanograins made of various particles, as well as incoming and outgoing material. All told, the observations sum up to approximately 10,000 kg of material falling from the rings onto Saturn every second. (H.-W. HSU ET AL., SCIENCE (2018) VOL. 362, ISSUE 6410, EAAT3185)
Those molecules included hydrogen, water, ammonia and hydrocarbons, among others.
Based on the data obtained from Cassini’s 22 orbital passes between Saturn’s rings and the planet itself, 10,000 kg of material per second fall down onto Saturn from the rings. Of that material, the majority is hydrogen and helium, followed by organics (such as complex hydrocarbons), water, carbon monoxide and nitrogen, methane, ammonia, and carbon dioxide. (NASA/JPL/SWRI)
Overall, 10 tonnes of ring matter rain down onto Saturn with each passing second.
A few of the major findings from Cassini’s direct sampling from numerous passes between the rings and the planet: complex organics rain down from Saturn’s rings; inner-ring particles take on electric charges and travel along magnetic-field lines; newly revealed electric-current system and radiation belt; and up-close measurement of Saturn’s near-zero magnetic-field tilt. (NASA/JPL-CALTECH)
Based on the observed rate, Saturn’s rings will completely disappear within 300 million years, at most.
Saturn and its spectacular rings, as imaged by the Hubble Space Telescope on July 4, 2020. Hubble takes an annual image of Saturn as part of the Outer Planets Atmospheres Legacy (OPAL) project. (NASA, ESA, A. SIMON (GODDARD SPACE FLIGHT CENTER), M.H. WONG (UNIVERSITY OF CALIFORNIA, BERKELEY), AND THE OPAL TEAM)
Across the Universe, ringed planets may only be transient rarities, after all.
Artist’s conception of the extrasolar ring system circling the young giant planet or brown dwarf J1407b. Worlds with extraordinary ringed systems can produce large flux dips when they transit, but Cassini’s data from Saturn indicates that these ringed systems might be short-lived compared to the lifetime of a solar system. (RON MILLER)
Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more. Ethan Siegel is the author of
Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.