Saturn’s rings are one of the jewels of the solar system, but it seems that their time is short and their existence fleeting.
A new study suggests the rings are between 400 million and 100 million years old – a fraction of the age of the solar system. This means that we are just lucky to live in an era where the giant planet has its beautiful rings. Research also shows that they could be gone in another 100 million years.
The rings were first observed in 1610 by the astronomer Galileo Galilei who, due to the resolution limits of his telescope, initially described them as two smaller planets on opposite sides of Saturn’s main sphere, apparently in physical contact with them.
In 1659 the Dutch astronomer Christiaan Huygens published System Saturnin which he was the first to describe them as a thin, flat ring system that did not touch the planet.
He also showed how their appearance, as seen from Earth, changes as the two planets orbit the sun and why they seem to disappear at certain times. This is because their viewing geometry is such that we periodically see them from the side on Earth.
The rings are visible to anyone with decent binoculars or a modest backyard telescope. Cast white against Saturn’s pale yellow orb, the rings are composed almost entirely of billions of particles of water ice, which sparkle from scattered sunlight.
Amidst this icy material are deposits of darker, dusty stuff. In space science, “dust” usually refers to small grains of rocky, metallic, or carbon-rich material noticeably darker than ice. It is also collectively referred to as micrometeoroids. These grains permeate the solar system.
Occasionally, you can see them entering the Earth’s atmosphere at night like shooting stars. The gravitational fields of the planets have the effect of magnifying or focusing this dusty planetary “incursion”.
Over time, this incursion adds mass to a planet and changes its chemical composition. Saturn is a huge gas giant planet with a radius of about 60,000 kilometers, about 9.5 times that of Earth, and a mass about 95 times that of Earth. This means it has a very large “gravitational source” (the gravitational field around a body in space) that is very effective in funneling the dusty grains to Saturn.
Collision course
The rings extend from about 2,000 kilometers above Saturn’s cloud tops to about 80,000 kilometers away, taking up a great deal of space. When infalling dust passes through it, it can collide with ice particles in the rings. Over time, due to the dust, the rings gradually darken and gain mass.
Cassini-Huygens was a robotic spacecraft launched in 1997. It reached Saturn in 2004 and entered orbit around the planet, where it remained until the end of the mission in 2017. One of the instruments on board was the Cosmic Dust Analyzer (CDA).
Using data from the CDA, the authors in the new paper compared current dust counts in Saturn’s space with the estimated masses of dark dusty material in the rings. They found that the rings are no older than 400 million years and may be as old as 100 million years. These may seem like long timescales, but they are less than a tenth of the solar system’s 4.5 billion-year age.
This also means that the rings did not form at the same time as Saturn or the other planets. They are, cosmologically speaking, a recent addition to the solar system. They were not present for more than 90 percent of Saturn’s existence.
Dead star
This leads to another mystery: How did the rings first form, given that all the major planets and moons of the solar system formed much earlier? The total mass of the rings is estimated to be about half that of one of Saturn’s smaller icy moons, many of which display massive impact features on their surfaces.
One in particular, the small moon Mimasnicknamed the Death Star, has a 130 kilometer wide impact crater called Herschel on its surface.
This is certainly not the largest crater in the solar system. However, Mimas is only about 250 miles across, so this impact wouldn’t have required much more energy to obliterate the moon. Mimas is made of water ice, just like the rings, so it’s possible the rings were formed by such a catastrophic impact.
Ring rain
However they formed, the future of Saturn’s rings is beyond question. The impact of the dust particles against the icy particles occurs at very high velocities, dislodging small pieces of ice and dust from their parent particles.
Ultraviolet light from the sun causes these fragments to become electrically charged through the photoelectric effect. Like Earth, Saturn has a magnetic field, and once charged, these tiny icy fragments are released from the ring system and trapped by the planet’s magnetic field.
Together with the giant planet’s gravity, they are then funneled into Saturn’s atmosphere. This “ring rain” was first observed from afar by the Voyager 1 and Voyager 2 spacecraft during their short flybys to Saturn in the early 1980s.
In a more recent paper from 2018 scientists used dust counts, again from the CDA, as Cassini flew between Saturn’s rings and cloud tops, to calculate how much ice and dust is being lost from the rings over time. This study showed that every half hour about one Olympic size pool of mass from the rings is lost to Saturn’s atmosphere.
This flow rate was used to estimate that, given their current masses, the rings will likely be gone within 100 million years. These beautiful rings have a turbulent history and unless they are replenished in some way, they will be swallowed up by Saturn.
Gareth Dorrianpostdoctoral researcher in space science, University of Birmingham
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