Here’s the good news: Saturn’s glorious rings might last as long as 300 million more years. The bad news? They might be gone much sooner than that.
New research confirms that Saturn is losing ring material in a steady “rain” falling into the planet’s upper atmosphere, under the influence of Saturn’s magnetic field and gravity, at the maximum rate estimated from the Voyager flybys decades ago.
“We estimate that this ‘ring rain’ drains an amount of water products that could fill an Olympic-sized swimming pool from Saturn’s rings in half an hour,” said James O’Donoghue, a researcher at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “From this alone, the entire ring system will be gone in 300 million years.”
But if data collected by NASA’s Cassini spacecraft are correct, “the rings have less than 100 million years to live,” O’Donoghue said “This is relatively short, compared to Saturn’s age of over 4 billion years.”
The new research indicates the rings are unlikely to be older than 100 million years or so and as such, “we are lucky to be around to see Saturn’s ring system, which appears to be in the middle of its lifetime,” O’Donoghue said. “However, if rings are temporary, perhaps we just missed out on seeing giant ring systems of Jupiter, Uranus and Neptune, which have only thin ringlets today!”
Saturn’s rings are mostly made up of chunks of water ice as small as microscopic grains to boulders several metres across. Their orbital velocity acts to prevent Saturn’s gravity from pulling them in. But smaller particles can become electrically charged by ultraviolet light or from plasma produced in collisions with micrometeoroids. When that happens, particles can be pulled into the planet’s atmosphere along magnetic field lines.
After vaporising, water molecules can chemically react with Saturn’s ionosphere, increasing the lifespan of electrically charged H3+ ions, made up of three protons and two electrons. Sunlight can cause these particles to glow in infrared light, which was observed by O’Donoghue’s team using instruments at the the W.M. Keck Observatory in Hawaii.
Analysing that light, the team found the amount of rain matches the high values derived more than three decades earlier. The researchers also found a glowing band at a higher latitudes in the southern hemisphere where Saturn’s magnetic field intersects the orbit of the moon Enceladus, which spews water ice into space from geysers fed by a sub-surface ocean.
“That wasn’t a complete surprise,” said Jack Connerney, who proposed in 1986 that electrically charged ring particles were flowing down magnetic field lines into Saturn’s atmosphere. “We identified Enceladus and the E-ring as a copious source of water as well, based on another narrow dark band in that old Voyager image.”
O’Donoghue’s team wants to find out how the ring rain changes as Saturn moves through different seasons during its 29.4-year obit of the sun. Those observations will help researchers pin down the ultimate fate of the rings.