Saturn’s rings should not various hundred million years outdated and won’t exist as vibrant, dense rings for greater than one other few hundred million years.

NASA’s Cassini spacecraft took this detailed picture of Saturn and its rings in August 2009. Cassini helped scientists decide that the rings are comparatively younger — and likewise short-lived — notably with measurements taken throughout its Grand Finale, threading the hole between planet and rings. Credit score: NASA/JPL-Caltech/House Science Institute

Do astronomers have any estimates of when Saturn’s rings will disappear?

Doug Kaupa
Council Bluffs, Iowa

All 4 of the photo voltaic system’s large planets have ring methods. The rings of Jupiter, Uranus, and Neptune are darkish, sparse belts or ringlets. Solely Saturn’s large predominant rings are dense and vibrant, made of virtually pure water-ice particles ranging in measurement from mud and pebbles to boulders. You possibly can visualize Saturn’s rings as an infinite swarm of snowballs and snowman-sized items orbiting the planet.

Most planetary scientists as soon as thought Saturn’s rings have been as outdated as Saturn itself: 4.5 billion years. However the Cassini spacecraft, which orbited Saturn from 2004 to 2017, gathered proof that the rings are comparatively latest. If early dinosaurs have been sensible sufficient to construct telescopes, they may not have seen rings round Saturn in any respect. Cassini’s findings additionally counsel the rings are short-lived, with a lifetime of tons of of tens of millions of years.

Astronomers arrived at these estimates primarily based on a number of measurements.

Cassini ended its mission in 2017 with its Grand Finale, 22 plunging orbits during which the craft swooped between Saturn and its innermost ring, the D ring. This allowed Cassini to find out the rings’ mass by evaluating the gravitational pull on the spacecraft throughout its close-in orbits to the pull it skilled when orbiting farther out, exterior to the rings. The mass of the ring system is akin to however lower than that of Saturn’s innermost icy moon Mimas.

On the similar time, Cassini noticed materials flowing from the rings into Saturn at many tons per second. Combining this mass loss with the rings’ present mass suggests a remaining ring lifetime (or mass-loss age) of just a few hundred million years.

Cassini additionally decided the rings’ so-called air pollution age. Throughout its 13 years orbiting Saturn, Cassini’s onboard mud detector measured the influence fee of interplanetary micrometeoroids. These mud particles are pulled in by Saturn’s gravity and strike the ring particles. Interplanetary mud is usually darkish, non-icy stuff. Over time then, the rings ought to develop polluted. How briskly they darken relies on the mud inflow fee and the ring mass. Based mostly on Cassini’s measurements, researchers decided that this inflow would darken the rings to the present noticed stage in at most a number of hundred million years. This independently decided air pollution age agrees with the mass-loss age.

The mass deposited within the rings by these meteoroids additionally reduces the orbital angular momentum of the ring particles and causes them to float inward. Such mud impacts can moreover exert a damaging torque. Moreover, micrometeoroids hit at tens of miles per second. That is like setting off a firecracker in a snowball. Particles will get thrown across the rings, and a few may even rain down immediately onto Saturn. The mixture of all these dynamical results produces influx at charges akin to the influx fee Cassini noticed.

So, the conclusion is that Saturn’s rings should not various hundred million years outdated and won’t exist as vibrant, dense rings for greater than one other few hundred million years. What is going to they grow to be? They might persist for billions of years, trying extra just like the sparse, darkish ring methods of the opposite large planets.

Richard H. Durisen and Paul R. Estrada
Professor Emeritus of Astronomy, Indiana College, Bloomington, Indiana,
and Analysis Scientist, NASA Ames Analysis Middle, Mountain View, California