From the March 2015 issue

How are Saturn’s rings replenished? Will the process shut down?

Doug Kaupa, Colorado Springs, Colorado
By | Published: March 30, 2015 | Last updated on May 18, 2023
The ringed planet’s shadow blocks out the Sun in this 140-image mosaic of Saturn’s surroundings captured by NASA’s Cassini mission.
The age, origin, and evolution of Saturn’s bright icy rings are some of the biggest mysteries in the solar system. Most theories have the main rings forming from the partial or total destruction of an icy moon (roughly 250 to 3,000 miles [400 to 5,000 kilometers] in diameter) some 3.8 to 4.5 billion years ago. We would expect the constant influx of micrometeoroids to have substantially darkened the rings over time, but their nearly pure white appearance leads to a paradox: The rings are likely old, but they look young!

One way to solve this paradox is to recycle the rings. If micrometeoroid contamination is limited to the ring particles’ surfaces, it is possible that when particles collide and re-accrete, or when large particles are broken up by meteoroid impacts, the pristine interior ice is exposed, making the particles look new again. Also, if the dense B ring has much more mass than expected, the ring can act as a reservoir for fresh ice, replenishing the entire ring system over time.

The mass of the B ring is unknown. The Cassini mission’s “Grand Finale” orbits in 2017 will place the spacecraft between the rings and Saturn and allow, for the first time, a direct measurement of the rings’ mass. This may help solve the paradox once and for all.

Recycling and replenishment use material already in the rings. Even though it may take billions of years to destroy them, the lifetime of Saturn’s rings is still finite.

Finally, we do know where two rings come from. The enormous faint E ring is produced from ice particles emitted by geysers on the moon Enceladus, while dust from the surface of the tiny moon Aegaeon replenishes the G ring.

Robert French
SETI Institute, Mountain View, California