Tiny moons lurking in the Saturnian ring system are born just as quickly as they die, giving one of the planet’s rings its odd appearance, suggest a comparison between images returned by two of NASA’s most successful spacecraft.
The SETI Institute’s Robert French and Mark Showalter discovered a marked change in the appearance of Saturn’s F ring, which lies outside the broad, luminous A, B and C rings, when they examined 30-year-old images returned by the Voyager spacecraft and more recent shots snapped by Cassini.
“The F ring is a narrow, lumpy feature made entirely of water ice,” says French. “It has bright spots but it has fundamentally changed its appearance since the time of Voyager. Today there are fewer bright lumps.”
These bright knots seem to come and go over the course of hours or days, which led the pair of astronomers to suspect that the birth and death of small moons might be the explanation.
“We believe the most luminous lumps occur when tiny moons, no bigger than a large mountain, collide with the densest part of the F ring,” explains French. “These moons are small enough to coalesce and then break apart in small order.”
Planetary scientists know the F ring to be quite a dramatic place, given the chance. If a moon gets too close to Saturn’s gaseous body, the difference in the tug of gravitational attraction on its near side compared to its far side is capable of tearing the rocky body apart. Experts call this region – whose distance relies very much on the mass of the planet – the Roche limit, after French astronomer Edouard Roche.
One of Saturn’s larger moons, Prometheus, is around 92 kilometres (57 miles) larger in size and is very likely to be adding extra oomph to the making of the moonlets. Prometheus’ bulk, the researchers reason, must be emphasising the gravitational influence of the F ring’s rings particles, which then go on to make the bite-sized satellites.
“These newborn moons will repeatedly crash through the F ring, like bumper cars, producing bright clumps as they careen through lanes of material,” adds Showalter. “But this is self-destructive behaviour, and the moons – being just at the Roche limit – are barely stable and quickly fragmented.”
“The sort of processes going on around Saturn are very similar to those that took place here 4.6 billion years ago, when the Earth and the other large planets were formed,” states French. “It’s an important process to understand.”
However, the pair aren’t fully sold on the idea of minimoons in Saturn’s ring system as of yet. If Prometheus is responsible for their birth and sudden death, then there should be an increase in moons being made, quickly followed by their destruction, over the next few years – something that the Cassini spacecraft will be keeping a close eye on.