Sky & Telescope - September 2021 - 15

between ring particles cause the rings to
spread out over time. Any material that
expands far enough from Saturn, past
the so-called Roche limit, can clump up
into moons. Meanwhile, the rings' inner
edges are constantly losing material to
Saturn's gravity; the Voyagers and Cassini
even observed this ring rain as it fell.
Second, the rings are extremely reflecRAVIOLI
MOONS Atlas (above) and Pan
(below) are two of the smaller moons. The ridges
around their equators are made of particles they
sweep up as they orbit within Saturn's rings.
tive, made of 90-95% water ice. " They
look like fresh snow, " says Luke Dones
(Southwest Research Institute). And just
as snow dirties within a day in a city,
thanks to the carbon-enriched smoke
from cars and trucks, interplanetary
meteoroids should darken the rings over
time. Jupiter, Uranus, and Neptune have
darker rings for this reason, yet Saturn's
rings are still bright.
Since less massive rings would darken more quickly, some
scientists have suggested that they must be quite young - less
than 100 million years old. That's recent enough that the
Triceratops and other dinosaurs of the Cretaceous Period
would have been roaming Earth when the rings form. Yet
the precipitating event - a moon-size collision - would have
been so rare that many scientists remain uneasy with the
idea of young rings. Even those in favor of a past collision
acknowledge this, such as John Dubinsky (then at University
of Toronto), who qualified such an event in a 2019 study as
" perhaps unlikely, but not wildly so. "
There's also a more general sense of discomfort with the
young-ring scenario. Despite the circumstantial evidence,
Dones counters, " there's no direct evidence that the rings
are young. " In fact, Dones worked with Aurélien Crida (Côte
d'Azur University, France) and others to outline why the rings
could be light, bright, and still be primordial - no moon
smash necessary.
Take the mass, for example. Crida's team acknowledged
that the spreading effect would cause the rings to lose mateYOUNG
RINGS
These simulation frames show
what would happen if a comet
that happened to pass through
the Saturn system smashed
into a moon. The resulting
debris would form wide, icy
rings around Saturn, close to
what we see now. However,
the small likelihood of the
smashup causes some scientists
to doubt the scenario.
(Watch the full animation at
https://is.gd/newrings.)
rial. But because more massive rings
spread faster, they argue, the rings would
have lost most of their material early on,
eventually converging to a mass very like
the one Cassini measured. So the rings'
current leanness could be a sign not of
youth but of age.
It's even possible that the ring rain
could preferentially " clean " the rings of
organic material, leaving them looking
shiny and new. Although the details of
how this would work remain unclear,
Cassini observations of the ring rain and
of Saturn itself confirm that much of
what is raining down consists of silicates and organics.
But Dones admits the debate is far from settled. " We've
never really come up with a good reason why Saturn should
be the planet that has a much more 'ginormous' ring system
than the others, " he says.
The unsettled age of the rings ties directly to the unsettled
origin of the midsize moons, because the two structures are
two sides of the same coin. Even as the rings' outer edges
spawn new moons (including moonlets we've spotted in the
act of forming), other moons donate material to the rings or
even make them wholesale, in moon-destroying impacts. If
the rings are young - and maybe even if they're not - then
some of the midsize moons might be newcomers, too.
" Children of the Rings "
There's little doubt that Saturn's innermost moons are indeed
" children of the rings, " as first proposed by Sébastien Charnoz
(then at Paris Diderot University). Observations bear this
out. The ravioli shapes of Pan and Atlas likely come from the
ice particles they've amassed as they orbit within and just
(continued on page 18)
500s
12.9h
23.9h
2.6d
8.6d
18.1d
sk yand tele scope .o r g * SEPTEMBER 2021 15
PAN AND ATLAS: NASA / JPL / SPACE SCIENCE INSTITUTE;
RING SIMULATION: JOHN DUBINSKI / ICARUS 2019
https://www.is.gd/newrings https://skyandtelescope.org/
Sky & Telescope - September 2021

Table of Contents for the Digital Edition of Sky & Telescope - September 2021
