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Saturn’s beautiful rings might be the most
recognizable thing in all of astronomy,
but it won’t be that way forever.
In 2018, planetary scientists
realized that the rings
are disappearing way faster
than previously expected.
In fact, some researchers went as far as to
say this was the “worst-case scenario.”
Now, before the wave of grief overwhelms you,
remember that this is astronomy.
In this case, “way faster” means that
the rings may only be around for another
hundred million years or so.
What might be even more surprising about this
research is that it suggests Saturn’s rings
might also only be around
a hundred million years old,
billions of years younger than
some astronomers have suspected.
That complicates our understanding of where
the rings might have come from in the first place.
And it’s making some people wonder if, maybe,
the ring system we see today
isn’t the only one Saturn has ever had.
Those 2018 results were thanks to NASA’s
Cassini mission, which wrapped up in 2017.
As the mission came to an end,
scientists sent the spacecraft between
Saturn and its rings for the very first time.
During these final orbits, Cassini detected
material falling from the innermost ring onto
the planet’s upper atmosphere, the first direct
measurements of what scientists called ring rain.
And the data were backed up by astronomers
using the giant Keck telescope in Hawaii.
Material falls from the rings for a bunch
of reasons, including random collisions between
ring particles and particles getting dragged
around by Saturn’s magnetic field.
We’ve actually seen evidence
of this process since the 1980s,
so that part of Cassini’s
discovery wasn’t surprising.
What did catch scientists off guard, though,
was how effective ring rain seems to be.
Cassini and Keck’s observations allowed
scientists to more accurately calculate the
rate Saturn’s rings are losing material,
and they found it could be anywhere from around
5,000 to 45,000 kilograms per second.
And that has some significant implications.
Researchers have used that new number to run
time forward and backward in computer simulations,
and they’ve found that it would only take
around a hundred million years
for what we see today to disappear.
But even more significantly, you’d only
have to go back about a hundred million years
for Saturn’s inner rings to have as much
material as the largest one does today.
Scientists think that’s a pretty good estimate
for how old the ring system might be.
It’s a rough guess, but that value matches
other lines of evidence,
like how clean the rings seem to be.
Either way, what’s clear is that the rings
are young compared to the four and a half
billion-year age of the solar system, and
that kind of throws a wrench in things.
See, Saturn’s rings are the outlier in the
solar system.
Not only are they much larger than
those of other outer planets,
but they’re also made of nearly
pure ice instead of dust and rock.
Whatever formed them needs
to account for this purity.
And that eliminates many options, since most objects
in space contain a mix of ice, rock, and metal.
The problem is, some of the best
hypotheses that do explain the
purity really only make sense if the
rings are as old as the solar system.
Like, one idea is that the rings are the icy
outer layers of a large moon
that was torn apart by Saturn’s gravity.
But in order for a moon that size to get close
enough to Saturn to be ripped apart,
there would have to be a disk of debris around Saturn,
and that disk only lasted a short
time after the planet formed.
Another possibility is that a large asteroid
or comet destroyed a medium-sized moon like
Mimas, which contains very little rock or
metal.
But again, that kind of big collision was
much more common billions of years ago
than a hundred million years ago.
Researchers aren’t totally out of ideas,
though.
One hypothesis that might make more sense
is a pair of smaller moons colliding in
the recent past, similar to how some scientists
think the rings of Uranus formed.
The gravity of a passing comet might be enough
to nudge one object into the orbit of another,
and most of Saturn’s small moons seem to
be nearly pure ice, anyway.
Unfortunately, the models for this idea aren’t
all that supportive so far,
so it might be back to the drawing board.
Scientists will likely be researching their
hypotheses for a while,
but there’s actually another, even bigger
question raised by Cassini’s discoveries:
If the rings are so young and short-lived,
why are we so lucky that we get
to be around when they are?
Astronomers hate feeling lucky,
because it sometimes means
that they’re not looking hard enough
for the deeper answer.
So many don’t think this is a coincidence.
Like, maybe these rings are recent, but what
we see is actually just the latest iteration
of a process that has happened multiple times
in Saturn’s history.
Maybe Saturn has spent the last four billion years
collecting and destroying one ring system after another.
If that’s true, it might also mean that
the other outer planets have had large rings
in the past, too, but they just don’t right
now.
There are a lot of questions left to answer
here, but they’re ones scientists are actively
investigating, and they could lead to a new
understanding of how ring systems form,
and what our solar system
looked like not that long ago.
Oh, and in case you’re still mourning the
future loss of the rings,
there is one last bit of good news:
Even once the main rings have disappeared,
Saturn won’t be entirely ringless!
The dusty G ring and Phoebe ring are constantly
being replenished by debris knocked off nearby moons,
and the E ring is composed of ice particles
ejected by the geysers on Enceladus.
So there still will be rings! Everything will be alright.
And who knows, maybe by then another planet
will have some new bling to show off!
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