Saturn's rings are one of the jewels of the Solar System, but it seems that their time is short and their existence fleeting.
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A color-exaggerated view of Saturn backlit by the sun.
(NASA/JPL/Space Science Institute) |
A new study suggests the rings are between 400 million and
100 million years old – a fraction of the age of the Solar System. This means
we are just lucky to be living in an age when the giant planet has its
magnificent rings. Research also reveals that they could be gone in another 100
million years.
The rings were first observed in 1610 by the astronomer
Galileo Galilei who, owing to the resolution limits of his telescope, initially
described them as two smaller planets on each side of Saturn's main orb,
apparently in physical contact with it.
In 1659, the Dutch astronomer Christiaan Huygens published
Systema Saturnium, in which he became the first to describe them as a thin,
flat ring system that was not touching the planet.
He also showed how their appearance, as viewed from Earth,
changes as the two planets orbit the Sun and why they seemingly disappear at
certain times. This is due to their viewing geometry being such that we on
Earth periodically see them edge-on.
The rings are visible to anyone with a decent pair of
binoculars or a modest back garden telescope. Cast white against the pale-yellow
orb of Saturn, the rings are composed almost entirely of billions of particles
of water ice, which shine by scattering sunlight.
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A page from the System Saturnium published in 1659. (US
Library of Congress) |
Amid this icy material are deposits of darker, dusty stuff.
In space science, "dust" usually refers to tiny grains of rocky,
metallic, or carbon-rich material that is noticeably darker than ice. It is
also collectively referred to as micrometeoroids. These grains permeate the
Solar System.
Occasionally, you can see them entering the Earth's
atmosphere at night as shooting stars. The gravitational fields of the planets
have the effect of magnifying or focusing this dusty, planetary
"in-fall".
Over time, this in-fall adds mass to a planet and alters its
chemical composition. Saturn is a massive gas giant planet with a radius of
some 60,000 kilometers, about 9.5 times that of Earth, and a mass of about 95
times that of Earth. This means it has a very large "gravity well"
(the gravitational field surrounding a body in space) that is very effective at
funneling the dusty grains towards Saturn.
Collision course
The rings extend from some 2,000 kilometers above Saturn's
cloud tops to about 80,000 kilometers away, occupying a large area of space.
When in-falling dust passes through, it can collide with icy particles in the
rings. Over time, the dust gradually darkens the rings and adds to their mass.
Cassini-Huygens was a robotic spacecraft launched in 1997.
It reached Saturn in 2004 and entered orbit around the planet, where it stayed
until the end of the mission in 2017. One of the instruments aboard was the
Cosmic Dust Analyzer (CDA).
Using data from the CDA, the authors in the new paper
compared the current dust counts in space around Saturn with the estimated mass
of dark dusty material in the rings. They found that the rings are no older
than 400 million years and may be as young as 100 million years. These may seem
like lengthy time scales, but they are less than one-tenth of the 4.5
billion-year age of the Solar System.
This also means that the rings did not form at the same time
as Saturn or the other planets. They are, cosmologically speaking, a recent
addition to the Solar System. For over 90 percent of Saturn's existence, they
were not present.
Death Star
This leads to another mystery: how did the rings first form,
given that all of the Solar System's major planets and moons formed much
earlier? The total mass of the rings is estimated to be about half as much as
one of Saturn's smaller icy moons, many of which exhibit enormous impact
features on their surfaces.
One in particular, the little moon Mimas, which is nicknamed
the Death Star, has a 130 kilometer-wide impact crater called Herschel on its
surface.
This is by no means the largest crater in the Solar System.
However, Mimas is only about 400 kilometers across, so this impact would not
have needed much more energy to obliterate the moon. Mimas is made of
water-ice, just like the rings, so it's possible that the rings were formed
from just such a cataclysmic impact.
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Saturn's moon Mimas, showing Herschel crater. (NASA/JPL/SSI) |
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