Saturn’s rings explained…
April 18, 2012 1 Comment
Of the five planets visible to the naked eye from Earth, Saturn is the furthest and slowest moving across the sky. That’s actually how Saturn got its name; the ancient Greeks named the planet after Chronos (Saturn to the Romans), the father of Zeus (Jupiter) and the God of time (a reason why chronos is a root of time-related words, as I explained in an earlier post). Galileo was the first to resolve and discover the rings of Saturn when he looked at it through his telescope in 1610. It’s incorrect status as the only ringed planet in the solar system survived until well into the latter half of the 20th century. In 1977, Saturn was joined in the ringed planet club by Uranus, after scientists observed a star passing behind the planet, a phenomenon called an occultation. Most unexpectedly, the star’s light blinked on and off nine times before disappearing behind the disc of the planet; this proved that although the rings were too dim to be seen from Earth, the material was present. Only a few years later, Voyager 1 discovered the rings of Jupiter and it wasn’t until the mid-1980’s that another stellar occultation proved the existence of Neptune’s ring system. So it’s true, all of the gaseous outer planets, or “gas giants”, have rings, but Saturn’s are by far the most visually impressive and the only ones visible from Earth. So that leads us to a very interesting and mysterious question…why? What makes Saturn’s rings special?
As telescopes on Earth have become more and more advanced and we continue our extensive exploration of the solar system, we have pieced together a much more comprehensive understanding of Saturn’s rings than Galileo had 400 years ago. Although the rings look solid and sheet-like from Earth, we now know that the rings of Saturn are actually comprised of billions of particles of rock, ice, and dust ranging in size from microscopic to meters-wide. The brighter, more dense regions of the rings have more material to reflect light while the dark regions or “gaps” are much more scarcely populated. The debris that makes up the planet’s rings are in a very well-defined plane, only a few tens of meters thick, but extending almost 130,000 km (80,778 mi) above the planet’s surface. The gallery below shows the evolution of our understanding and imaging of Saturn’s rings.
But many questions still remain about why Saturn’s rings are so much brighter than the other gassy planets. The answer scientists think, lies with the sixth-largest of Saturns 60+ moons: a small, icy world called Enceladus (shown below).
Enceladus is roughly 500 km in diameter (14% of the Moon) and 1.1 × 10²º kg in mass (0.2% of the Moon), but what it lacks in stature it makes up for in output. Literally. Discovered by William Herschel in 1879 (only 2 years before his discovery of the planet Uranus), Enceladus first came into the spotlight for scientists in early 1980. As John Spencer recalls in a recent article in Physics Today, that was
“…when scientists using Earth-based telescopes acquired new images of a faint outer ring of Saturn—the E ring—which had been discovered in the 1960’s. Those images revealed that the E ring’s brightness peaked at the orbit of Enceladus. They also showed that unlike Saturn’s other rings, the E ring scattered sunlight more efficiently at shorter wavelengths, which indicated that the ring was dominated by particles not much larger than the wavelength of light. Sputtering by charged particles in Saturn’s magnetosphere would erode away such micron-sized particles on time scales of decades to hundreds of years, so something had to be replenishing the ring on comparable time scales. The peak in ring density at Enceladus pointed to that moon as the likely source.”
Since then interest in the small ice-world increased exponentially and as a result Enceladus became a primary target of investigation for the joint NASA/ESA mission of the Cassini spacecraft. After only slightly whetting their appetite with two flybys of the small moon in early 2005, researchers decided to make a third flyby at a much closer range, 170 km (105 mi) instead of the planned 1000 km (621 mi). The dramatic results from this third Enceladus flyby in July 2005 were released in a special March 2006 issue of the journal Science. It was this flyby that got the high-resolution images seen above and first discovered the “four prominent parallel fractures, dubbed tiger stripes, surrounded by an intensely tectonically disrupted landscape” that the image depicts. In an even more interesting find, Cassini caught evidence of multiple plume jets erupting from the four tiger stripe fractures seen near Enceladus’s south pole.
These plumes (shown above), currently ejecting mass at an astounding 200 kg/s, have two observable components: micron-sized ice grains and gas (99% water vapor). It is speculated that the water vapor and ice crystals that are being deposited into Saturn’s ring system by Enceladus are what have kept the planet’s rings so bright and reflective for so long.
In spite of all of this other extremely intriguing science, the most interesting thing for scientists is Enceladus’ potential for life. As one of the few places in the solar system where we know water exists, Enceladus has become a key target in the search for extraterrestrial life. Scientists speculate that liquid water might occur in several places on the tiny moon: as a global ocean between the silicate core and the ice crust, as a more local south polar sea beneath the ice shell), or as localized bodies of water in the ice shell itself.