How To Make A Flying Saucer, Saturn-style

A sextet of moons orbiting within and beyond the planet's rings are collected in this recently released image taken October 6, 2010 by the Cassini spacecraft. All photos credit: NASA/JPL/Space Science Institute

It’s not often a spacecraft nabs six moons around another planet in one photo, but there they are – Enceladus (313 miles across), oblong Janus (111 miles), Atlas (19 miles), Pan (17 miles), Daphnis (5 miles) and Epimetheus (70 miles) – nestled in and around Saturn’s rings. We see the unlit southern side of the rings in this picture, which was taken at a distance of 1.7 million miles. The spacecraft was closest to Epimetheus (ep-ee-MEE-thee-us) at the time.

Notice that both Pan and Daphnis are located inside dark gaps within the rings; these are zones where the gravitational interactions between the moons and the ring plane have cleared away most of the icy ring particles. They’re but two examples of how small moons, acting alone or in consort with other moons, have sculpted many of the planet’s rings. Although Saturn has 53 named moons, astronomers think there are dozens if not hundreds more tiny ones embedded in the ring plane too small to be resolved with Cassini’s camera.

The two pictures at top feature the moon Atlas; the two at bottom are of Pan. The lower right photo shows Pan crossed midway by Saturn's rings.

Small moons aren’t massive enough to crunch themselves into spheres through pressures created by their own self-gravity. The borderline between being spherical or irregular in shape is around 300-360 miles for a rocky body and somewhat smaller for icy objects.

Two of Saturn’s more curious moons shown in the first photo are Pan and Atlas, both of which are shaped like flying saucers. Scientists think that each started out as a fragment from an originally larger moon that was shattered by a long-ago collision with a comet or meteoroid. Over time, the remnants attracted icy material from the rings which accumulated along their equators to form a 360-degree shelf or ridge of ice. This scenario neatly explains the fact that the icy extensions lie exactly in the ring plane.

Learn more about Pan and Atlas in this short video with a distinctly French accent

Five disks of remant dust and gas, called protoplanetary disks, around newborn stars in the Orion Nebula photographed with the Hubble Space Telescope Credit: NASA/ESA

The process of these little moons gathering material around their equators is remarkably similar to what we believe happened during the birth of the solar system over 4.5 billion years ago. The Earth and planets, which lie in a fairly flat plane around the sun, are thought to have formed when bits of rocky and icy materials in a disk surrounding the early sun (analogous to Saturn’s rings) collided, stuck together and grew into the worlds we know today. Was Earth once a smaller, saucer-shaped body that eventually fattened up enough to absorb its ridges? Planets and moons wouldn’t be the only bodies siphoning dust and ice from their surroundings. Newborn stars are also cocooned inside disks of dust and gas from which they draw fresh material in a manner similar to Saturn’s Pan and Atlas. Studying these moons and how they interact with the rings may teach us lessons that apply to phenomena visible across the broader universe.

5 Responses

  1. J. Major

    Hm! Interesting article. So a football (US-version) shape may be common for proto-worlds of a certain size? Or more accurately, M&M-shaped? I suppose too small and they’d just be irreguar and lumpy, and any larger and they’d become spherical.

    What about Iapetus’ central ridge? The remains of an earlier saucer-shape?

    1. astrobob

      Good thought on Iapetus. It’s interesting that the ridge formed along the moon’s equator. I’ve heard a couple hypotheses speaking to its origin. Either it’s icy material oozed out of Iapetus that soon solidified, or it might have formed when Iapetus rotated much faster than it does now. Heat from radioactive decay of elements early on in the moon’s formation could have softened and liquified icy materials that then spread outward along the equator through rapid rotation. No definitive answer yet.

  2. Jim Schaff

    Hi Bob,

    Neat concept- the saucer shaped moons. It makes sense.

    I was out looking at Jupiter tonight. I think I spotted a tendril of south equatorial belt that was maybe around 30 degrees long. The seeing wasn’t the greatest but the area seemed sort of fuzzy at the leading edge with this tendril extending back in a direction opposite the rotation. The tendril seemed to be at the southern most boarder of the belt. It was on the meridian at 8:30. I will have to keep an eye on it.

    By the way, I was out viewing in short sleeves. Although I did wear long pants. Its getting chilly down here in Florida. 🙂

    take care,

    1. astrobob

      Hi Jim,
      You must have seen the SEB outbreak. Your timing put it right along the target longitude of 290 degrees. (CM = 262 at 8:30 your time. It would have looked like a tendril just as you described. Good going! Hah – long pants. Yeah, it’s cooling off in Duluth finally with the temp sitting at 8 above at the moment. Maybe I should put a hat on. Naw, it’s not below zero yet!

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