Saturn's shadow slices the rings

By Phil Plait | August 29, 2012 6:30 am

There is a whole lot of awesome in a picture of Saturn and its rings just released from the Cassini spacecraft. Check this out:

Mmmmm, ringalicious.

Cassini was about 2 million kilometers (1.2 million miles) from Saturn when it took this picture, so we’re seeing a decently wide-angle view. At the time, the spacecraft was below the plane of the rings, looking north (up, if you like). The Sun is off mostly to the left and up a bit.

The first cool thing is obviously the shadow of the planet itself cast on the rings. It cuts across like a black scythe! As I looked at the picture my eyes and brain kept trying to fill in the missing arc of rings, which was amplified by a slight afterimage as my eyes moved around. It’s a difficult illusion to ignore.

Second, I love how you can see all the different rings in the picture, including the thin, lumpy F-ring outside the main band. The big gap is called the Cassini Division; it’s not really an empty space since there are many faint thin rings inside it. They’re just hard to see here. The Cassini Division is fairly easy to spot even through a small telescope, looking from Earth like someone took a knife to the rings and sliced them.

Third, you can see the tiny moons Janus (below the rings on the left) and Epimetheus (above the rings on the left) as well. I wonder how hard it is to get a picture like this without seeing any moons in it? Saturn has quite the fleet of them.

Fourth, look to the left, just where the inner arc of the rings cuts across Saturn. You can see the planet right through the rings! The rings aren’t solid; they’re composed of gazillions of particles of nearly pure water ice. There are spaces between the particles, so we can partially see through them, like looking through a screened window.

Fifth, and perhaps most cool of all: the part of Saturn we’re seeing here is the night side, entirely unlit by the Sun. The bottom (southern) part of Saturn is only noticeable by its absence! But what’s that glow in the north?

That, my friends, is ringshine! Although this part of Saturn is in nighttime, the Sun is still shining on the rings (wherever you don’t see Saturn’s shadow across them). The ring particles are very bright and shiny. They reflect the sunlight, which then illuminates the northern hemisphere of Saturn. The southern half is still dark because the ice particles tend to reflect light back up, like a mirror. Since the Sun is coming from the north, that’s the way the light gets reflected. I’ll note that most of the light gets reflected away from Saturn, to the upper right in this picture, but enough is reflected back to make the cloud tops glow softly.

This happens on Earth too, when sunlight reflects off the Earth and illuminates the dark part of the Moon. This is called Earthshine, also poetically called "the old Moon in the new Moon’s arms." It’s quite lovely.

And it’s science! Which is lovely, too.

Image credit: NASA/JPL-Caltech/Space Science Institute


Related Posts:

- Lightning strikes in a storm bigger than worlds
- Saturn eclipses an icy moon
- An unusual view of the Death Star moon
- Saturn, raw (this one’s a jaw-dropper)

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (15)

  1. This is to weird. Here I am eating my lunch and watching “How the Universe Works” on Discovery. It´s about Moons.
    There they are talking away how the rings fromed. And meanwhile I think let´s have a look if there is something new on BA.
    And what is new? An article about Saturn.

    Anyhow, good timing, because I was just wondering if the rocks closer to Saturn are bigger/smaller than the outer ones.

  2. Chris

    It’d be cool if we could blow a massive hole in the rings of Saturn. I wonder if it would be possible and I wonder what type of patterns we could create? What if?

  3. Chris

    At the time, the spacecraft was below the plane of the rings, looking north (up, if you like)…

    Actually a little fun fact. The magnetic field on Saturn and Jupiter is the opposite of the Earth. So if you were on Saturn with a compass pointing north, you’d actually be heading south!
    http://en.wikipedia.org/wiki/Magnetosphere_of_Saturn

  4. So they are not outer-planets, but aussie-planets.
    Thanks for the info.

  5. “Ringshine”. Ok, that has really piqued my “how I wish Earth had rings” feelings. =^_^=

  6. kahlisana

    Looks like a Cylon Raider to me…

  7. sheldonc

    What a fantastic image.

    Saturn’s rings are so cool! The fact that they have waves and dark “spokes” and there’s even little moonlets embedded in the A and B rings just makes them more and more mysterious and amazing.

    One of my favorite images is Saturn eclipsing the sun as seen from Cassini.

    http://apod.nasa.gov/apod/ap090111.html

  8. @sheldonc
    That pic is a stunner, specially because we are photobombing it.

  9. Pete Jackson

    As I tend to do with pics like these, I have desaturated the blackness:

    http://chinesemeridiansblueprintsoflife.com/Images/Saturn.jpg

    This is now really cool. The part of Saturn’s disk below the rings can now be seen by light that has scattered through the rings and then scattered again off the disk of Saturn. Note that the equator of Saturn is the darkest, because the rings cannot be seen either by reflected light or scattered light there. And the shadow of Saturn on the rings is still completely black. In theory one should see some light there that has been reflected off the rings to the northern part of Saturn’s disk and then reflected back to the rings, and then scattered through the rings but the exposure time on the camera was presumably not long enough to see that deep.

    I’m definitely going to make a poster of this image. Thanks for posting it, Phil!

  10. Clint L

    I have a question.
    So we solve enough of the problems with space travel and head out to Saturn. The rings are made in part of water ice, which we need to survive out there.
    With our ship we dive into the rings and scoop up a few hundred tonnes of ice cubes. This would remove several kilometres of “ring”.
    Will the remaining ring material spread out to fill the gap or would there forever be a gap?

  11. It will fill up again.

  12. Clint L
  13. @kahlisana

    At first I thought “tie interceptor”, but a raider fits the bill too!

    Imagine if we had ring shine here on earth, some parts of the planet would not need street lights!

  14. Matt B.

    Sixth, even though the cut of the shadow seems straight, you can discern a slight curve to it, due to Saturn being a sphere.

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