Titanic slice

By Phil Plait | June 28, 2011 11:00 am

A quirk of physics can lead to some real drama. Two quirks of physics can lead to very dramatic pictures.

"Why Phil, what could you possibly mean?", I hear you thinking*.

This is what I mean: Saturn’s moon Titan, sliced in twain by the planet’s rings:

[Click to enchronosenate.]

Due to a quirk of physics (aha!) all the moons and rings of Saturn orbit the big planet in the same plane. There are two reasons for this: one is that they almost all formed out of the same disk of material orbiting the Sun. As the pieces clumped together, they naturally all stuck to the same plane. A second reason is that any object that tries to stray out of that plane (or that gets captured by Saturn like some of its bizarre outer moons) feels a torque on it, forcing it back down. That torque is provided by Saturn itself, which has tides that tend to circularize and flatten all the orbits of the moons, confining them to the planet’s equatorial plane. The physics of this is a little hairy, but I do have a simplified explanation using the Earth and Moon as an example.

The Cassini spacecraft orbits Saturn, but it has rockets on board that can push it around into any orbit the engineers and scientists back home want. That usually means a tilted path, plunging it through the equatorial plane twice each orbit. When it is precisely in that plane it sees the rings edge-on, and if the geometry is right it can spot one or more moons.

In this case, the gigantic moon Titan was in its sights, and the angle was such that the rings slice right across it.

Titan is so big it has a substantial atmosphere, murky and thick, which hides its surface. So why do we see details on the moon in this picture?

Due to a second quirk of physics (aha aha) infrared light can penetrate Titan’s soupy air. Different surface features (like lakes of liquid methane and ethane!) emit and reflect different amounts of IR light, and that goes up through the clouds and into Cassini’s detectors, which were cleverly designed to detect that specific wavelength of light.

And that, you see, is how some nifty physics can provide you with a very dramatic picture. Well, that and a few hundred million dollars, a team of very smart people, and a decade or two to design, build, and get your spacecraft to its target.

… which, again, involves an awful lot of physics. So really, if you want to see just how spectacular and awesome the universe is, science is the way to go. We deliver.

Image credit: Credit: NASA/JPL/Space Science Institute

* All professional bloggers can read minds. It’s how we know exactly what to say to tick off the largest fraction of our readers.

Related posts:

The stark beauty of Cassini’s Saturn
Two alien worlds, superposed
A titanic wink confirms otherworldly lakes
Titan’s shadow

CATEGORIZED UNDER: Astronomy, Pretty pictures, Science

Comments (36)

  1. Scott P.

    Those damned Martians are up to something again. I think this calls for Titan Force Five!

  2. VinceRN

    Saturn’s moons are definitely the coolest part pf the solar system to me. Though this rock we live on is pretty cool too.

    I surprised that the ring is so perfectly flat in that view. It almost looks artificial.

  3. Paul

    It looks like Titan is actually slightly out of the ring plane.

  4. Martin AA6E

    In the same plane? Why don’t the rings cut the moon in half then?

  5. Digital Atheist

    YAY! Phil, believe me when I say that I love all of the astronomical pictures you put up, but my favorites are always anything to do with the planets or their moons of our solar system. This holds true even when i break out my telescope. I like looking at nebulae and such, but I always wind up spending more time looking at our neighbors. I guess I’m just a frustrated amateur planetologist. 😀

    P.s. HiRISE, anaglyphs, OH MY! 😉

  6. Digital Atheist

    Actually it appears that the picture was taken slightly out of plane since you can see an obvious gap that grows larger the closer to the center of the picture. If the photo was taken from an angle (apparently) below the rings, then moving upward should also move the rings closer to the equator of Titan. Persepective is everything.

  7. any object that tries to stray out of that plane

    Hey! Don’t you have an article about why anthropomorphizing is bad?

  8. Paul

    Ah, I read the orbital inclination of Titan is 0.35 degrees. That explains why the ring plane doesn’t precisely bisect the moon.

  9. Keith Bowden

    Titan looks so wicked in this shot! Love it!

  10. Joel

    So are the surface features on Titan we can see there actually limited only to the tropics (the rest being covered in ice or something, maybe?), or all over, and it’s just a quirk of the quirk of physics that lets us see them in the first place?

  11. Schuyler

    Is it just me, or is there a bit of an illusion in the photo? Whenever I focus my eyes on the rings, it appears that the moon moves away from me slightly…

  12. Christopher Shoup

    I’m wondering what Joel is. Is the amount of Titan’s surface which is visible somehow related to the angle at which the camera is observing the moon?

  13. JJA

    Digital Atheist: If that black stripe were an open gap, one would expect to see a bit of Titan’s surface through it, not so? Perhaps it’s being blocked by the material forming the gray haze around the brighter sections of the ring.

    Schuyler:If you look at Titan, you see a predominantly bright area, and when you look at the rings, you see a predominantly black area. I think your iris is adjusting accordingly and this produces the illusion of a receding moon. (Or perhaps it’s a contrast illusion brought about by Titan’s blurry-looking circumference?)

    To me, the ring appears slightly bent upwards where it crosses Titan, an effect which I can confirm is an optical illusion by using a straightedge.

  14. Digital Atheist


    You may be right as regards haze blocking the view, or mit may be a darker section of ring material? my reasoning for this being a slightly below ring view is the fact that the gap/dark area mid-ring increases as viewed left to right. It would be nice to see the whole uncropped version to see if the gap closes on the right side though.

  15. Christopher Shoup

    @JJA and @Digital Atheist

    I think the gap you’re referring to is not a gap in ring material but the gap between the near and far side of the rings. I think the farther left is simply the curve of the ring and thetop part of the “gap” is the near or far part of the rings and the bottom is the opposite as the rings circle the planet.

    If this is the case then it wouldn’t explain the lack of visible bits of Titan through the gap though.

    If I were a conspiracy theorist and not simply a skeptic I’d say this was proof of a bad photoshop job and that there is no such thing as “Cassini” OR this “Saturn” nonsense.

  16. Digital Atheist


    PFFFT! Any true conspiracy theorist would tell you that not only is there no Titan or Saturn or Cassini, there is no way that any planet could ever have rings or be made mostly of gas/liquid/ice. Anything farther than the Moon is OBVIOUSLY a hoax perpetrated using holgraphic laser technology.

  17. Messier Tidy Upper

    Great image. :-)

    Wonder whether it would be even better in a colour version or not?

    @16. Christopher Shoup :

    @JJA and @Digital Atheist : I think the gap you’re referring to is not a gap in ring material but the gap between the near and far side of the rings.

    Hmmm .. I assumed that was the Cassini division myself, or maybe the Encke one but I could be mistaken, naturally.

  18. Digital Atheist


    I presumed the same thing actually.

  19. Christopher Shoup

    It could be the Cassini division, I just think it gets a touch too wide compared to the rest of the line.

  20. Jeffersonian

    Thanx for ‘splaining the physics.

  21. Nigel Depledge

    Christopher Shoup (13) said:

    I’m wondering what Joel is.

    At a wild guess, I’d say Homo sapiens.


  22. chris j.

    why is our moon still in the ecliptic, instead of orbiting over earth’s equator?

    i understand why the moon’s rotation is tidally locked with its revolution, and that eventually the earth and moon will be locked together (assuming it happens before the sun goes red giant). what i don’t understand is why, after more than 4 billion years, the moon’s orbit and the earth’s equatorial plane have not matched up.

    given how far the moon has moved away from the earth during that time, and assuming the impact theory is correct, shouldn’t the moon’s orbit have drifted out of the ecliptic by now?

    is it because the earth/moon mass ratio is so much lower than the corresponding ratios for the gas/ice giants and their moons?

  23. The photo reminds me of the old “Twilight Zone” TV series opening scene.

  24. DennyMo

    When I saw the title, I wondered why BA was writing an article on my golf game…

    Nigel, I’m glad I’m not the only one who chuckled at Christoper’s phraseology.

    BA, neat pic, thanks for posting this one. But I’m also “wondering what chris is”. Why isn’t our moon in an equatorial orbit?

  25. Aubri

    >Titanic slice

    o/~ Nearer, my God, to thee… o/~
    Too soon? :)

  26. Gary Ansorge

    25. DennyMo

    “The plane of the ecliptic (also known as the ecliptic plane) is the plane of the Earth’s orbit around the Sun”

    Whereas earths equatorial plane is inclined 23 degrees to the plane of the ecliptic. The Moon is not just in orbit of earth. It also orbits the sun(in the plane of the ecliptic).

    Gary 7

  27. Nigel Depledge

    @ Chris J (23) –
    Good question.

    It’s set me to thinking, and I don’t have an answer . . . yet.

    Maybe some knowledgeable other commenter will be able to give us the answer (and save me the bother of looking it up – hey, I’m a biochemist and celestial mechanics make my head hurt).

  28. Nigel Depledge

    @ Gary (27) –
    Er, I’m not sure I get it.

    Could you go into a bit more detail?

  29. Joel

    @22. Nigel Depledge –

    Good guess, but no 😛

  30. truthspeaker

    Why would you expect the moon’s orbit to be above the earth’s equator?

  31. truthspeaker

    And I hope #5 isn’t a serious question.

  32. chris j.

    truthspeaker @31, though i can’t claim to have a complete grasp of the physics or the terminology, my understanding is that as the moon revolves around the earth, the point on the earth that is directly below it oscillates between -23 deg and +23 deg latitude (the tropics of cancer and capricorn). since the earth is somewhat fatter at the equator than at the poles (due to its rotation), the moon’s drift between the tropics should create a torque that either pulls it into the earth’s rotational plane, or pulls the earth’s rotation into the moon’s orbital plane.

  33. Bob

    Very cool photo! But the orbits of two of Saturn’s moons are not the same plane as the rings. In my June almanac for the Westchester Astronomers*, I mentioned that it was hard to find charts for Iapetus, since its’ orbit is tilted from the plane of the other moons, but it’s easier to see Saturn’s rings from Iapetus, since its orbit takes it above and below the ring plane. At my blog site, today I posted some simulated photos produced from JPL’s Solar System Simulator site to show how the view of the rings is “better” from Iapetus.

    Blog site: http://bkellysky.wordpress.com

    Look for the NEWSLETTER section – click on June 2011 for the PDF.

  34. Messier Tidy Upper

    @ ^ Bob : Cheers – good animation & explanation there. :-)

    @3. VinceRN : “Saturn’s moons are definitely the coolest part of the solar system to me. Though this rock we live on is pretty cool too.”

    Nah, Pluto – make that Eris, no make that *Sedna* – has to be the coolest place in our solar system! 😉

    NB. Apparently one of the Lunar polar craters has the record for coldest place currently known in our solar system – but I’m betting that we’ll find Pluto or another of the ice dwarfs or a cometary nucleus has colder spots!

    @8. Ken B :

    “any object that tries to stray out of that plane”
    Hey! Don’t you have an article about why anthropomorphizing is bad?

    Well animals (eg. pet dogs & cats & cows) tend to stray as much or more than humans do – so I’m not sure “straying” quite equals anthropomorphising!

    PS. About the whole “golf clap” thing (no, I don’t get it either) – could it have something to do with the ‘slice’ word maybe? 😉


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