Midnight on a ringed world

By Phil Plait | August 24, 2010 8:05 pm

Check. This. Out. What an amazing shot from Cassini!

cassini_saturn_enceladus

Holy Haleakala! How gorgeous is that?

This picture really threw me for a minute. I couldn’t figure it out! Obviously, you’re seeing the night side of Saturn; the planet itself is an almost entirely dark disk. The bright curve you see at the edge is sunlight scattered by the upper atmosphere — the break in the curve near the top is from the shadow of the rings!

But you can also see the moon Enceladus, too: the plumes of water geysers off the southern pole (at the bottom) are obvious. When this shot was taken, Cassini had to look past Saturn to see Enceladus; that is, the moon was farther from Cassini than the planet was.

OK, cool enough, but the problem is Enceladus looks like it’s full, as if we’re seeing the side completely lit by the Sun. How can that be? If we’re seeing the dark part of Saturn, the Sun must be on the other side — in other words, we’re facing toward the Sun in this picture, and it’s blocked by Saturn itself. But if that’s true, there’s no way Enceladus can be fully lit. The Sun would have to be between the planet and the moon! Draw yourself a picture if that helps.

I was honestly baffled about this, until I read the caption for the picture:

Enceladus and its plumes have been brightened by a factor of two relative to the planet and rings.

Aha! The astronomers artificially brightened the part of the image with Enceladus to make it easier to see. That’s why it looks so bright. And that also explains why it looks full: it’s being illuminated by Saturn itself! If you were standing on Enceladus in the middle of the part we see here, it would be midnight, and you’d see a full Saturn directly overhead. It’s a big, bright planet, so it would illuminate the ground just like a full Moon lights up the ground here on Earth.

Phew! I thought I was losing it for a second. The geometry of the Cassini pictures can be pretty confusing sometimes, and I thought I was totally lost with this one. Nice to know my confusion was earned honestly.


Related posts:

A moon skating in its own ice
An otherwordly eclipse
Crescent planet, crescent moon
Enceladus is erupting!


CATEGORIZED UNDER: Astronomy, Pretty pictures
MORE ABOUT: Cassini, Enceladus, Saturn

Comments (38)

Links to this Post

  1. The dark side of Saturn « Textículos | August 25, 2010
  1. And if I’m not mistaken, BA, you can see a nice large plume coming off the bottom of Enceladus.

  2. Great picture. Glad they made it clear in the caption. I find that rare nowadays and I think any science-based graphic should explain whether it is an illustration, actual picture, and if the colors or aspect ratios have been altered. The computer generated stuff is getting so good you can’t tell what is real and what isn’t.

  3. I figured the Saturn-shine angle immediately, but didn’t guess that they’d jiggered with the exposure in PhotoShop! Tricksy Hobbitses!

  4. So that means Enceladus is being lit up by “Saturn Shine”. Cool!

  5. Jamey

    That’s kind of bogus, really. I wish they’d left it as the real planet shine level – diddling the image this way just gives the Moon Landing Deniers that much more ammo.

  6. 24601

    Saturn-light as seen from Enceladus would be comparable to the light from a full moon seen from Earth? It takes a little mind-bending, but I can kinda see how it might work out, since Saturn is so much further away from the sun. It does seem like Saturn-light should be brighter, though, since it is much bigger.

  7. Jack Mitcham

    I’m too lazy to do the math at the moment, but it seems to me that Saturn over Enceladus would be brighter than Luna over Earth.

  8. Messier Tidy Upper

    Holy Haleakala! How gorgeous is that?

    *VERY!*

    Very gorgeous indeed – even if it is artificial rather than true to life. :-)

  9. Jon

    Wait, was that a DJ Shadow reference?

  10. Ross Cunniff

    Jack Mitcham@7 – given the following data:

    Moon albedo: 0.12
    Saturn albedo: 0.47
    Moon angle (seen from Earth): 0.5 degrees
    Saturn angle (seen from Enceladus): 28 degrees
    Saturn dist from sun: 9.54 AU

    I get that Saturn as seen from Enceladus is about 140 times as bright as the full moon, or roughly 5.3 visual magnitudes:

    (0.47 / 0.12) * (28^2/0.5^2) * (1/9.54^2)

    The first ratio being the brightness difference; the second being the size difference (square degrees); and the third being the solar intensity difference.

    Corrections to my math are welcome :-)

  11. Click to deplutorise

  12. Jack Mitcham

    Alright, I can’t sleep without doing the math, hold on.

    Saturn is 10x further from the sun than the earth-moon system, so it receives 100x less solar radiation. Saturn’s albedo is 0.47 compared to 0.12 of the moon. The diameter of Saturn is approximately 35x greater than the moon. Enceladus is approximately 180,000 km from Saturn while the moon is approximately 380,000 km from earth.

    If I did my math correct, Enceladus gets approximately 2.8 times more light flux from a “full Saturn” than Earth gets from a full moon. Of course, our eyes don’t pick this up linearly, so we’d say it’s comparable.

    I’m not sure about my magnitude calculation, so maybe I can get some help here. If the full moon is magnitude -12.74, a full Saturn over Enceladus would be magnitude -13.86

    It’s 1am, now I sleep.

  13. Jack Mitcham

    Nevermind, I forgot to square the distance between the moons and their planets.

    6 times more flux

    -14.69 magnitude

    Please correct my math if I’m wrong. Good night.

  14. Messier Tidy Upper

    @ ^ Jack Mitcham : Maths is not my forte & its interesting to get this info calculated – much appreciated. Thanks. :-)

    Enceladus (along with Triton and Europa) is one of the most reflective, shiniest moons in our solar system isn’t it?

    Does his help or cast any light (ha!) on this? ;-)

  15. «bønez_brigade»

    WTF… When I clicky, it don’t embiggy. Methinks you linked to the thumbnail, Phil.

  16. «bønez_brigade»:
    If you click on the picture that opens this link
    http://www.ciclops.org/media/ir/2010/6231_15537_0.jpg

    and the picture is not increased. You can change the 0 to 1 and then obtain a larger picture.
    http://www.ciclops.org/media/ir/2010/6231_15537_1.jpg

    I’m so done :-)

  17. Messier Tidy Upper

    Off topic but here’s some awesome news that I hope gets a blog post of its own here soon :

    http://www.cosmicdiary.org/blogs/nasa/franck_marchis/?p=899

    Seven Exoplanets – most (5) probably Neptune type planets but one Saturn mass outer gas giant and one low mass (1.4 Earths minimum!) rocky SuperMercury* have been discovered around the sun-like star HD 10180 in Hydrus (better dubbed “The SevenWorld Star” or something!) using the HARPS spectrograph on the 3.6m telescope at ESO-La Silla. :-)

    I’ve emailed / facebooked messaged the BA about it but I’m not sure if he’s recieving my messages so I’ll note it here as well. I’m sure other folks are also letting him know ’bout this too but still ..

    ———

    *I think calling this world a SuperMercury (or for that matter SuperVenus) is a better, more likely accurate term than “SuperEarth” which it’ll no doubt get called anyhow.

    Although there’s also a certain appeal to terming it a Mustafar-class world with reference to Star Wars! (http://en.wikipedia.org/wiki/Mustafar) ;-)

  18. Pete Jackson

    Methinks you are right, too, bonez_brigade.

    To see the embiggened version, go to:

    http://photojournal.jpl.nasa.gov/jpeg/PIA12705.jpg

  19. Nigel Depledge

    Sweet.

    Like #2, I figured Saturnshine explained how we could see Enceladus so clearly, but I also didn’t reckon on it needing to be embrightened (but only by a factor of 2, so it would still have been visible in the original image).

    I also expected Saturn to be brighter as seen from Enceladus than Luna is as seen from Earth, partly because of the larger albedo and greater size, and partly because there is so much less daylight out there than there is here on Earth that the nighttime Saturn would seem very bright in comparison to the (still substantially brighter) sun in daytime (that’s day and night as experienced on Enceladus, obviously).

    So, even if Saturn delivered the same amount of light to Enceladus as Luna does to Earth, I think it would seem to be a lot brighter because of the lesser day / night contrast in light level.

  20. Grand Lunar

    Reminds me of the scene from ‘2010’, where we see the night side of Jupiter and the spacecraft Leonov aerobraking through the atmosphere.

    Of course, THIS picture looks way cooler!

  21. pumpkinpie24

    Phil, I love that with all the thousands upon thousands of brilliant images you’ve seen, you still can get blown away by a new one! The universe is a wondrous place.

  22. XPT

    I renew my availability to be sealed in a rocket and launched on a one-way mission to freaking go there!!!

  23. MattF

    Jamey @ 5: That’s kind of bogus, really. I wish they’d left it as the real planet shine level – diddling the image this way just gives the Moon Landing Deniers that much more ammo.

    I understand your point, but why on Earth should we deprive ourselves of awesomeness because of a self-lobotomized minority?

    We understand reality. We understand that the photo was doctored, and can appreciate it in light of that. It seems rather backward to insist that we should limit our ability to present wonderful things because a few people known to be lunatics will use it as an excuse to further their lunacy.

    There are many good ways to fight bad thinking. “Just shut up — They might hear you” isn’t one of them. If your intent is to deprive them of ammunition, keep in mind that mindsets like these only gain traction by spewing a warped interpretation of reality itself; the only way to completely remove their ability to spew garbage using these tactics is to remove them from reality.

    Which, come to think of it, might be a benefit. But I’d rather try to drive this sort of crackpottery to extinction or irrelevance by helping people as a whole become more equipped to recognize it for what it is, teaching them how to process arguments and evidence; trying to starve out individual lunacies without addressing the underlying problem only leaves room for other lunacies to spring up in their absence.

  24. Jamey (#5) and MattF (#24), re: “moon landing deniers”

    Just remember… First, they don’t need “ammo”, since they just make it up as they go, anyway. Second, nothing has been “added” or “removed” here — just the brightness of one part of the image was increased to make it easier to see. It’s not even like they stretched the contrast to the breaking point, like “the hoaxers” love to do to find “hidden” things in the pictures.

    Besides, I don’t think NASA had Photoshop in 1969. :-)

  25. Tom Ames

    There is not and never has been any such thing as “objective truth” in a photograph. There is ALWAYS a manipulation of some kind or another. (Even the “raw” data do not contain some kind of absolute reality.)

    The best you can do (as NASA has consistently done) is to be transparent about your processing steps and make the data and algorithms public. This way others can explore the effects of your processing.

    (This is actually an important point for ALL scientists–especially those at Duke University–to keep in mind.)

  26. MattF

    Ken B: First, they don’t need “ammo”, since they just make it up as they go, anyway.

    That was kind of my point. :)

    Ken B: Second, nothing has been “added” or “removed” here — just the brightness of one part of the image was increased to make it easier to see.

    That’s changing something, and for some, that counts as an addition. But I couldn’t care less, since the people who took the picture were transparent about the changes they made. That’s the difference between these people and the Moon hoaxers, who aren’t straightforward about the changes they make as they emphasize compression artifacts and the like in pictures. As Tom points out:

    Tom Ames: There is not and never has been any such thing as “objective truth” in a photograph. There is ALWAYS a manipulation of some kind or another. (Even the “raw” data do not contain some kind of absolute reality.)

    Absolutely. Well said.

  27. Jearley

    Smullin, in ‘Three Roads to Quantum Gravity’, points out that everything is process, so photos, in attempting to ‘stop time’ are completely artificial. Without getting all philosophical, consider the reality behind this image- everything was moving, and at different rates. Everything in the picture, and the spacecraft itself, were evolving, aging and changing in relationship to each other. Taken as a whole, that makes this even cooler!

  28. Hmmm, this photo must have been doctored by the same team that faked all those moon landing photos.

    (snicker)

  29. @ XPT:

    Sorry, so long as these primitive, backward savages insist on using these ridiculously low-Isp chemically-propelled rockets, the cost per pound to transport human flesh to the outer solar system is just too high. Even for a one-way trip.

  30. Folks,

    While you’re enjoying this fab image, just wanted to boast a little and say that we folks here in my research group planned this image for the purpose of its sheer glory. (I’m studying the Enceladus jets and so am responsible for planning the Enc jet images.) We are constantly on the lookout for the most dramatic, awe-inspiring, gob-smacking scenes, and this was one of them.

    Glad you all liked it so much. There will surely be more to come!!

    Best,
    Carolyn Porco

  31. Jeff

    Enceladus’s albedo is so high, it probably reflects Saturnshine pretty effectively. That probably made it very easy to pull off this trick.

  32. Nigel Depledge

    @ Carolyn Porco (31) –

    I hope I speak for everyone here when I say a great big Thank-You to you and your team. Not only is Cassini delivering awesome science, it is delivering awesome imagery too.

    Somehow, protein biochemistry briefly doesn’t seem quite so exciting when I see these images of distant worlds.

  33. Nigel Depledge

    @ Jeff (32) –

    This word that you use – “easy”. I do not think it means what you think it means.

  34. I’d’ve thought they wouldn’t need to make Enceladus brighter for the image. Is there a version where that wasn’t done?

  35. «bønez_brigade»

    Aleksey & Pete,
    Thanks for the image info. I located the embiggened version soon after but still wanted to file the obligatory complaint.

  36. Ross Cunniff

    Just for fun, I “unbrightened” Enceladus by 50% (since the original was brightened by 2x) and put the result on flickr:

    http://www.flickr.com/photos/12202840@N08/4931255252/

    Since this has been JPEG uncompressed and recompressed, it is certainly of no use as a scientific image. But I thought the folk here would be interested.

  37. Brent

    Ross Cunniff @10: You maths look correct (I hope). So if Saturn would appear as -18 magnitude from Enceladus. The Sun appears as -21.9 magnitude from Enceladus. That is only a 3.9 magnitude difference or about an 8000 brightness difference. Doesn’t seem like much. Might look into it deeper later.

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