Don't have gravity? Take your lumps.

By Phil Plait | May 21, 2010 9:12 am

It might seem like a tautology — and that’s because it is — but sometimes the only word you can use to describe an image from the Cassini Saturn probe is otherwordly:

cassini_rhea_epimetheus

[Click to engasgiantize.]

This otherworldy picture was taken on March 24, 2010. The big moon is Rhea, seen from 1.2 million kilometers (750,000 miles) away, and the little one below it is Epimetheus, from 1.6 million km (990,000 miles) away. Perspective makes them look right next to each other, but in reality the distance between them is the same as the Moon from the Earth! Saturn and its rings provide the backdrop for this stunning alien portrait.

To me, the most striking thing about this picture is the difference between the two moons. Rhea is a ball, a sphere, while Epimetheus is clearly a lumpy rock. Rhea is also clearly a lot bigger, even accounting for perspective in the picture; it’s about 1520 km (940 miles) across, while Epimetheus is 144 x 108 x 98 km (86 x 64 x 58 miles) in size.

Why is Rhea round, and Epimetheus lumpy? Gravity. Rhea, being so much bigger, has a lot more mass, so its gravity is much stronger. Objects bigger than a few hundred kilometers across have enough mass that self-gravity becomes important in shaping them. A rock you might see lying on the ground is small and has very little gravity, so the important things that shape it are its chemistry, the crystal structure inside it, and its history (getting banged by another rock, erosion, and so on).

But as the mass increases, so does the influence of gravity. Eventually, gravity wins: it doesn’t matter what the composition is (metal, ice, rock) or the history (getting knocked around), because gravity is strong enough to shape the object into a sphere. Sure, other forces can be at play (for example, rotation can flatten an object out a bit), but gravity is the one with the biggest influence.

Gravity is an inward force, trying to draw everything into the center of the mass. That’s why big objects are spheres; anything large enough to stick up very far gets pulled down. Look at mountains on Earth: they can only get to a certain size before slumping. They can’t support their own weight! Olympus Mons on Mars is much bigger than any mountain could ever be on Earth, because Mars has less gravity.

So this is more than just a beautiful picture from Cassini; it’s an object lesson in gravity. And as science tells us over and again, size matters.

Credit: NASA/JPL/Space Science Institute


Related posts:

Rhea:
Happy Valentines Day. Love Rhea
A marvelous night for a (Saturn) moon dance
Peek-a-moon
Epimetheus:
The real Pandora, and two mooning brothers
Cassini eavesdrops on orbit-swapping moons

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Comments (32)

  1. And as science tells us over and again, size matters.

    It’s a good thing that we’re all mature adults here. Otherwise, someone would probably post a juvenile joke about that statement.

    Wait… Let me give it a shot…

    I bet that you’v(*&^Jhg*^%.++—)(&hx../
    NO CARRIER
    :-)

  2. Messier Tidy Upper

    Such a gray image that picture. Not bad but you do have to look more closely to spot Epimetheus.

    Perspective makes them look right next to each other, but in reality the distance between them is the same as the Moon from the Earth! Saturn and its rings provide the backdrop for this stunning alien portrait.

    I never would have guessed the two moons were *that* far apart – it looks almost like Epimetheus is orbiting Rhea which is orbiting Saturn! ;-)

    Plus are those Saturns rings we’re seeing there or the *shadow* of Saturn’s rings cast upon the planets bulk?

  3. Rhea is a ball, a sphere, while Epimetheus is clearly a lumpy rock.

    Actually, in this image, I don’t see Epimetheus as “clearly a lumpy rock”, as it’s too small to make out much detail.

    On a side note, Firefox tags “Epimetheus” as a spelling error, and suggests “Epimethius”.

  4. Messier Tidy Upper:

    Plus are those Saturns rings we’re seeing there or the *shadow* of Saturn’s rings cast upon the planets bulk?

    Good question. My layman’s guess would be that Rhea is in front of the rings themselves, while the part of the image below the moons are the shadows. (Notice the different angles.)

  5. It is suspended so beautifully. I am a sucker for rings too.

  6. Click the image to see a bigger version. Epimetheus is easier to see. And yeah, my spellchecker was unhappy as well, but I use the correct spelling here for the moon.

  7. Messier Tidy Upper:

    Plus are those Saturns rings we’re seeing there or the *shadow* of Saturn’s rings cast upon the planets bulk?

    I fired up Celestia, and the answer appears to be… both! The topmost, horizontal band is the rings seen edge-on, while the lower, angled bands are the shadow of those rings on Saturn.

  8. I love these perspectives. The scale between the moons, the rings and Saturn is just mind-blowing! I had made a kinda-sorta color-calibrated version of this scene (taken from a slightly different angle) here: http://twitpic.com/1c3j8y.

    Nothing wrong with being a little lumpy either. Now I can just blame physics. ;)

  9. I always assumed the f/stop on the Cassini imaging camera was essentially infinity–that everything in frame would be equally in focus., But that doesn’t appear to be the case here. The backdrop of the rings seems to be slightly out of focus, the effect you get in photography when you use a low f/stop to photograph a person: person is in focus, background is blurry. Anybody got any idea about this? Do you see what I see?

  10. @Gogblog – I think it’s just that the rings themselves aren’t sharp edged objects. When you get close to them they get fuzzy since they’re made up of trillions of smaller objects. So maybe it’s a clever illusion that makes them look out of focus. Just an educated guess. :)

  11. HvP

    Gogblog,

    It has been suggested that what we are seeing may actually be the shadow of the rings cast across the cloud tops of Saturn. If that is so, then the bands would be intrinsically fuzzy looking.

  12. AliasUndercover

    I want to go there and see all of this with my own built-in optics, please.

  13. I just looked up “tautology”.

    I concur.

  14. @ gogblog:

    I suspect it also has to do with the exposure time. The spacecraft was no doubt aimed at the moons and tracking their movement, while the planet and rings in the background were happily spinning by. Of course, I’m not sure if a few seconds difference at that huge distance would make a distance, and that might be where my theory collapses into a decidedly non-spherical pile.

  15. Autumn

    So what’s the upper limit on mountain height on Earth? At what point will the Himalayas start to squoosh out onto Asia?

  16. gogblog

    RE: blurry rings and sharp moons
    ALL very educated guesses, thanks! But this is gonna bug me. I should email the imaging team with my handy “nasa.gov” email domain. They may just answer…. will report back.

  17. Jefferson

    Phil,
    I just have to say I love your variations on “en-whatever-ize”. lol I bet your spell check doesn’t like those either.
    – J

  18. That is one of the coolest pictures evaaar. I love it!

  19. David

    http://www.boston.com/bigpicture/2010/05/checking_in_on_saturn.html

    While we humans carry on with our daily lives down here on Earth, perhaps stuck in traffic or reading blogs, or just enjoying a Springtime stroll, a school-bus-sized spacecraft called Cassini continues to gather data and images for us – 1.4 billion kilometers (870 million miles) away. Over the past months, NASA’s Cassini spacecraft has made several close flybys of Saturn’s moons, caught the Sun’s reflection glinting off a lake on Titan, and has brought us even more tantalizing images of ongoing cryovolcanism on Enceladus. Collected here are a handful of recent images from the Saturnian system. (30 photos total)

  20. lookylou

    Not a great image to point out the role of gravity in making things round(though it is a GREAT image, thanks). even at full resolultion, Epimetheus is not all THAT lumpy. of course, small things don’t HAVE to be non-round, it’s just that most likely they will be. For a look at a large object that is decidely NOT round, check out models of Haumea (2003 El61). With a rotation period of about 4 hours, it’s shape is similar to that of a rugby ball.

  21. As others have pointed out, boston.com has a great edition of “The Big Picture”, dedicated to the Cassini mission. I was completely blown away by this one: http://www.boston.com/bigpicture/2010/05/checking_in_on_saturn.html#photo28 , which shows the gravitational effect of Prometheus and Daphnis on Saturn’s A and F rings. That is so amazing!

  22. Grand Lunar

    Pah! Gravity is overrated! Were it not for gravity, we’d be on the moon now!
    Down with gravity!

    And thus ends my dry attempt at astronomical humor for today.

  23. Grumpy

    Hard to tell without context that we’re looking down at Saturn. Given our common experience of seeing the Moon from below, I assumed there was empty sky in the background of this image. Except it’s not empty, it’s cloudy. How could there be a cloudy sky behind the moons??

  24. Messier Tidy Upper

    @ 8. HJ Hornbeck & 5. Ken B : Thanks. :-)

  25. WJM

    And as science tells us over and again, size matters.

    = = =

    That’s what she said?

  26. Autumn:

    There is unequivocal evidence to show that the Indian and Asian continental plates collided between 54 and 49 Ma (million years) ago. Since then, India has continued to move northwards into Asia at a rate of between 40 and 50 mm a year−1. This has resulted in folding and thrusting of the Indian crust in the Himalayas and southern Tibet. The homogeneous deformation of Tibet entailed shortening and doubling the thickness of the crust, elevating the Himalayas and Tibetan plateau to an average height of between 5000 to 5500 m above sea level. The tectonic forces resulting from the northward movement of India were not, however, in the opinion of some geologists great enough to support the thickened crust, and the plateau began to spread under its own weight. Currently, the plateau is spreading, extending east–west on normal faults, by about 1 per cent of its extent every million years.

    Himalayan–Tibetan uplift and global climate change

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