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:
[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
May 21st, 2010 at 9:21 am
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
May 21st, 2010 at 9:22 am
Such a gray image that picture. Not bad but you do have to look more closely to spot Epimetheus.
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?
May 21st, 2010 at 9:26 am
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”.
May 21st, 2010 at 9:27 am
[...] May 21, 2010 · Leave a Comment via blogs.discovermagazine.com [...]
May 21st, 2010 at 9:29 am
Messier Tidy Upper:
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.)
May 21st, 2010 at 9:31 am
It is suspended so beautifully. I am a sucker for rings too.
May 21st, 2010 at 9:40 am
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.
May 21st, 2010 at 10:04 am
Messier Tidy Upper:
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.
May 21st, 2010 at 10:35 am
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.
May 21st, 2010 at 11:03 am
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?
May 21st, 2010 at 11:18 am
@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.
May 21st, 2010 at 11:26 am
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.
May 21st, 2010 at 12:28 pm
I want to go there and see all of this with my own built-in optics, please.
May 21st, 2010 at 1:00 pm
I just looked up “tautology”.
I concur.
May 21st, 2010 at 1:49 pm
@ 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.
May 21st, 2010 at 2:07 pm
So what’s the upper limit on mountain height on Earth? At what point will the Himalayas start to squoosh out onto Asia?
May 21st, 2010 at 2:32 pm
Gravity. Oh, My!
Also, check this out, too: Time-Lapse Video of Space Shuttle Discoveryand Checking in on Saturn
May 21st, 2010 at 3:20 pm
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.
May 21st, 2010 at 5:20 pm
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
May 21st, 2010 at 6:51 pm
That is one of the coolest pictures evaaar. I love it!
May 21st, 2010 at 7:45 pm
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)
May 21st, 2010 at 8:08 pm
[...] Blogs / Bad Astronomy –> « What’s the matter? Never mind WANT Part IX: Levitating TARDIS edition » Don’t have gravity? Take your lumps. It might seem like a tautology — and that’s because it is — but sometimes the only word you can use to describe 17 Responses to “Don’t have gravity? Take your lumps.” 1. Ken B Says: May 21st, 2010 at 9:21 am 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 Says: May 21st, 2010 at 9:22 am 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 Read ahead [...]
May 21st, 2010 at 10:35 pm
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.
May 22nd, 2010 at 10:14 am
[...] Adrianus V | 22 mei 2010 om 18:13u. …1 Bron: Bad Astronomy. Noot:Van zo’n foto ben je toch sprakeloos? Daarom de info in deze voetnoot – woehaha [...]
May 22nd, 2010 at 11:16 am
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!
May 22nd, 2010 at 11:49 am
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.
May 22nd, 2010 at 1:56 pm
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??
May 22nd, 2010 at 11:02 pm
@ 8. HJ Hornbeck & 5. Ken B : Thanks.
May 24th, 2010 at 5:57 pm
[...] Don’t have gravity? Take your lumps. — Once again, BA shows his blogging pumpitude with a post on a recent stunning photograph taken from Saturn’s orbit by the Cassini probe: a vista of the Kronian moons Epimetheus and Rhea, shown at about the same distance from each other as from the Earth to Luna. He gives a good rundown on the importance of gravity due to a body’s mass on its shape using these two moons, one a sphere and the other a rocky lump, demonstrating rather nicely that when it comes to celestial bodies, size really is important… [...]
May 24th, 2010 at 8:07 pm
[...] Bad Astronomy. http://www.ciclops.org/view_media/31979/Big_and_Small_Before_RingsEpimetheus [...]
May 24th, 2010 at 10:01 pm
And as science tells us over and again, size matters.
= = =
That’s what she said?
May 26th, 2010 at 9:33 am
Autumn:
Himalayan–Tibetan uplift and global climate change