A billion km distant ice mountain against the black

By Phil Plait | July 14, 2010 3:03 pm

Carolyn Porco just tweeted about a beautiful image from Cassini, showing the icy moon Tethys hanging in space:

cassini_tethys_crater_

How forbidding and lovely!

Tethys is big, about 1100 km (660 miles) across (about 1/3 the diameter as our own Moon). Its density is actually a bit less than that of water, so it’s most likely predominantly composed of water ice. The surface is bombarded with craters, including the big one at the bottom called Melanthius. It’s 250 km across (150 miles!) and sports a massive central peak, common in larger craters. The crater itself is from a gigantic impact on the moon, and the central mountain forms when material is first displaced by the impact, then flows back. Under those titanic stresses, solid material can actually flow as the impact shock wave passes through, so these peaks are seen on lots of big objects in the solar system.

Cassini was 670,000 km (415,000 miles) from Tethys when it took this shot, which is nearly twice the Earth-Moon distance. The Sun is shining on Tethys from the left (the angle between the Sun, Tethys, and Cassini was about 41°, for those keeping track at home). You can see just how beaten Tethys is by looking at the terminator, the day/night dividing line, where shadows highlight the cratering. Even so, sandblasting by particles in Saturn’s rings has smoothed the moon’s surface, making it highly reflective — it’s shiny!

If there ever comes a day when I tire of seeing pictures like this, write my obituary. I’ll be done.

Image credit: NASA/JPL/Space Science Institute

CATEGORIZED UNDER: Astronomy, Pretty pictures
MORE ABOUT: Cassini, Melanthius, Tethys

Comments (40)

  1. Chris

    Its belly button is an outie!

  2. Jens

    I´m done.

    Seeing the well known rocks of our Solar System at ever more varieng angles and backgrounds is just getting old.

    Sorry, but I want us to actually Go There. And beyond.

  3. Craig

    That’s no moon.

  4. Can a surface be bombarded with craters?

  5. Jens

    It is. Yet another rock, possibly with a bunch of ice. And no amount of exclamation points will that anymore exciting than that. Until we go there.

  6. Russell

    @Craig #3

    Your right “That’s no moon” … It’s a navel orange!

    How exciting it would be to see images from a space craft in orbit of this!!!
    Lets go! I packed already!

  7. RCSI

    Though I wonder, what other popular 1980’s icon will appear next in a press release about Tethys.

    Still, it really is an awesome shot.

  8. Michael Suttkus, II

    Wow… that’s…

    I remember growing up in the seventies, seeing the first Viking pictures, the first Voyager shots…I can’t believe this has become old hat to anyone.

    Come on, people. We built an eye out of metal, put it on top of one of the largest explosions in human history, and threw it out into space so precisely it went exactly where we wanted it. Go there? WE ARE THERE! This is a dream.

    Is that what growing up is like? I’m still stunned by tiny chunks of rock floating in space, but then I’m still amazed by every blade of grass. You can’t get tired of this stuff. This is everything.

  9. Grimbold

    Am I the only one who gets the Imperial Stormtrooper March playing through his head when he sees pictures of moons that look like this (Mimas particularly)?

    DUN DUN DUN … DUN daDUN… DUN daDUN!!

  10. Josie

    Jens…Craig was making a Star Wars reference based on the resemblance of Tethys to the Death Star.

    The quote is a pretty famous one for those of us who still get off on this kind of stuff.

  11. Chet Twarog

    Hey, what about moving it into Earth LEO or GEO–all that water ice!
    Or, impact Mars–re-flood it after 3 by!

  12. How often do you think Carolyn Porco changes her wallpaper?

  13. John Page

    Hi. Off topic question. It’s eleven pm eastern time in Maine and I just saw a shooting star moving wnw across the sky. It was incredibly bright with a thick white plume trailing it. Is there any meteor showers expected in my area or was it just a random one?

  14. um

    it looks like a boob :( sorry for being like that everybody.

  15. rabidmob

    @Jens: I’d rather not go there to be honest. I’d rather send a robot there, does not look like a fun place to go.

    @Chet: Do you want to be responsible for a the gravitational changes cause by moving a planetoid around the solar system?

  16. KSK

    Great, Saturn has *two* Death Stars. We are so hosed…

  17. Joe

    Hm, thought this was Mimas at first because of the big Death Star-esque crater… Nice picture!

  18. Messier Tidy Upper

    Great image – Cassini never fails to deliver stunning images & neither does Carolyn Porco and I, for one, just love that! To them I say a big “thankyou.” :-)

    @6. Russell Says:

    Your right “That’s no moon” … It’s a navel orange!

    A gargantuan scoop of vanilla ice cream. :-)
    That’s what it looks like to me.

    @17. KSK Says:

    Great, Saturn has *two* Death Stars. We are so hosed…

    Yes, but they’re both frozen solid and covered over with a layer of ice so I presume what means they can’t fire or they’d blow themselves up! ;-)

    Actually this means there’s a whole lot of hyper-advanced tech just sitting up there orbiting Saturn – from another distant galaxy far away at that!Sounds like a great excuse for a human mission to the Saturnean system pronto! ;-)

  19. Messier Tidy Upper

    @ 14. John Page Says:

    Hi. Off topic question. It’s eleven pm eastern time in Maine and I just saw a shooting star moving wnw across the sky. It was incredibly bright with a thick white plume trailing it. Is there any meteor showers expected in my area or was it just a random one?

    I’d recomend checking out the Meteor showers online website here :

    http://meteorshowersonline.com/index.html

    its got a lot of good info and resources that may be handy for you in answering that.

    A quick glance through my 2010 Sky At Night astronomy calendar that lists the main meteor showers for the northern hemisphere+ reveals no likely candidate ones – the closest being the Southern Delta Aquarids which peak on July 28th. So it is most likely a sporadic meteor but I could be wrong.

    You’re lucky – and observant. There is certainly something awe-inspiring, magnificently beautiful and so incredibly satisfying about seeing a bright “shooting star.” They are nature’s firework’s (along with lightning) set off at random and surprising us with spectacle. I love watching them myself. :-)

    ************************************

    + I’m a Southern Hemispherean myself but I do get the english magazines & esp. Patrick Moore’s one! ;-)

  20. kevbo

    I love the smaller, almost duplicate crater just north of the main gun…

  21. John Page

    Thanks Messier. It was a gorgeous sight.

  22. ouu, what a cute planet :p

  23. Nigel Depledge

    Ooooh . . . Shiny!

  24. Andrew

    Predominantly composed of water? Like Europa, but colder, so the surface is not completely smooth? Notwithstanding, the surface looks less cratered than you might expect – compare the Moon, or Mercury. Has anyone estimated how long ago Melanthius might have been created?

    Melanthius is about 245 km across, roughly a quarter of the diameter of Tethys. Tethys has an other large crater, Odysseus (not visible in this image), which is getting on for twice as large – 445 km across.

  25. Mapnut

    Disappointed, I thought Phil was going to give us an estimate of the height of that central peak. I’ll have to try it myself. Tethys scales at 1100km = 102 mm on my screen. The mountain scales at least 2 mm high (big margin of error here), which works out to 22 km. Olympus Mons on Mars is about 27 km high, so I think this peak needs to be measured to see if O.M. retains its title. And it needs a name.

  26. Messier Tidy Upper

    @ ^ Mapnut : Thanks for that. :-)

    As for a name – Melanthius Mons?

  27. I don’t care if people here are jaded. I find it amazing. I want to go to “beyond” too but I’m still just amazed that we even get pictures like this. If you want to go there, then go there. I find sophisticated cynicism boorish.

  28. T_U_T

    looks like an egg to me. An gargantuan egg. Wonder what is inside, and what hatches from it when the time comes

  29. mike burkhart

    Anyone for mountian cilming? should be easy if this moons gravity is low .

  30. Messier Tidy Upper

    @ ^ mike burkhart :

    Yes the gravity certainly would be low! ;-)

    As the BA noted in the Opening Post here :

    Tethys is big, about 1100 km (660 miles) across (about 1/3 the diameter as our own Moon). Its density is actually a bit less than that of water, so it’s most likely predominantly composed of water ice.

    A quick look at a couple of relative sizes posters (from Scientific American magazine years ago) shows that Tethys is smaller than Pluto and even the Ouranosian moon Ariel. Among Saturn’s major moons Tethys is smaller than Titan, Rhea and Iapetus about the same size as Dione and looks about twice as big as Enceladus and the slightly smaller again Mimas – which is the original “death star” moon! ;-)

    Mountain climbing on the central peak of Melanthius crater? I’d love to see – or better yet *do* that! ;-)

    @ 28. Nerdista : I agree! :-)

  31. rabidmob

    @Nerdista: I’m not jaded, I’m more of a realist. It’s much more realistic to send a robot than a human for these kinds of missions. Unless we’re trying to build a colony of some sort, I feel that sending humans to the location is daunting and inefficient.

  32. David

    All we need to do now is to make a giant glass of scotch for this ice sphere!

  33. Donovan

    It looks so cool. Hahaha Cool, get it?



    Fine, be that way, don’t laugh.

  34. Aaron F.

    “[Tethys’s] density is actually a bit less than that of water…”

    How do we know that?

  35. Aaron,

    I think we can calculate its mass, first, from how it orbits Saturn. Knowing its mass and then measuring its size yields its density.

  36. Alabama_Al

    26. Mapnut Says:
    July 15th, 2010 at 7:51 am
    Disappointed, I thought Phil was going to give us an estimate of the height of that central peak. I’ll have to try it myself. Tethys scales at 1100km = 102 mm on my screen. The mountain scales at least 2 mm high (big margin of error here), which works out to 22 km. Olympus Mons on Mars is about 27 km high, so I think this peak needs to be measured to see if O.M. retains its title. And it needs a name.

    ————————————–

    I googled another photo of Tethys which showed the moon from almost directly above the Melanthius crater. The first photo shows Mount Melanthius (for want of another name for the impact peak in the center of the crater) edge on; the second photo reveals Mount Melanthius to be a ridge approximately 18 miles in length. Assuming my scale estimate is correct, the Melanthius peak appears in the second photo to be casting a shadow about four miles in length. Estimating that the Sun as seen from the Melanthius crater floor was appearing 20 degrees above the Tethys horizon, we can use simple Trig to calculate that Mount Melanthius rises around 7,000 to 7,400 feet above the crater’s floor. Standing at its base, Mount Melanthius would be an impressive sight.

  37. Messier Tidy Upper

    @ ^ Alabama_Al : Thanks for calculating that – an impressive mountain indeed. Wonder how it does compare with Olympus Mons more precisely and other volcanoes on the Martian Tharsis bulge and Elysium plateau plus on the weird fractured Ouranosian moon Miranda too? :-)

  38. roy

    the crater is close in size to the chixulub impact site on the yucatan. the hit must of raised the mother of all snow falls.

  39. Truly awesome photo, but about that on/off switch at the bottom…

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