A moody moon turns its face

By Phil Plait | June 20, 2011 1:00 pm

Just two days ago, the Cassini spacecraft flew by Saturn’s tiny moon Helene at a distance of only 7000 km (4300 miles). As it swept past, Cassini’s first view was of the dark, unilluminated portion of the moon, getting this lovely, moody shot:

[Click to embiggen.]

Helene is tiny, only 36 x 32 x 30 km (22 x 19 x 18 miles) in size. That’s far too small to shape itself into a sphere by gravity, so it’s lumpy and irregular. You can see a large flattened region on the left lit at low angle by the Sun, and a depression at the bottom which appears to be lit by reflected light from Saturn itself.

One thing I noticed after the beauty of this picture gave way to scientific curiosity is that there aren’t any smaller craters in that lit area. Most of Saturn’s moons are covered in craters, so what’s going on? The mystery deepens a bit in the next picture, captured by Cassini as it moved on, the viewing angle changed, and it saw the sunlit part of the moon more fully:

Again, the lack of smaller craters is pretty obvious on the moon. At least, on this part of it. Helene orbits Saturn well beyond the rings and spins once every orbit, keeping the same face toward Saturn. The side of the moon facing Saturn has lots of small craters, while the side facing away does not.

There could be many reasons for this; perhaps small chunks of ice from the direction of Saturn get launched outward by gravitational interactions from the inner moons, making small craters on the Saturn-facing side of Helene (that seems unlikely to me; that’s not seen on other outer moons). Maybe there’s dust that falls in toward Saturn that sand-blasts the outward-facing half (again, why don’t we see this on the other moons?). Maybe it’s something else entirely… Helene’s sister moon Dione also shows such a two-faced personality, and there’s evidence a big impact may have spun Dione around 180°! It could be that’s what happened to Helene as well, with the cratered side facing Saturn and the smooth side facing away. I’m guessing here, but hopefully images like these will give the real experts some ideas on how this happened.

Emily Lakdawalla at the Planetary Society Blog has an atlas of Helene created using images from a previous Cassini encounter, and a more thorough discussion of this weird little moon and this crater dichotomy.

I’ve said it before, and as more data come in I’ll say it again and again: Saturn’s moons are damn weird. We sent Cassini to investigate these moons, as well as Saturn itself, its atmosphere, rings, and space environment. I wish we had a thousand such spacecraft orbiting Saturn… but, as you can see here, Helene has a face — two faces, in fact — that make it worth launching at least one ship.

Tip o’ the nose cone to Cassini Imaging Team Leader Carolyn Porco.

Related posts:

Helene of Saturnian Troy
Just in case you need reminding how nice a place to live Earth is
The stark beauty of Cassini’s Saturn
Side view of a Death Star moon

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

Comments (22)

Links to this Post

  1. Las Dos Caras de Helena. | Pablo Della Paolera | June 20, 2011
  2. TIQY » Lastest Astronomy News | June 24, 2011
  1. Helene

    Obviously, I am absolutely loving all of these articles. :) I had no idea I had a namesake, and I’M SO MYSTERIOUS! 😀

  2. Pete Jackson

    Could it just be a big dust pile so that impacts just throw dust off into space in all directions, rather than forming crater walls?

  3. Justin

    It actually looks like there ARE craters but they are filled in with dirt. Especially at the top just to the left.

  4. Keith Bowden

    I was playing ball with my 7 year old nephew yesterday and I started telling him about gravity and how the earth orbits the sun, the moon orbits the earth (and how he’d only weigh about 7.5 pounds on the moon). Then we got into earth’s rotation and the phases of the moon. It was a fun hot day! (Who said school is out?) He is, naturally, curious about everything and responds best when it’s a conversation or even “Hey, look at this cool thing!” (if he suspects you’re trying to teach him something, watch out!) He had all kinds of questions about how hot the sun is and if you could stand on the sun, and if you can actually fly on the moon (he’s seen the big jumps that astronauts took).

    Doing my part!

  5. berndb

    I would agree with Peter in that impact debris would behave quite differently on Helene compared to bigger worlds with greater gravity. It might form a cloud around the whole moon, then gradually settle back onto it, creating the “smooth finish” by even distribution.

  6. Mejilan

    Gorgeous shots. Loving the shadow-play at work. There’s an awesome wind-swept texture to the images that I find very appealing.

  7. Cameron

    Keith #4: I remember my daughter asking about the pahses of the moon. I ended up taking her into a dark cupboard with a basketball, tennis ball and a flash light. The amazing thing was that she seemed to understand what was going on, and we managed to cover seasons while we were at it!

  8. VinceRN

    Amazing stuff out there. The more we learn, the more we find that we don’t know yet. The universe is pretty darned cool.

  9. Loooks almost like an asteroid caught within the gravitational pull of saturn..

  10. Arnd

    For me the second picture looks a lot like a skull. You can see the eyes and the nose. Pretty scary place :).

  11. andy

    Well it looks to my untrained eye that there are a lot of landslides all over the satellite. Perhaps a result of the vibrations from impacts shaking the moon? What’s the density: consistent with being a rubble pile?

    Incidentally Helene is co-orbital with Dione, it is located near the leading Trojan point (L4). The even smaller moon Polydeuces is located near Dione’s trailing Trojan point (L5).

  12. Messier Tidy Upper

    @1. Helene : “Obviously, I am absolutely loving all of these articles. :-) I had no idea I had a namesake, and I’M SO MYSTERIOUS! 😀 ”

    Well girl’s names are pretty well represented in the listing of solar system moons – There’s Elara, Carme (Jupiter), Dione, Phoebe, Pandora (Saturn), Miranda* , Ariel, Titania (Ouranos), Despina, Larissa, (Neptune) and more! 😎

    Not forgetting Venus! 😉

    Us blokes get .. ummm .. Oberon king of the fairies? 😉

    * No, not the planet of Reavers from the Serenity movie. 😉

  13. Messier Tidy Upper

    @ ^ Oh and, of course, I remembered just a second too late to add :

    LUNA! (Earth) 😉

    To that list.

    Also Selene – another name for our moon, plus girls get called Stella (& variants Astella, Estelle) pertaining to stars, Astrid ( ditto) and constellation names Carina the Mia part of Miaplacidus*! 😉


  14. Messier Tidy Upper

    @ ^ A-a-a-nd, how could I forget Celeste!!! 😉


    CORRECTION / EXPANSION : (Out of editing time.)

    To that list as well.

    Also Selene – another name for our moon, plus girls get called Stella (& variants Astella, Estelle) pertaining to stars and ( ditto) Astrid. More feminine astronomical names include the star name Gemma* the ‘Mia’ part of Miaplacidus** and the ‘Bella’ part of Bellatrix***!

    Plus the ladies get the constellation names Carina, Andromeda, Cassieopeia, Norma and Comae Berenices too. Not fair! Or perhaps very ‘fair’ depending on mythological hair colours of choice! 😉


    * Gemma = Alpha Coronae Borealis and also has the alternative name of Alphecca. Located east of Arcturus in the sky and 75 light years away in space, this is a Sirian star with a sunlike companion.

    ** Miaplacidus = Beta Carinae, a Sirian sub-dwarf 28th brightest in the sky apparent magnitude~wise and together with Atria (Alpha Trianguli Australis) the closest bright star to the south Celestial pole.

    *** Bellatrix = Gamma Orionis, sometimes also called the “Amazon star” as its name translates as ‘female warrior.” I’m pretty sure J.K. Rowling must’ve known that too. ;-)Bellatrix is a blue giant, class B2 III and is situated 240 ly distant making it one of the closer bright stars of Orion and in our skies sits opposite Rigel and on the same side of Orion’s belt as Betelgeux.

  15. Other Paul

    @Arnd – I thought nobody was going to say it out loud.

    It’s clearly – and this is why there isn’t any appreciable crater damage – the skull of a giant Grey. Domed head at top left via the two eye sockets, the nose and cheekbones along the sothwest-northeast diagonal, and the pointy-chinny jaw just rotated into the shadows at the bottom right.

    They’re here, in our system, and they’re a lot bigger than we thought. Run, I tells ya, run!

  16. Messier Tidy Upper

    Source for the info above : http://stars.astro.illinois.edu/sow/sowlist.html

    Specifically including :



    http://stars.astro.illinois.edu/sow/nor-t.html among others.

    (NB. Gemma is listed there under its alternate name.)

    Not fair!

    There is at least one star named for a bloke tho’ : Regor or Gamma-2 Velorum.
    Dosn’t sound like a blokes name? Just reverse the letters & what do you get! 😉
    This honours Roger Chaffee, one of the astronauts who died in the Apollo 1 fire. As far as I know, there’s no star named ‘De’ or ‘Sug’ or more perhaps euphoniously ‘Etihw’ or ‘Mossirg’ for the other two who perished then.

    Plus there’s also Saulocin (Alpha Delphini) and Rotanev (Beta Delphini) similarly named backwards for Nicholas Venator, Piazzi’s assistant. As well as Hercules of course! 😉

  17. Arnd

    @OtherPaul: I don’t think Grey’s get that big. But i know the solution: It’s a monument for some accomplished Grey! Maybe it was the discoverer of the solar system. They HAD to build a monument for him.

  18. #14 MTU:
    “Us blokes get .. ummm .. Oberon king of the fairies?”

    And not to mention Ganymede, who was also… Er, nuff said. :(

  19. g

    Regor (Roger) – Roger Bruce Chaffee
    Navi (Ivan) – Virgil Ivan “Gus” Grissom
    Dnoces (Second) – Edward Higgins White, II



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