A moody moon turns its face

The entire universe in blog form
June 20 2011 1:00 PM

A moody moon turns its face

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:

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


[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:



Slate Plus Early Read: The Self-Made Man

The story of America’s most pliable, pernicious, irrepressible myth.

Rehtaeh Parsons Was the Most Famous Victim in Canada. Now, Journalists Can’t Even Say Her Name.

Mitt Romney May Be Weighing a 2016 Run. That Would Be a Big Mistake.

Amazing Photos From Hong Kong’s Umbrella Revolution

Transparent Is the Fall’s Only Great New Show

The XX Factor

Rehtaeh Parsons Was the Most Famous Victim in Canada

Now, journalists can't even say her name.


Lena Dunham, the Book

More shtick than honesty in Not That Kind of Girl.

What a Juicy New Book About Diane Sawyer and Katie Couric Fails to Tell Us About the TV News Business

Does Your Child Have Sluggish Cognitive Tempo? Or Is That Just a Disorder Made Up to Scare You?

  News & Politics
Sept. 29 2014 10:00 PM “Everything Must Change in Italy” An interview with Italian Prime Minster Matteo Renzi.
Sept. 29 2014 7:01 PM We May Never Know If Larry Ellison Flew a Fighter Jet Under the Golden Gate Bridge
Dear Prudence
Sept. 29 2014 3:10 PM The Lonely Teetotaler Prudie counsels a letter writer who doesn’t drink alcohol—and is constantly harassed by others for it.
  Double X
The XX Factor
Sept. 29 2014 1:52 PM Do Not Fear California’s New Affirmative Consent Law
  Slate Plus
Slate Fare
Sept. 29 2014 8:45 AM Slate Isn’t Too Liberal, but … What readers said about the magazine’s bias and balance.
Brow Beat
Sept. 29 2014 9:06 PM Paul Thomas Anderson’s Inherent Vice Looks Like a Comic Masterpiece
Future Tense
Sept. 29 2014 11:56 PM Innovation Starvation, the Next Generation Humankind has lots of great ideas for the future. We need people to carry them out.
  Health & Science
Bad Astronomy
Sept. 29 2014 12:01 PM This Is Your MOM’s Mars
Sports Nut
Sept. 28 2014 8:30 PM NFL Players Die Young. Or Maybe They Live Long Lives. Why it’s so hard to pin down the effects of football on players’ lives.