Return of the Sponge Moon

One of the coolest things about Saturn is its funky moons. Titan's the most Earthlike. Enceladus has those awesome ice volcanoes, and Iapetus has black and white halves that are so different, the moon looks like an Oreo cookie. Growing up, my favorite was Mimas — not enough people know that the Death Star is alive and well in our solar system.

Hyperion
Hyperion highly pitted surface was seen up-close for the first time when Cassini flew less than 1,000 kilometers (620 miles) from Saturn's moon on September 26, 2005. The image scale is 362 meters (1,200 feet) per pixel.
Courtesy NASA/JPL/Space Science Institute

But as my tastes got more refined, my new favorite became Hyperion. This week two papers in Nature try to explain a bit more about this oddball satellite. The first strange thing about Hyperion is that it spins chaotically — you never know what side of it you're going to see whenever you snap a picture. In the parlance of scientists, mapping Hyperion is "a royal pain."

Second, the thing looks more like a sea sponge than a rocky satellite. Back in 2005, I wrote a news story speculating why there was a moon masquerading as a piece of coral around Saturn. It turns out, according to Peter Thomas (Cornell University) and his team, that my 2005 story is dead wrong.

Back then the experts thought that Hyperion's pitted appearance was due to "Sun cups". The effect happens all the time in the polar regions on Earth. A bit of soot or dark material falls on a white surface, then this material heats up in sunlight, melts, and creates a pit where more dirt can accumulate. The added dirt then heats up even more, more ice melts, lather, rinse, repeat, and you get a deep crater-like spot.

Try again. Now it seems that the craters really are craters, and it's Hyperion's amazingly low density that causes the deep pits. The body is incredibly porous overall (more than 40%), so when a rock hits it, there's little ejecta from the impact. In fact, given this moon's weak gravity, Thomas and his team write, "It may be difficult for large fractions of any ejecta produced to escape." Much of the ejecta actually falls into the original hole. They go onto say that Sun cups don't work because the craters are just too big and deep.

The second paper this week, written by Dale Cruikshank (NASA/Ames Research Center) and others, reports that the dark stuff in the bottom of the craters contains complex hydrocarbons, some of the basic building blocks for life. Can't say that that came as a shock — hydrocarbons are showing up everywhere including on comets and meteorites. But it's always nice when a result makes perfect sense.

Want to learn more? You can read the various press releases here and here. Meanwhile, I'm suddenly in the mood to go snorkeling.

2 thoughts on “Return of the Sponge Moon

  1. Roland Dechesne

    I’m not sure whether the Hyperion article was supposed to be an ‘oped’ or a news article. There’s too much fluff (and I’m not talking about the behaviour of Hyperion – like an aerogel) and a key conlusion of the published papers was apparently misquoted in David Tytell’s piece (as far as I can tell):

    In fact, given this moon’s weak gravity, Thomas and his team write, “It may be difficult for large fractions of any ejecta produced to escape.”

    What I read in the original articles indicated that:

    ” the moon’s low gravity means that any material ejected from craters on Hyperion has a good chance of escaping completely and not re-impacting the surface,…”

    I’m hoping that this article is not indicative of the lowering of standards of the S&T website, both stylistically and factually, as it goes to blog format.

  2. David Tytell

    Roland — You make an excellent point and I realized upon rereading my post that the physics are kinda confusing. Here’s what’s going on:

    When a crater forms, part of the hole is due to stuff being blasted out and some of the hole is due to compression. When a rock hits a very porous body — like Hyperion — there is much more compression than ejecta. I checked in with Thomas and he reminded me that because of Hyperion’s gravity, much of the ejecta that is blasted out actually falls back into the original hole it came from. Ejecta isn’t spread out and we don’t see ejecta blankets like we see on the Moon or Mars. So the quote refers to the materials falling back into the crater, which wasn’t made clear in my original post. I’ll fix that next.

    In regards to your final comment, we haven’t lowered our standards at all. Rather, this new format gives us immediate reader feedback. That way we can interact with readers and clarify things if there is confusion. So thanks for feedback!

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