Iapetus and the Cassini Regio: 365 Days of Astronomy
By now my second podcast for 365 Days of Astronomy should be live, and here is the post to support it – containing links I mentioned in the podcast. So go listen already!
In this podcast I tell a story about Iapetus and the mystery shrouding her. And yes, I mean tell a story. I was inspired by Jay O’Callahan and his fact-tales.
By the way, this is the only thing I’ve put out on the internet that I haven’t licensed under Creative Commons. (Things that aren’t mine, but are posted by me may be excepted as well. For example, NASA retains the rights to their images though they allow generous usage of them, Jason retains the rights to his photographs, and there are others). I do, in fact, hold the copyright on this story. I’ve granted 365 Days of Astronomy the rights to replay it – and you can replay it as well and use it for personal and educational purposes, but don’t claim it as your own. Let me know if you want to use it for something else, I’ll probably says yes and be flattered.
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Emily Lakdawalla about Iapetus
The Story
Once upon a time there was a moon named Iapetus. She orbited Saturn at a distance of over 3 and a half million kilometers, and there were only two larger moons of Saturn, but still all the other moons made fun of her.
They made fun of her because her front hemisphere was a lot darker than her back hemisphere. This darker area was called the Cassini Regio, but the other moons laughed at her and said she looked like a spherical Oreo. Iapetus thought this wasn’t really fair, since she wasn’t even spherical herself, more lumpy in places.
One July, the Cassini Spacecraft showed up. He noticed how the other moons wouldn’t let Iapetus play with them, and how they always made fun of her. “Come over here, Iapetus,” he said, “I have a story to tell you.”
“Me?” asked Iapetus, “You have a story to tell me?”
“Yes, but only for you, your other friends don’t get to listen to this story.”
Suddenly the play-space became silent. The other moons stopped their games, their hula-hoops fell off, the ones running on the track slowed down, and they all turned around to look at Cassini and Iapetus talking quietly as they orbited around Saturn.
“Once upon a time there was a moon named Iapetus,” started Cassini.
“No, no! We already did that part, Cassini!” protested Iapetus, “get to the good part!”
“Well,” said Cassini, “the first time anyone from Earth saw Iapetus was in 1671, and that man’s name was Cassini.”
“Hey, that’s your name too! Wait, which one is Earth, is that the third or the fourth one out from the Sun?” asked Iapetus.
“Fourth, silly!” said Titan, one of the other moons, stepping closer to Cassini. “Don’t you know anything?”
“I’m not silly!” yelled Iapetus.
“Titan, Earth is the third one. Now if you’re going to listen you both need to sit down and be quiet like Phoebe,” said Cassini gently. “Back then no one knew what mysteries awaited them on the surface of Iapetus.”
“See? I’m not silly, I’m mysterious!” said Iapetus, sticking out her tongue at Titan.
“Shh! No more interruptions.” Cassini frowned at the two of them.
“Since Voyagers 1 and 2 first glimpsed Iapetus’s interesting surface there has been much speculation by scientists all over Earth about how Iapetus came to be this way. I will tell you a few of these ideas, and then Iapetus can tell us what really happened.
“The first idea involves Phoebe. Where is Phoebe? Ah, there she is. A scientist named Hamilton proposed that micrometeors could have knocked some dark dust off Phoebe, then Iapetus could have swept up this material such that it all collected on the front hemisphere.”
“But Cassini, I’m a different color than either Iapetus’s dark side or Iapetus’s light side. I don’t think we’re related!” protested Phoebe.
“Yes, that’s a problem with this idea, as the Earth scientists found out in 1998,” said Cassini.
“What about me?” asked Hyperion, “I’m close to Iapetus too, maybe I’m part of this.”
“That was the very next idea I was going to mention, Hyperion, thank you for bringing it up. There are two different theories relating to you. The first thing though is to find out if the dust can actually get from Hyperion to Iapetus. The scientist Marchi and his colleagues think that’s pretty easy, but how do you get the dust off Hyperion in the first place?”
“Hit it with something!” chorused all the moons of Saturn, making a terrible racket and almost waking the Sun up from her mid-afternoon nap.
“I see you know the secret,” agreed Cassini. “If you need to get something from one place to another in the Solar System, you usually need to slam two things together.”
“And look,” said Hyperion, holding his arm up next to Iapetus’s dark side, “we’re basically the same color on this side of Iapetus.”
“So you are,” observed Cassini. “That makes this idea seem plausible. One last puzzling idea is that perhaps this dark material is from somewhere else, and was collected on both of you.
“One of my jobs in coming here is to take a better look at you Iapetus, and see if I can provide any useful data for the scientists to use in figuring out where your Cassini Regio came from. Do you know the answer?”
“Wow, everyone’s looking at me?” asked Iapetus, “I dunno, I can’t remember when it happened. I am pretty sure that the light side of me is ice, because my backside is always a little chilly. Anyway, if I did know where the dark stuff came from, shouldn’t I leave it as a puzzle for you to find out?” With that she ran off to play hula-hoop by herself, but Hyperion and some of the smaller moons followed her and they all started a game of Occultation.
The End
~ A l i c e !
