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April 25, 2011


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Russell Arben Fox

Doug, this was a totally fascinating read; a great way to start out my Monday morning. Born (yet frustrated) intellectual critic that I am, I couldn't help but think while reading it that this post is exactly the sort of thing which drives the vision of some of the true internet enthusiasts out there: the idea that what the world wide web gives us is the opportunity for everyone's expertise to contribute to everyone else's overall knowledge. You have some real awareness here, and now you have a platform from which to share it.

One actual question about Castillo though: exactly what kind of "organic compounds" could that moon have possessed which could have been broken down by radiolysis? I await your learned response.

Doug M.

It's not a learned response so much as a hastily researched one. But: organic compounds are everywhere. After all, carbon is a pretty common element, and tolerably reactive even at low temperatures. Combine it with hydrogen, and you easily get methane and ethane. Combine it with oxygen, you get carbon dioxide and carbon monixide. Combine it with nitrogen, you get nitriles and other cyanide compounds. These, in turn, will react with each other pretty readily to form a wide range of simple organic compounds.

In the particular case of Callisto, Galileo found spectra of CO2, sulfur, cyanide, and an odd spectrum that could mean the presence of "tholins", the miscellaneous organic compounds that form the "smog" in the atmosphere of Titan.

A 2008 paper gave this analysis:

"A significant effect of ion bombardment of ices is the creation of new species through radiolysis... Ionizing radiation not only forms more complex molecules, it can efficiently decompose organics. At low doses, CO and CO2 are the principal decomposition products of a number of organics and carbonates... A carbon cycle may be occurring on Callisto, with CO2, carbonates and carbon suboxides as principal end-products. The overall dark gray visible appearance of Callisto is consistent with laboratory measurements of carbonization of organics through radiation. Surfaces made up of organics exposed to high amounts of radiation (e.g., inner solar system, or older surfaces in the outer solar system) get darker and blacker (or grayer), whereas organic-containing surfaces exposed to less radiation are redder."

-- This last is why a lot of Kuiper Belt objects have a reddish color; that seems to be the default color of more-or-less primordial organics.

Notice that over time, you have different effects feeding into each other. Radiolysis breaks primordial "red" organic compounds down into darker sooty stuff. This in turn lowers the moon's albedo, causing it to get warmer. This increases sublimation, changing the topography as discussed above. Sublimation also releases water vapor and carbon dioxide into Callisto's very thin and tenuous atmosphere. These in turn may get ionized and cracked by radiation; the hydrogen would be lost to space quickly, but the carbon and oxygen may linger long enough to react with the surface.

There are probably several feedback loops at work here, gradually evolving over geologic time.

There's still a lot we don't know. And won't know for a while to come. The next probe to the Jupiter system -- JUNO -- should launch later this year, but JUNO will be looking at Jupiter itself rather than the moons. Nobdy's planning to visit Callisto again until 2025 at the earliest, and that's if EJSM / Laplace gets fully funded, which at the moment is not looking like a good bet. "Sometime between 2025 and 2030", would be my guess.

Planetary science is awesome! but you need to be very patient.

Doug M.

Bill Higgins-- Beam Jockey

Callisto would get some sympathy from Mercury, which is often dismissed as a "boring old dead-for-billions-of-years world."

Andy Nathan

Great Post! I really liked your description of Callisto. I have read a lot about Titan, but not Callisto. Thanks!

Air Hexagonal

Your post is my first read this Monday morning. I don't know a lot about space thing, but Callisto remind me about something enemy name :D *no offence

Callisto will thank you for defensing it from "boring old dead-for-billions-of-years world."

I'll read about Callisto soon after this.

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