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LPSC, Day 2: Impacts onto icy moons

Posted by Emily Lakdawalla

03-03-2010 15:29 CST

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There has been big news from Moon and Mars here at the Lunar and Planetary Science Conference, but I can't find the time to wrap that stuff up into a properly illustrated blog post; while I'm still on site at the conference I'll be tossing the easier-to-digest bits into the blog. So here's some stuff that's less newsy but still, I think, really interesting. [Ha. That intro I wrote yesterday. So much for finding time even for the easier-to-digest bits.]

One big change at this year's LPSC is that rather than having one session for Mars, one for Enceladus, and so on, they've organized the sessions by process. This is very cool because it brings together the people who are, for just one example, exploring dune migration on Mars and on Titan; in most other meetings you'd never get those people into the same room. One challenge though is that if I do want to find out something about what people are doing on, say, icy satellite research, I have to pore through the program to locate the very few icy satellites talks that made it onto the schedule.

I found three such talks yesterday, all in the impact models & experiments session. One was given by Mark Price of the University of Kent, who asked an odd question: is it possible to make organic molecules in ices just by blasting them in impacts? His question was, he said, motivated by the discovery of glycine (the simplest amino acid, NH2CH3COOH) in Stardust's analysis of grains from comet Wild 2. He wondered how this was formed -- was it by solar ultraviolet radiation hitting ice, or could it be from "shock synthesis?"

Here's how shock synthesis would work. You start with a mixture of ices common in the outer solar system, things like water, ammonia, and carbon dioxide. Slam something into it, simulating a collision between two bodies (a much smaller one hitting the larger icy thing). When an impact occurs, a shock wave travels outward from the site of the impact. A shock wave is a pressure wave; wherever it travels, materials get compressed into a much smaller volume. Compression heats the ices, cooking them into a hot soup. Just as soon as the shock wave passes, though, the ices re-expand, cool, and resolidify, except that the cooking has left them with new compounds composed of those ices.

Price did experiments, blasting ices with pellets, creating pressures of 50 or 60 Gigapascal. And he did find he made something interesting -- crystals of compounds that could be glycine, cyanoacetic acid, cyano-guanidine, or other simple organic compounds. He doesn't know what they are yet, but he's pretty sure they weren't there before he blasted the ices. But experiments can never be kept totally clean of contamination; so he needs to do further work to prove that he really made amino acids and other simple organics just with the force of an impact. One possible way forward is to use isotopically tagged ammonia (ammonia made of an unusual isotope of nitrogen); if the unusual isotope were found within the amino acids, he'd have the proof he needs.

Two other interesting talks followed. In one, R. Kraus did a bunch of mathematical modeling work that suggests that when a big impact hits big icy moons, nearly all of the melt (70 to 80 percent) produced during the impact actually stays within the crater; it doesn't splash out. In a related talk, C. Elder looked at what happens to the melt that does stay inside the crater. Lots of Ganymede craters have central pits; she explored whether the pits could originally have been filled entirely with impact melt that drained through fractures in the floor. To make a long story short, she found the answer to be yes.

 
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