Real Stars Break Down Alcohol Through Quantum Mechanics, Not Their Liver

When most people think of astronomy, they think of physics. Many astronomers are technically astrophysicists, and even if that’s not their title, most have a physics background. (If you’re really in the know, you might know that planetary science is a distinct field that draws a lot on geology as well as astronomy.)  But another aspect of space science that’s grown a lot over the last decade or so is astrochemistry. Astronomers have been able to study chemical compounds in celestial bodies since the the middle of the 20th century, when radio telescopes could detect spectral emissions unique to certain molecules (both nearby and across the galaxy) and even more so when space probes could directly analyze celestial bodies in our solar system. But there’s also a lot of chemicals just out in the middle of space, and the list keeps getting longer and includes increasingly more complicated compounds. Astrochemistry looks at these chemicals and tries to understand how they could form in astronomical environments.

One of the bigger puzzles for astrochemists has been understanding how alcohols are formed and destroyed in space. Space is too cold for methanol to break up into the highly reactive methoxy radical in a way similar to most reactions on Earth. While UV radiation exciting molecules enough to break them apart can explain how some chemicals are formed (and why UV light gives you cancer), lab tests couldn’t detect methoxy after exposing methanol to UV radiation. Dust wasn’t even acting as a catalyst. It actually turns out the reaction works best when the methanol is in its gaseous form at low temperatures because those conditions are optimal for quantum tunneling.

The procession of a chemical reaction in normal, bulk circumstances (on the left) and via quantum tunneling (on the right). From Richard Helmich.

Tunnelingis a phenomenon that only occurs in quantum mechanics. There’s really no good analogy in the classical physics we’re most familiar with. To very quickly sum up, if an electron is in place A and can also be in place C, but A and C are separated by a region B where it shouldn’t be able to travel, it can sometimes still end up in C by tunneling through B. This is also generalized to more than just physical space. Tunneling means particles can do things they shouldn’t have the energy for, like the reaction picture above. Quantum mechanics just says that it won’t happen very often and it can take some time. This is where the low temperature comes in. As we’ve talked about before, temperature reflects molecular motion. At the low temperatures of open space, the methanol and hydroxide are moving relatively slowly. When they bump into each other, this means they won’t bounce off immediately, and in that longer frame of time, an electron is more likely to jump from the methanol to the hydroxide. It turns out this tunneling reaction is really efficient at lower temperatures: lab experiments showed methanol reacted 50 times faster at -210 degrees Celsius than at room temperature. The researchers are also confident that quantum tunneling can explain many other reactions in space.

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