Extraterrestrial civilizations need a great deal of energy as they advance up the Kardashev scale. Fossil fuels are finite, wind and solar energy are carbon free but not as efficient as fossil fuels, and traditional nuclear fission power depends on a supply of fissionable material and has a waste problem. Thus, any advanced alien species may well turn to nuclear fusion for their ever-increasing energy needs (unless they've discovered even better energy processes we don't yet know about).
Deuterium (D) fusion is one of the simplest forms of nuclear fusion, where D fuses with tritium or another D. As life needs water as far as we know, oceans on an advanced world could supply plenty of it in ocean water.
On Earth, water contains a natural miniscule amount of heavy water, with deuterium replacing one or both hydrogen atoms to exist as HOD or DOH and rarely as D2O. Extracting deuterium from an ocean would decrease its ratio of deuterium-to-hydrogen, D/H, including in atmospheric water vapor, while the helium produced in the nuclear reactions would escape to space. Could low values of D/H in an exoplanet's atmosphere be a technosignature of long-lived, uber-advanced extraterrestrial life?
That's what David C. Catling of the University of Washington started wondering a while back. "I didn't do much with this germ of idea until I was co-organizing an astrobiology meeting last year at Green Bank Observatory in West Virginia," he said.
When a brainstorming session on SETI was in the works, he did some preliminary analysis that eventually led to a three-party collaboration and a paper on the subject that will appear in the Astrophysical Journal and is published on the arXiv preprint server.
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