Potential technosignature from anomalously low deuterium/hydrogen (D/H) in planetary water depleted by nuclear fusion technology

Abstract

Deuterium-deuterium (DD) fusion is viewed as an ideal energy source for humanity in the far future, given a vast seawater supply of D. Here, we consider long-lived, extraterrestrial, technological societies that develop DD fusion. If such a society persists over geologic timescales, oceanic deuterium would diminish. For an ocean mass and initial D/H that are Earth-like, fusion power use of only $\sim$10 times that projected for humankind next century would deplete the deuterium-hydrogen ratio (D/H) in $\sim$(a few)$\times 10^8$ years to values below that of the local Interstellar Medium (ISM). Ocean masses of a few percent Earth's would reach anomalously low D/H in $\sim 10^6$ to $10^7$ years. The timescale shortens with greater energy consumption, smaller oceans, or lower initial D/H. Here, we suggest that anomalous D/H in planetary water below local ISM values of $\sim 16\times 10^{-6}$ (set by Big Bang nucleosynthesis plus deuterium loss onto dust or small admixtures of deuterium-poor stellar material) may be a technosignature. Unlike SETI from radio signals, anomalous D/H would persist for eons, even if civilizations perish or relocate. We discuss wavelengths of strong absorption features for detecting D/H anomalies in atmospheric water vapor. These are vibrational O-D stretching at 3.7 $\hat{I}¼$m in transmission spectroscopy of Earth-like worlds, $\sim 1.5$ $\hat{I}¼$m (in the wings of the 1.4 $\hat{I}¼$m water band) in the shorter near-infrared for direct imaging by Habitable Worlds Observatory, and 3.7 $\hat{I}¼$m or $\sim 7.5$ $\hat{I}¼$m (in the wings of the broad 6.3 $\hat{I}¼$m bending vibration of water) for concepts like the Large Interferometer for Exoplanets (LIFE).

Submitted to Astrophys. J. on March 12, 2024. Accepted for publication on November 27, 2024