SETI Researchers Want to Sift One Cubic Meter of Moon Dust for Alien Debris. Even Finding Nothing Would Rule Out Any Civilization That Ever Shed More Than a Tenth of an Earth's Mass of Machinery.

The search for extraterrestrial intelligence has a timing problem. Radio telescopes can only hear a civilization that is transmitting now, and the Milky Way is about 13 billion years old. A civilization that rose and fell a billion years ago would have left no signal for anyone to catch. A new paper from researchers at the SETI Institute proposes a way around that: stop listening for messages and start looking for garbage, specifically in the soil of the Moon.

The paper, led by Lewis J. Pinault, an associate researcher at the SETI Institute, is posted as a preprint on arXiv and submitted to the International Journal of Astrobiology. Its argument does not depend on aliens ever having visited the solar system. It depends on the Moon being an unusually good collector of whatever drifts through it.

The authors describe two kinds of particle worth hunting. "Arkhipov particles," named for the Ukrainian astronomer Alexei Arkhipov who raised the idea in the 1990s, are fragments of industrial debris shed unintentionally. A civilization that builds structures around its star, a Dyson swarm for instance, will see pieces of it destroyed over time, and some of that wreckage could be flung out of its home system. The Sun completes an orbit of the galaxy roughly every 230 million years, so the solar system has swept through a great deal of the Milky Way already. "Bracewell particles," named for physicist Ronald Bracewell, are the deliberate kind, tiny probes sent to other stars as a form of smart dust.

Earth is the bigger target, but it destroys evidence. Its atmosphere vaporizes small incoming grains, and its oceans, weather and plate tectonics bury or erase whatever survives. The Moon has none of that. Micrometeoroid impacts churn its top layer in a process called impact gardening, which buries small particles under up to a few meters of regolith, shielding them from the cosmic rays that would otherwise break them down. The paper estimates that grains made of refractory materials such as advanced ceramics, graphene or titanium-tungsten superalloys could survive the trip through interstellar space for 100 million to 1 billion years.

Arrival is the hard part. A grain falling toward the Sun would reach about 42 kilometers per second by the time it crossed Earth's orbit, and anything that hits the Moon faster than about 5 kilometers per second vaporizes. The authors calculate that for grains of a particular size and density, the pressure of sunlight itself could slow them enough to land intact, either as recognizable micron-scale fragments or trapped inside agglutinates, the glassy droplets that form when impacts flash-melt lunar soil.

Then comes the search. One cubic meter of regolith weighs about 1.5 metric tons and contains more than a trillion micron-sized grains. No human can inspect that. The proposal is to run the material through a high-resolution scanning electron microscope and feed the images to a computer-vision model, including one called YOLO-ET that the team used in earlier work, flagging anomalies for closer inspection with focused ion beams or nano-CT scanners.

Even a null result would be worth something. The authors calculate that finding no artificial grains in one cubic meter would rule out any scenario in which Sun-like stars across the Milky Way have collectively dispersed more than 0.1 Earth masses of artificial dust over the galaxy's history. It would also mean no civilization has been intentionally sending probes at a rate above 0.4 kilograms per billion years. Those are loose bounds, and the authors admit it. But they are the first bounds of their kind, and as lunar missions multiply, a ton of dirt is not an unreasonable thing to ask for.