Scientists Simulate a Lifeless Earth for 5 Billion Years to Learn What Fake Life Signs Look Like
A University of Washington model reproduces 19 planetary properties without biology, giving astronomers a baseline for judging alien atmospheres.
Astronomers have cataloged more than 6,000 planets beyond our solar system, and they are now searching the atmospheres of the most promising ones for chemicals that might be byproducts of life. The hard part is telling a habitable world from an inhabited one. To do that, researchers need to know what an atmosphere can look like with no life at all.
University of Washington researchers have built a tool for exactly that: a virtual Earth with one crucial tweak, no life. In a paper published Sept. 18 in The Planetary Science Journal, they simulated Earth's geochemical systems for 5 billion years, from the planet's formation 4.5 billion years ago to roughly 500 million years in the future. The model tracks the exchange of chemicals among the core, mantle, crust, surface and atmosphere. It accurately predicted 19 planetary properties, such as heat flow through the mantle, while leaving out any influence from living things.
The simulation can be generalized to predict how an atmosphere might evolve on any rocky planet, and to flag atmospheres that do not fit the lifeless model. "This model can help us see how habitable but nonliving planets might mimic living planets," said lead author Samantha Gilbert-Janizek, who did the work as a UW doctoral student in astronomy and is now a postdoctoral fellow at UC Riverside. "When we interpret observations of an exoplanet's atmosphere, this tool will help us determine the probability that what we observe indicates life on the surface."
Atmospheres are shaped by many nonbiological factors, including heat and gases from a planet's interior and radiation from its star. Life leaves its own marks. On Earth, plants and animals pump steady streams of oxygen and methane into the air. Those two gases destroy each other when they mix, so an alien astronomer studying our planet would suspect something alive is replenishing both. That reasoning led Gilbert-Janizek to add surface and atmosphere modeling to an existing Earth simulation built in the lab of senior author Rory Barnes, a UW associate professor of astronomy. Earlier versions covered the core, mantle, crust and stellar radiation but not the surface or atmosphere.
The finished model can be tuned for any rocky exoplanet of interest. Researchers plug in basic details about the planet and run the simulation to see whether it can reproduce the observed atmosphere without life. If it cannot, that world becomes far more interesting.
Barnes said that after more than a decade of work by many scientists, the team has a model of Earth that simultaneously reproduces properties of its core, mantle, surface and atmosphere. "That calibration gives us confidence that future simulations of exoplanets will be accurate," he said. Gilbert-Janizek said the model can also help shape the design of future telescopes, including the proposed Habitable Worlds Observatory, a mission whose goals will take decades and many generations of scientists to realize.
