Astronomers from the University of Washington (USA) have built a computer model of Earth that simulates geochemical processes over 5 billion years - from the planet's formation 4.5 billion years ago to roughly 500 million years in the future. The main difference from earlier versions: this simulation has no life at all. The work is published in The Planetary Science Journal.
The model describes how chemicals move between the core, the mantle (the layer beneath the crust), the crust, the surface and the atmosphere. The scientists checked its accuracy: the simulation correctly predicted 19 properties of the planet, for example the flow of heat through the mantle. At the same time, the influence of living organisms on these systems was deliberately not taken into account.
Why is this needed? Today more than 6000 planets are known beyond the Solar System. Astrobiologists search their atmospheres for chemical "signatures" of life. But to speak confidently about the presence of living organisms, one must know what a planet would look like without them. The new model provides exactly such a reference point.
"The model helps to see how habitable but lifeless planets can masquerade as living ones," explains lead author Samantha Gilbert-Janizek, who carried out the research during her doctorate at the University of Washington and now works at the University of California, Riverside.
The planet's atmosphere forms under the action of many non-biological factors: heat and gases from the interior, and radiation from the star. On Earth, living organisms constantly replenish supplies of oxygen and methane. These gases destroy each other when they meet, so their simultaneous presence in the atmosphere is a hint that someone is replenishing them. That is precisely why the surface and atmosphere were added to the model, to cover the entire cycle of matter.
Researchers can "tune" the simulation to any rocky exoplanet (a planet beyond the Solar System) and check whether the model reproduces its observed atmosphere without the involvement of life. If it cannot, the planet becomes even more interesting to study.
"Gilbert-Janizek's work is an important step toward understanding where life may exist in the Universe," says senior author Rory Barnes, an assistant professor of astronomy at the University of Washington. "After more than ten years of work by many scientists, it has completed a model of Earth that simultaneously reproduces the properties of the core, mantle, surface and atmosphere. Such calibration gives confidence in the accuracy of future exoplanet simulations."
The authors note: the model can be applied to any rocky planets, and also used in the design of future telescopes, for example the proposed Habitable Worlds Observatory. This is a statistical relationship between the model and real observations, which by itself does not prove causality, but helps scientists test hypotheses.