In the search for life on other planets, the presence of oxygen in the planet's atmosphere is one potential sign of biological activity that could be detected by future telescopes. The new study, however, describes several scenarios in which a lifeless rocky planet around a sun-like star could evolve to have oxygen in its atmosphere.
The new findings, published April 13 in AGU Advances, highlight the need for next-generation telescopes that are able to characterize the planetary environment and search for multiple lines of evidence for life in addition to detecting oxygen.
"This is useful because it shows there are ways to get oxygen in the atmosphere without life, but there are other observations you can make to help distinguish these false positives from the real deal," said first author Joshua Krissansen-Totton, a Sagan fellow in the Department of Astronomy and Astrophysics at the University of California, Santa Cruz. "For each scenario, we try to say what your telescope should be able to do to distinguish this from biological oxygen."
In the coming decades, perhaps by the late 2030s, astronomers hope to have a telescope capable of taking images and spectra of potentially Earth-like planets around the sun-like stars. Co-author Jonathan Fortney, a professor of astronomy and astrophysics and director of UCSC's Other Worlds Lab, said the idea would target planets similar enough to Earth that life could arise on them and characterize their atmospheres.
"There's been a lot of discussion about whether oxygen detection is a 'sufficient' sign of life, " he said. "This work really states that it is necessary to know the context of your detection. What other molecules are found in addition to oxygen, or are not found, and what does this tell you about the evolution of the planet?
This means that astronomers will want a telescope that is sensitive to a wide range of wavelengths to detect different types of molecules in the planet's atmosphere.
The researchers based their findings on a detailed, end-to-end computational model of the evolution of rocky planets, starting with their molten origins and stretching through billions of years of cooling and geochemical cycling. By changing the initial list of volatile elements in their model planets, the researchers obtained a surprisingly wide range of results.
Oxygen can begin to build up in the planet's atmosphere when high-energy ultraviolet light splits water molecules in the upper atmosphere into hydrogen and oxygen. Light hydrogen mostly goes into space, leaving oxygen behind. Other processes can remove oxygen from the atmosphere. Carbon monoxide and hydrogen released from the release of molten rock, for example, will react with oxygen, and the weathering of the rock will also subtract oxygen. These are just some of the processes that the researchers have incorporated into their model of the rocky planet's geochemical evolution.
"If you run a model for Earth, with what we think was the initial inventory of volatiles, you reliably get the same result every time, without life you don't get oxygen in the atmosphere," Krissansen-Totton said. "But we also found a few scenarios where you can get oxygen without life."
For example, a planet that is otherwise like Earth but starts with a lot of water will end up with very deep oceans, exerting tremendous pressure on the crust. This effectively shuts down geological activity, including all processes such as melting or weathering of rocks that will remove oxygen from the atmosphere.
In the opposite case, when the planet starts with a relatively small amount of water, the magma surface of the originally molten planet can quickly freeze while the water remains in the atmosphere. This "vapor atmosphere" puts enough water in the upper layers of the atmosphere to allow the accumulation of oxygen as the water decays and the hydrogen escapes.
"The typical sequence is that the magma surface solidifies at the same time as the water condenses into the oceans on the surface," Krissansen-Totton said. "On the Ground, once the water condenses on the surface, the evacuation rate was low. But if you keep the vapor atmosphere after the molten surface solidifies, there's a window of about a million years when oxygen can create, because there are high concentrations of water in the upper atmosphere and there's no molten surface to consume the oxygen produced by the hydrogen escape."
A third scenario that could lead to oxygen in the atmosphere involves a planet that is otherwise like Earth but starts with a higher ratio of carbon dioxide to water. This results in a runaway greenhouse effect, making it too hot for water to ever condense from the atmosphere to the planet's surface.
"In this Venus-like scenario, all the volatiles start in the atmosphere, and only a few are left in the mantle to be outgassed and swabbed with oxygen," Krissansen-Totton said.
He noted that previous studies have focused on atmospheric processes, while the model used in this study examines the geochemical and thermal evolution of the planet's mantle and crust, as well as the interaction between the crust and the atmosphere.
"It's not computationally intensive, but there are a lot of moving parts and interconnected processes," he said.