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Oxygen ions in planets' ionospheres could be indicators of life

The presence of charged ions in upper atmospheres is due to photosynthesis by plants and algae.
By Laurel Kornfeld | Feb 22, 2018
Scientists looking for signs of life on worlds beyond Earth should consider searching for high oxygen content in planets' ionospheres, according to a new study published in the journal Nature Astronomy.

A planet's ionosphere is the thin, electrically charged portion of its upper atmosphere, where interaction with the solar wind ionizes or strips away molecules' electrons.

Funded by a National Science Foundation (NSF) grant, Michael Mendillo, Paul Withers, and Paul Dalba, all of Boston University, compared the ionospheres of planets in our solar system and found Earth's ionosphere unique in that it harbors high levels of a specific type of ionized oxygen--single atoms containing a positive charge.

Because of its closeness to the Sun, Mercury has no ionosphere. Pluto has a tenuous one with a high concentration of methane.

The solar system's other planets' ionospheres are composed of complicated charged molecules produced by carbon dioxide and hydrogen, with the exception of Mars, where most ionosphere molecules have escaped into space.

Earth's ionosphere has a high oxygen concentration due to the green plants and algae on its surface, the researchers determined.

"It's because we have this atomic oxygen that traces its origin back to photosynthesis," Mendillo explained. "We have atomic oxygen ions, O+, in the ionosphere as a direct consequence of having life on the planet. So why don't we see if we can come up with a criterion where the ionosphere could be a biomarker, not just of possible life but of actual life," he said regarding exoplanets.

Molecules of oxygen exhaled by plants rise upward into Earth's atmosphere. At approximately 93.2 miles (150 km) above the planet's surface, ultraviolet sunlight splits them into two parts. Once they reach the ionosphere, the single atoms of oxygen have electrons ripped from their outer shells by the Sun's X-rays and ultraviolet light, leaving the air with charged oxygen atoms.

The search for evidence of life beyond Earth has centered on finding water, which life needs, but the researchers suggest it should instead be centered on finding high levels of oxygen ion in exoplanets' ionospheres.

While it is unclear how much water a planet must have to support life, there is less ambiguity regarding ionospheric oxygen.

"If you look at the ionosphere, you don't need to know the number. You just need to know that if the maximum electron density is associated with oxygen ions, then you've nailed it--you've got a planet where there's photosynthesis and life," Mendillo emphasized.

Today's space telescopes are not capable of detecting ionospheres on exoplanets, but scientists are optimistic the technology for doing this will become available within ten years.

 

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