The presence of large quantities of oxygen ions may be able to distinguish habitable exoplanets with life from barren exoplanets in the habitable zone (resembling Venus or Mars):
Like Earth, Venus and Mars are small rocky planets; they have permanent atmospheres like Earth, and their atmospheres are exposed to the same solar radiation as Earth's. Data from the Pioneer Venus Orbiter and the Viking descent probe on Mars show that they have very similar ionospheres to each other—which don't contain a lot of atomic O+ ions. Know what else Venus and Mars are missing? Photosynthesis.
[Astronomy PhD candidate Paul] Dalba's contention is that photosynthesis on a planet's surface, which generates a surfeit of molecular oxygen, is the only thing that can account for these atomic O+ ions in a planet's ionosphere. The mere existence of life throws a planet's atmosphere out of chemical balance. O+ would be a neat biomarker because there isn't a numerical cutoff required—just the dominance of O+ among the ionic species in the upper atmosphere would indicate "thriving global biological activity" on the planet below.
Dalba claims that Venus and Mars act as negative controls, demonstrating that planets like Earth but lacking life don't have this O+ layer. Some may think that continuous volcanic activity on the surface could also generate enough oxygen, but Dalba doesn't. Chemistry involving water and UV light [open, DOI: 10.1038/srep13977] [DX] can also release oxygen. But the amount of water on Earth is insufficient to account for the requisite oxygen content, so he thinks that the presence of water on other planets wouldn't make enough oxygen there either.
Atomic oxygen ions as ionospheric biomarkers on exoplanets (DOI: 10.1038/s41550-017-0375-y) (DX)
Related: Nitrogen in Ancient Rocks a Sign of Early Life
Oxygen Ions From Earth Escape to the Moon
Researchers Suffocate Hopes of Life Support in Red Dwarf "Habitable Zones"
Seven Earth-Sized Exoplanets, Including Three Potentially Habitable, Identified Around TRAPPIST-1
Cosmic Methyl Chloride Detection Complicates the Search for Life on Exoplanets
Mars Colonists Could Produce Oxygen by Making a Plasma Out of Atmospheric Carbon Dioxide
Analysis of Microfossils Finds that Microbial Life Existed at Least 3.5 Billion Years Ago
To Detect Life on Other Planets, Look for Methane, Carbon Dioxide, and an Absence of Carbon Monoxide
(Score: 5, Insightful) by requerdanos on Monday February 19 2018, @01:10AM
There's a relevant passage from Michael Crichton's 1969 novel "The Andromeda Strain [michaelcrichton.com]":
His commentary through his characters' voices has aged pretty well, considering that it's almost 50 years old.
It has flaws and is imperfect, but so too do our definitions of life have flaws and are imperfect.
Here, they are confronted with the difficult question of whether virii are life, and today we're past virii (without resolution) and now wondering whether prions are life, given that they are just chemicals (until you notice that they know how to operate DNA).
I believe that we will with alien life that misses our definition treat it like we often treat the virus: As a special exception that's definitely life, even though we can't say why, because we know it when we see it. I don't think that while-vs-light brown-vs-dark brown skin will be an issue in that area.
Of course, after the alien life is declared to be "life" or even "sentient life" should the case arise, that doesn't mean that killing them to steal their possessions and sell them to make a fortune, for example, or enslaving them to do man's bidding, will necessarily be automatically illegal nor even automatically condemned. Sigh.