A team of researchers at MIT has found that phosphine, which is among the stinkiest, most toxic gases on Earth, cannot be produced in any other way except by anaerobic organisms, making it a pure ‘biosignature gas.’
Artist’s conception of what life could look like on the surface of an exoplanet. Image credit: NASA.
Life makes use of thousands of volatile compounds that could contribute toward a biosphere. Some of these compounds may accumulate in a planetary atmosphere and be remotely detectable — these are commonly called ‘biosignature gases.’
Prominent biosignature gases on Earth are those that are both relatively abundant and spectroscopically active (primarily oxygen, ozone, methane and nitrous oxide).
“Here on Earth, oxygen is a really impressive sign of life,” said Dr. Clara Sousa-Silva, a research scientist in MIT’s Department of Earth, Atmospheric and Planetary Sciences.
“But other things besides life make oxygen too. It’s important to consider stranger molecules that might not be made as often, but if you do find them on another planet, there’s only one explanation.”
In the new research, Dr. Sousa-Silva and her colleagues found that phosphine is a promising marker for life if detected on a rocky exoplanet.
But to be sure, they had to rule out any possibility that this gas could be produced by anything other than life.
To do this, they spent the last several years running many species of phosphorous — phosphine’s essential building block — through an exhaustive, theoretical analysis of chemical pathways, under increasingly extreme scenarios, to see whether phosphorous could turn into phosphine in any abiotic way.
The researchers worked out the chemical pathways and thermodynamics involved in multiple scenarios on Earth to see if they could produce enough energy to turn phosphorous into phosphine.
“At some point we were looking at increasingly less-plausible mechanisms, like if tectonic plates were rubbing against each other, could you get a plasma spark that generated phosphine?” they said.
“Or if lightning hit somewhere that had phosphorous, or a meteor had a phosphorous content, could it generate an impact to make phosphine?”
“And we went through several years of this process to figure out that nothing else but life makes detectable amounts of phosphine.”
The scientists found that phosphine has no significant false positives, meaning any detection of phosphine is a sure sign of life.
According to the team, if phosphine were produced at relatively small amounts equivalent to the amount of methane produced on Earth today, it would produce a signal in the atmosphere that would be clear enough to be detected by an advanced observatory such as the upcoming James Webb Space Telescope, if that planet was within 16 light-years from Earth.
“Aside from establishing phosphine as a viable biosignature in the search for extraterrestrial life, our results provide a pipeline, or process for researchers to follow in characterizing any other of the other 16,000 biosignature candidates,” Dr. Sousa-Silva said.
The team’s paper was published in the journal Astrobiology.
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Clara Sousa-Silva et al. Phosphine as a Biosignature Gas in Exoplanet Atmospheres. Astrobiology, published online November 22, 2019; doi: 10.1089/ast.2018.1954
This article is based on text provided by the Massachusetts Institute of Technology.
