Venus Life Is Still A Longshot

View of planet Venus from space. Space, nebula and planet Venus. This image elements furnished by … [+] NASA.

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Since being reported yesterday in the journal Nature Astronomy, the putative detection of phosphine in Venus’ upper atmosphere has been gobbled up like raw astrobiological meat. But here’s the rub. The putative detection of the toxic phosphine compound —- in and of itself an impressive technical achievement —- doesn’t automatically point to any sort of biological source. 

Phosphine in the temperate upper cloud decks of Venus’ hyperacidic atmosphere is just as likely caused by something mundane. In fact, the textbook method for making phosphine (PH3) is to heat phosphorous acid (H3PO3) to 205-210 degrees Celsius, which splits into phosphine and phosphoric acid, Kevin Zahnle, a planetary scientist at NASA Ames Research Center at Moffett Field, Calif., told me. 

The droplets heat and evaporate as they fall to hotter depths in Venus’ atmosphere, he says. These temperatures are seen on Venus, Zahnle notes.  When asking if phosphine means life, he wonders, one might ask which seems more likely, heating phosphorous acid or life in the clouds?

But David Catling, a planetary scientist at the University of Washington in Seattle, points out that the Soviet Vega 1 and Vega 2 spacecraft, which descended through Venus’ atmosphere in 1985, did detect phosphorus in some unknown compound in the clouds.

On Earth, phosphine is connected to life as a decay product of dead organisms or their excrement (such as penguin guano), says Catling. But the presence of phosphorus is not always taken as a sign of life, such as in the atmospheres of Jupiter and Saturn, he says.

Three impact craters are displayed in this three-dimensional perspective view of the surface of … [+] Venus created with data from the probe Magellan. Visible are the Aglaonice, Howe, and Danilova craters. | Location: Venus. (Photo by © CORBIS/Corbis via Getty Images)

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Hot water vapor and phosphorus trioxide (P4O6), a gas that smells like garlic, might also directly form phosphine, says Catling.  In fact, after the Vega spacecraft results, phosphorus trioxide was postulated as a likely gas on Venus, he says.

To be fair, the paper’s authors —- who represent an international team of researchers from the U.K., U.S. and Japan —- therefore note that even if confirmed, the detection of phosphine is not robust evidence for life, only for anomalous and unexplained chemistry.  Their spectroscopic detection was made using the James Clerk Maxwell Telescope (JCMT) in Hawaii, and the Atacama Large Millimeter Array (ALMA) observatory in Chile.

Their detection comes from a single absorption feature in the microwave region of the spectrum, near one millimeter’s wavelength, Catling cautions. In particular, the observed phosphine line coincides with a sulfur dioxide (SO2) line, he says.  

Because the line is bigger than expected for SO2 alone, the authors interpret the line as a blend of phosphine and SO2, says Catling who notes that a single spectroscopic line is the bare minimum for a detection. “So, more data is needed from a spacecraft orbiting Venus or from perhaps from probes flying in clouds of Venus,” he says.

However, independent evidence for phosphine on Venus may already exist in data taken by the European Space Agency’s Venus Express mission. 

Phosphine has an absorption band in the infrared spectrum at roughly 3.05 microns, Julie Castillo, a planetary scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, told me. So, one should check the observations of the VIRTIS instrument on Venus Express for the possible presence of that phosphine in its VIRTIS observations, she says.

If this phosphine is indeed a byproduct of life in Venus’ cloud decks, did it migrate from the surface or was did it form in situ?

Biology in the atmosphere could be the last surviving members of a prior Venusian biosphere, Stephen Kane, a planetary geophysicist at the University of California, Riverside, told me. Advanced climate models are certainly pointing towards a previously temperate climate for Venus as recent as a billion years ago, he says.

Laurie Barge, an astrobiologist at NASA JPL, agrees. 

Most origin of life scenarios for Earth postulate processes that would be occurring in confined environments such as the seafloor or on land where there are plenty of organic precursors, and mineral catalysts, Barge told me. “Atmospheric gases and chemistry are thought to be important to the process – but it’s not likely that the whole process occurred in the atmosphere,” she said.

But Catling is skeptical that life could survive in Venus’ clouds regardless of where it originated.

Catling says life itself is very unlikely to survive in the sulfuric acid cloud droplets of Venus simply because it’s so dry. He notes that this is one reason honey made by bees on Earth doesn’t need refrigeration. Terrestrial microbes simply cannot live in honey, he says. 

“Once you have a substance roughly drier than honey, life cannot live in it,” said Catling, who notes that Venus’ concentrated sulfuric acid cloud droplets are so dry, that there are just not enough water molecules around for biochemistry.

Even so, our hellish neighbor next door may hold many of our solar system’s astrobiological keys.

As “Earth’s twin,” Barge says that Venus might be the only other planet that can help us understand why there is life on Earth at all.