NASA Administrator Wants to Prioritize Venus After Promising Sign of Life Detected
NASA Administrator Jim Bridenstine has called for the immediate prioritization of studying Venus after astronomers at the Royal Astronomical Society announced the detection of phosphine gas, a byproduct of biological activity on Earth, in the planet’s atmosphere. The extensive study that led to the discovery was published in Nature Astronomy on Monday.
“Life on Venus? The discovery of phosphine, a byproduct of anaerobic biology, is the most significant development yet in building the case for life off Earth,” Bridenstine tweeted Monday afternoon. “About 10 years ago NASA discovered microbial life at 120,000ft in Earth’s upper atmosphere. It’s time to prioritize Venus.”
Life on Venus? The discovery of phosphine, a byproduct of anaerobic biology, is the most significant development yet in building the case for life off Earth. About 10 years ago NASA discovered microbial life at 120,000ft in Earth’s upper atmosphere. It’s time to prioritize Venus. https://t.co/hm8TOEQ9es
— Jim Bridenstine (@JimBridenstine) September 14, 2020
As with an increasing number of planetary bodies, Venus is proving to be an exciting place of discovery, though it had not been a significant part of the search for life because of its extreme temperatures, atmospheric composition and other factors.
Venus, often to referred to as “Earth’s twin” due to their similar masses, is the hottest planet in the Solar System. Its inhospitable surface roasts at 471 degrees Celsius (880 degrees Fahrenheit) under a dense atmosphere of carbon dioxide and sulfuric acid, which traps radiation from the Sun in what scientists call a “runaway greenhouse effect.” The crushing atmospheric pressures on the Venusian surface are equal to being under 1.6 kilometers (1 mile) of water.
But the cloud layers 48 kilometers (30 miles) above Venus harbor a much more Earth-like environment, and scientists have long wondered whether microbial life could have found a way to evolve there.
The detection of phosphine gas lends enormous credence to that possibility, according to the team of researchers involved with Monday’s findings:
The presence of PH3 is unexplained after exhaustive study of steady-state chemistry and photochemical pathways, with no currently known abiotic production routes in Venus’s atmosphere, clouds, surface and subsurface, or from lightning, volcanic or meteoritic delivery. PH3 could originate from unknown photochemistry or geochemistry, or, by analogy with biological production of PH3 on Earth, from the presence of life. Other PH3 spectral features should be sought, while in situ cloud and surface sampling could examine sources of this gas.
Phosphine gas has been found in the atmospheres of the outer gas giant planets; it is forged under the enormous pressures and temperatures in their deep interiors. On Earth, however, life is the only known source of phosphine, and astrobiologists believe that is likely the case on smaller rocky planets, although one of the challenges scientists still face is distortion of chemical spectra by Earth’s atmosphere:
Trace PH3 in Earth’s atmosphere (parts per trillion abundance globally) is uniquely associated with anthropogenic activity or microbial presence – life produces this highly reducing gas even in an overall oxidizing environment. PH3 is found elsewhere in the Solar System only in the reducing atmospheres of giant planets, where it is produced in deep atmospheric layers at high temperatures and pressures, and dredged upwards by convection. Solid surfaces of rocky planets present a barrier to their interiors, and PH3 would be rapidly destroyed in their highly oxidized crusts and atmospheres. Thus PH3 meets most criteria for a biosignature-gas search, but is challenging as many of its spectral features are strongly absorbed by Earth’s atmosphere.
The findings were subject to “double false positive” tests, the study explains, and therefore the presence of phosphine is conclusive:
We are unable to find another chemical species (known in current databases) besides PH3 that can explain the observed features. We conclude that the candidate detection of PH3 is robust, for four main reasons. First, the absorption has been seen, at comparable line depth, with two independent facilities; second, line measurements are consistent under varied and independent processing methods; third, overlap of spectra from the two facilities shows no other such consistent negative features; and fourth, there is no other known reasonable candidate transition for the absorption other than PH3.
If phosphine is coming from living sources, the biggest mystery is how, because of the chemical processes required to generate the gas in the first place:
The presence of PH3 implies an atmospheric, surface or subsurface source of phosphorus, or delivery from interplanetary space. The only measured values of atmospheric phosphorus on Venus come from Vega descent probes, which were only sensitive to phosphorus as an element, so its chemical speciation is not known. No phosphorus species have been reported at the planetary surface.
The study cautions that there could still be other sources of phosphine on Venus, because many unknowns about the Venusian environment remain:
If no known chemical process can explain PH3 within the upper atmosphere of Venus, then it must be produced by a process not previously considered plausible for Venusian conditions. This could be unknown photochemistry or geochemistry, or possibly life. Information is lacking—as an example, the photochemistry of Venusian cloud droplets is almost completely unknown.
Much more research and experimentation are required before any determination can be made about whether life exists in Venus’ atmosphere, and the only thing astronomers can say for sure is that they have new questions to ask and answer:
Even if confirmed, we emphasize that the detection of PH3 is not robust evidence for life, only for anomalous and unexplained chemistry. There are substantial conceptual problems for the idea of life in Venus’s clouds—the environment is extremely dehydrating as well as hyperacidic. However, we have ruled out many chemical routes to PH3, with the most likely ones falling short by four to eight orders of magnitude. To further discriminate between unknown photochemical and/or geological processes as the source of Venusian PH3, or to determine whether there is life in the clouds of Venus, substantial modelling and experimentation will be important. Ultimately, a solution could come from revisiting Venus for in situ measurements or aerosol return.
NASA was not a participant in the study, however, numerous space agencies from around the world – NASA, ESA, JAXA, and the Soviet Venera program – have been probing and photographing Venus since the early 1960’s, and there are promising candidates for life elsewhere in the Solar System.
Mars, for example, has subsurface water and fossilized molecules that resemble organic compounds on Earth were discovered in the 1990’s. Jupiter’s icy moon Europa, as well as Saturn’s Enceladus, have global oceans underneath their frozen water-ice surfaces. In 2018, NASA’s Cassini spacecraft detected organic compounds in ejecta from cryovolcanoes on Enceladus that shoot water into space. These plumes form one of Saturn’s rings.
Even on tiny, distant Pluto, possible signs of life were observed by New Horizons and announced by NASA in 2017:
Data from the New Horizons flyby finished downloading to Earth in October, and while it will take many years for scientists to complete their inventory and model the results, early studies offer intriguing hints of its complex chemistry, perhaps even some form of pre-biological processes below Pluto’s surface. Complex layers of organic haze; water ice mountains from some unknown geologic process; possible organics on the surface; and a liquid water ocean underneath — all of these features point to a world with much more vibrancy than scientists have long presumed.
Intriguingly, reddish material was also spotted near Pluto’s ice volcanoes, or calderas. It’s possible that the dwarf planet could have a subsurface ocean similar to that suspected on Titan, Saturn’s Enceladus and Jupiter’s Europa. These moons, however, have a tidal source of energy within, created by orbiting their huge central planets and interacting gravitationally with other moons. Pluto is bereft of such heating, but it’s possible that radioactivity in its interior could be keeping the inside liquid.
“The connection with astrobiology is immediate — it’s right there in front of your face. You see organic materials, water and energy,” Michael Summers, a planetary scientist on the New Horizons team, said at the time. “These are the things you need for life: organics, raw material and energy.”
The potential that Pluto is or has been a home to life is, at its best, a remote one. But for Summers, it is nonetheless a tantalizing thought:
“I’ve been studying Pluto all my life, and never expected to talk about these things being there.”
Above all else, the study is a reminder that Earth is a unique place in the Universe. It is the only planet known to have active biology 4.5 billion years after its formation.