For nearly a year, scientists have been locked in a debate about whether or not Venus has signs of life hidden in its clouds — the theory is based on traces of phosphine gas in its upper atmosphere, a sign of biological activity.
Jonathan Lunine, chair of the Department of Astronomy at Cornell University, and his graduate student Ngoc Truong are among those unconvinced of the habitability of this scorching planet. Instead, they suggest that Venus’ phosphine may have made its way to the planet’s upper atmosphere by way of volcanic eruptions. And in a new paper, they want to settle things once and for all.
“That's really why we went ahead and pursued this,” Lunine tells Inverse.
“We felt that this is the most plausible source of the phosphine, it’s volcanism and not biology.”
In the study, published Monday in the Proceedings of the National Academy of Sciences, the team suggests that rather than be habitable, Venus is actually a geologically active world. Specifically, they find evidence for volcanism occurring today or in the planet’s recent past — this would account for the phosphine gas.
While the latest spin on the phosphine debate puts a slight dampener on the case for life on Venus, it will help scientists in their search of other Earth-like planets — which may harbor life — orbiting around different stars.
WHAT'S NEW — Following a landmark discovery made last year that scientists claimed was clear evidence Venus may be harboring life because it had phosphine in its clouds, Lunine and his team began to explore an alternative explanation for the telltale gas.
Using observations from the ground-based James Clerk Maxwell Telescope in Hawaii and the Atacama Large Millimeter Array (ALMA) in northern Chile, Lunine and his team created a model of Venus — one in which the planet had volcanic activity.
The model suggests that some phosphorus-bearing compounds may lurk in Venus’ mantle, which lies underneath the surface of the planet. These compounds — called phosphites — could ostensibly find their way to the surface and then be ejected by explosive volcanism into Venus’ stratosphere.
This is a layer of the planet’s upper atmosphere where phosphites would react with other chemicals in the atmosphere — like sulphuric acid and water. If that occurred, the phosphites would be converted to phosphine.
“The original hypothesis was that the phosphine that was discovered points to life,” Lunine says. “We don't think so, we think of points to volcanism.”
HERE’S THE BACKGROUND — In September 2020, a team of scientists claimed to have detected traces of phosphine gas in Venus' atmosphere.
Phosphine is considered a biosignature gas on Earth, meaning that it is typically produced by a living organism. When searching for signs of life on other planets, scientists typically look for traces of “bio-signature” gases to help them identify if a planet could possibly host life — phosphine is on the list.
The initial results were met with equal excitement and skepticism, as some scientists doubt that Venus could host life now or ever. This is to do with Venus itself.
The surface of Venus boasts temperatures that reach up to 900 degrees Fahrenheit due to a dense carbon dioxide atmosphere that traps heat. At the same time, the scorching world spins slowly and in the opposite direction to most planets, but its winds blow as fast as hurricanes, sending Venus’ acidic clouds for a spin around the planet once every five days.
Basically, on the list of habitable worlds, Venus rarely ever makes the cut.
Digging into the details — Since the September 2020 paper, other scientists have suggested alternative reasons for the presence of phosphine on Venus, including that it’s not phosphine at all, but perhaps sulfuric dioxide.
But the new study bases its alternative theory on both the new findings and on past evidence that Venus hosts volcanic activity.
“There’s been a lot of circumstantial evidence over the years,” Lunine says.
In 1978, NASA’s Pioneer Venus orbiter mission discovered variations of sulfur dioxide in Venus’ upper atmosphere. Sulfur dioxide is released from volcanoes, and the amount discovered at the time would put the scale of volcanic activity on Venus similar to that of the catastrophic 1883 Krakatoa volcanic eruption in Indonesia. This eruption was so powerful it changed the Earth’s climate for several years after it happened.
Meanwhile, radar images from the Magellan spacecraft in the 1990s showed geological features on Venus that were indicative of recent volcanic activity, too.
“A lot of the surface of Venus seems to be relatively young,” Lunine says.
“It has a few impact craters, but it looks like there may have been an episode recently of large-scale volcanism.”
Venus is similar to Earth in a lot of ways, with similar size, density, and composition. It makes sense that Venus would get rid of its heat the same way that Earth does — through volcanoes.
WHY IT MATTERS — Although the recent study does not point to further evidence of Venus’ potential habitability, it does give scientists a rare opportunity to study a planet much like Earth but at a different stage of its evolution.
Venus’ atmosphere consists mostly of carbon dioxide and traps heat in the same way that greenhouse gases do here on Earth. That’s why Venus often serves as an eerie vision of Earth’s future.
Scientists believe Venus may have started off like Earth during its early history with water flowing on its surface. However, as the planet heated up, the oceans evaporated, and its surface temperature became so hot that any life would have been destroyed.
“It allows us a chance to study a planet the size of the Earth, where the geology is extremely different,” Lunine says. “It also gives us a bit of ground truth for looking at Earth-sized planets around other stars.”
WHAT'S NEXT — With so much focus on Mars, Venus has been neglected in the search for life off Earth for more than 30 years. But that’s all about to change soon.
The VERITAS mission, which stands for Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy, will map the geological features of Venus.
The spacecraft will create 3D topographic maps of the planet’s surface and measure its gravitational pull. The purpose of understanding Venus’ geological history is to determine why the planet, which started off very similar to Earth, developed in a much different way over time.
Abstract: We hypothesize that trace amounts of phosphides formed in the mantle are a plausible abiotic source of the Venusian phosphine observed by Greaves et al. [Nat. Astron., https://doi.org/10.1038/ s41550-020-1174-4 (2020)]. In this hypothesis, small amounts of phosphides (P3− bound in metals such as iron), sourced from a deep mantle, are brought to the surface by volcanism. They are then ejected into the atmosphere in the form of volcanic dust by explosive volcanic eruptions, which were invoked by others to explain the episodic changes of sulfur dioxide seen in the atmosphere [Esposito, Science 223, 1072–1074 (1984)]. There they react with sulfuric acid in the aerosol layer to form phosphine (2 P3− + 3H2SO4 = 2PH3 + 3SO4 2-). We take issue with the conclusion of Bains et al. [arXiv:2009.06499 (2020)] that the volcanic rates for such a mechanism would have to be implausibly high. We consider a mantle with the redox state similar to the Earth, magma originating deep in the mantle—a likely scenario for the origin of plume volcanism on Venus—and episodically high but plausible rates of volcanism on a Venus bereft of plate tectonics. We conclude that volcanism could supply an adequate amount of phosphide to produce phosphine. Our conclusion is supported by remote sensing observations of the Venusian atmosphere and surface that have been interpreted as indicative of currently active volcanism.