On March 1, 2020, humanity’s only spacecraft orbiting Venus—Japan’s Akatsuki—saw a mysterious flash in the planet’s alien skies. The flicker could provide crucial evidence in a 40-year quest to answer a perplexing planetary puzzle: Is there lightning on this cloud-shrouded world?
Lightning is found all over the solar system. Spacecraft have detected extraterrestrial lightning strikes in the clouds of Jupiter, Saturn, and Uranus. Cloaked in thick clouds, “we expect there to be lightning on Venus” as well, says Noam Izenberg, a planetary geologist at Johns Hopkins University and deputy chair of the Venus Exploration Analysis Group.
The flash seen by the spacecraft Akatsuki, which means “Dawn” in Japanese, was revealed by planetary scientist Yukihiro Takahashi of Hokkaido University at this year’s gathering of the American Geophysical Union. Takahashi’s team suspects it was either a powerful lightning strike, roughly 10 times more energetic than lightning on Earth, or a large meteor that exploded in the planet’s atmosphere.
The flash was spotted by the craft’s Lightning and Airglow Camera, an instrument that has been scanning the clouds of Venus for five years—only now picking up its first flash of light. It’s one of the most promising signs of lightning on Venus, but the team is still analyzing the data, and the members have declined to talk about the research until it has been published in a peer-reviewed paper.
“The existence of lightning in Venus has been controversial for many decades,” Takahashi said during his talk.
Tantalizing evidence of Venusian lightning has been spotted before, from electromagnetic pulses measured by spacecraft to blips of light seen from Earth. But each time, scientists have questioned whether the signals were coming from lightning or another source, such as flashes of particles from deep space known as cosmic rays, or from noise created by the scientific instruments themselves.
To identify the source of the recent flash, astronomers are hoping to see another one. “It’s intriguing, and they are doing the work to rule out other things,” says Izenberg, who is not involved in the new research. But “the proof is going to be in the pudding of seeing it again.”
If the flash was lightning, it’s discovery would be a major step toward deciphering the mysterious nature of Venus’s thick clouds—including providing a clue as to whether such an environment could support life. “[Lightning] can break apart atoms, and it gives you free radicals which recombine and form molecules which you otherwise wouldn’t expect to get,” says Colin Wilson, a planetary scientist at the University of Oxford.
Whistles in the maelstrom
Scientists have been looking for lightning on Venus for nearly half a century, searching through telescopes and monitoring for telltale electromagnetic sizzling with spacecraft. NASA’s Cassini spacecraft, which could easily detect lightning on Earth, flew by Venus twice in the late 1990s on its way to Saturn and didn’t pick up any flashes.
But earlier clues exist. Some of the Soviet Union’s Venera landers, launched between the 1960s and 1980s, registered suspicious pings on magnetic and acoustic sensors. The United States’ Pioneer Venus Orbiter picked up energetic bursts in the 1980s, as did the radio attached to the Galileo probe as it zipped by in 1990 on the way to Jupiter. A ground-based telescope also saw several faint luminous splotches on Venus in the mid-1990s.
“None of these have been completely convincing,” says Karen Aplin, a physicist at the University of Bristol who studies planetary lightning. “In general, it has been difficult to exclude the possibility of other explanations.”
The European Space Agency’s Venus Express spacecraft, which orbited the planet from 2006 to 2015, heard many “whistler-mode” radio waves coming from the planet. On Earth, these signals—named by radio operators during World War I who heard whistling sounds on the radio and worried they might be incoming grenades—can be generated by lightning.
However, “whistler-mode waves can be generated by any kind of instability or disturbance within the atmosphere,” says Shannon Curry, a planetary physicist at the University of California, Berkeley. They are heard regularly emanating from Venus and Mars, and it’s possible that these signals are coming from elusive lightning, but astronomers can’t be sure.
Seeing is believing
Most of the optical searches for lightning—looking for the visible flashes—have yielded no results. One possibility, Wilson says, is that “the source of the lightning is below the top of the clouds, which means the radio waves get out, but a lot of the light is blocked.”
The Akatsuki spacecraft can search for faint flashes of light escaping Venus’s clouds. However, because the craft experienced an engine malfunction and failed to enter orbit around Venus in 2010, it had to circle around the solar system and try again in 2015. Although Akatsuki successfully entered Venus orbit on the second try, it had to settle for an overly elongated orbit that keeps the craft far away from the planet most of the time.
After half a decade, though, Akatsuki spotted a flash of light. “I’m surprised they didn’t see it again,” Curry says. “The fact that they only saw it once is what bothers me,” as lightning should appear in clusters. But “the detection itself I’m willing to believe.”
The flash doesn’t look like it could have been caused by a cosmic ray, though the Akatsuki team believes it could have been a bolide—a meteor that explodes in the atmosphere with a bright flash. The odds of Akatsuki seeing a bolide, based on what we know about how often they hit planets, however, is exceedingly unlikely.
For now, the leading explanation is lightning.
“An isolated case of instrument error that happens to look like a real signal would be an incredible coincidence,” says Ricky Hart, a graduate student at the University of California, Los Angeles, who studies signals from possible lightning on Venus. The flash, he says, “adds a good deal of support to the argument for lightning on Venus.”
Mysteries in thick alien clouds
If the flash is lightning, what’s making it? Astronomers hunting for the answer to this question believe it could revolutionize what we know about the skies of Venus.
The planet’s sulfuric acid clouds are unique in the solar system, so traditional models of lightning generation don’t apply to Venus, Aplin says. One issue is that its clouds are thought to conduct electricity relatively well, which may prevent electricity from accumulating in one place to the point where lightning is triggered.
Earth’s clouds separate electrically charged water droplets and ice crystals through convection—when warmer clouds move up and cooler clouds sink down—leading to lightning. But it’s not clear how much vertical mixing happens in Venus’ clouds, says Paul Byrne, a planetary scientist at North Carolina State University. And Akatsuki can’t place the altitude of the flash, so if it was lightning, it could have struck from anywhere between the upper atmosphere and the main cloud deck tens of miles deeper.
One possibility is that flashes of lightning on Venus follow volcanic eruptions. Although no eruption has yet been directly observed through the planet’s obscuring clouds, circumstantial evidence has many planetary scientists convinced that eruptions are nevertheless taking place. Explosive events producing ashy, electrically excited plumes could generate bolts of lightning.
Whether this detection turns out to be genuine or not, planetary scientists will continue to search for more flashes, eager to know if lightning’s alchemic power is at work on Venus.
“Lightning is charismatic, as a process. It’s active,” Izenberg says. It “could be one of the potential engines for prebiotic chemistry on Venus,” meaning the blasts of energy could stitch together molecules that are needed for life. If this process occurs in parts of the atmosphere that are known to be watery, temperate, and sunlit, it could create a potential haven for photosynthetic microbes.
Lightning may also be responsible for making the gas phosphine, a compound that was recently detected on Venus—though some experts have questioned whether the detection is valid—and is known to be produced by microbes on Earth. If this gas really does exist in the Venusian clouds, some of it may be forged by lightning interacting with the atmosphere.
Dual observations made by terrestrial telescopes and Akatsuki would go a long way to convincing the community that lightning has been identified, Curry says. But until humanity sends a new mission to Venus to dive through the atmosphere or fly close to the cloud tops, the presence of lightning will likely remain an open question, Byrne says.
We know surprisingly little about Venus, a world that is about the same size and composition as Earth but whose evolution has differed dramatically. This flash, Izenberg says, is “yet another argument that says we have to go back.”