“On Earth, there’s never been that scale of change to a volcanic vent without an accompanying eruption of lava,” said Herrick, a research professor at the Geophysical Institute at the University of Alaska Fairbanks who published the findings in Science and presented them at the 54th Lunar Planetary Science Conference on Wednesday.

“The odds are that what this means is Venus is probably erupting every few months or so,” he added — though outside scientists said that the frequency of eruptions remains one of the big unknowns.

For years, scientists have been revising the notion that our nearest planetary neighbor is geologically dead, as different studies converge on the conclusion that some of its volcanoes are active even without plate tectonics, which fuels most of them here on Earth.

The question about what’s happening in Venus’s interior isn’t just a matter of neighborly nosiness. It builds on a deep mystery: How did Venus, so similar in size and starting ingredients to habitable Earth, turn out so savage? Venus is sometimes called Earth’s “evil twin” because its scorching hot surface is swathed in a dense carbon dioxide atmosphere that traps heat, its skies filled with clouds made up of droplets of sulfuric acid.

“This study is really important,” said James W. Head, a Venus expert and professor of geological sciences at Brown University who was not involved in the research. “Could this be the way Earth was in its earliest history? Or the way Earth is headed in its future?”

Three upcoming missions to Venus over the next decade will help scientists probe its geology and evolution. They’ve become a priority in part because understanding the interactions between Venus’s interior and its atmosphere could help scientists as they investigate the potential for life on similar-size planets in other star systems.

One of the ongoing questions about Venus is whether it was once habitable and Earth-like but then transitioned to a thick, hothouse atmosphere, driven by volcanic activity. Getting a better estimate for how frequent and widespread eruptions have been will help scientists refine their models for how the planet evolved into such a hot mess.

To do the study, Herrick and his co-author, Scott Hensley of NASA’s Jet Propulsion Laboratory, scoured decades-old archives. NASA’s Magellan mission took radar images of Venus in the 1990s, mapping the planet’s surface in detail before it plunged through the thick atmosphere to its demise. The computational power to examine those images was crude by modern standards. Data was sent out to scientists in boxes and boxes of compact discs, and Herrick recalled that getting a single image to load on a screen could take a good 10 seconds.

In the years since, the images have been mosaicked together, allowing scientists to zoom across Venus’s surface like someone panning through Google Earth and examine old data in new ways. Herrick decided to see if, buried in the Magellan images, there was any evidence of volcanic activity from the times when the spacecraft looked at the same spot more than once.

He likened his quest to “searching for a needle in a haystack, without any guarantee the needle existed.”

It was a long shot, because Magellan wasn’t looking continuously at all areas of the planet — essentially, as it orbited Venus and the planet rotated below, the spacecraft mapped some places more than once, like taking a “before” and “after” shot. But most volcanoes are not constantly erupting, so even if a similar spacecraft hovered over one volcanically active spot on Earth, it could come away with the finding that nothing much was happening based on just a few snapshots.

Herrick decided to study a specific area that was considered a likely candidate for volcanic activity — near Maat Mons and Ozza, two of the largest volcanoes on the planet. To his delight, he found a volcanic vent that clearly changed shape over time. In the later image, there was also evidence of potential new lava flows downhill. The second shot could have been after the eruption, or even during it.

Interpreting changes in radar images is difficult, and this kind of evidence falls short of a video of an erupting volcano on Venus. But several outside scientists said it was convincing, particularly in light of other recent studies.

“As I like to say, to talk about the possible volcanism on Venus, one [piece of] evidence is not enough,” said Jérémy Brossier, a planetary geologist at Italy’s National Institute for Astrophysics. “This alone would not be convincing. You need this, plus other papers. All of this is justifying this huge thinking that: Ah, Venus is maybe still active today.”

In the first image, the volcanic vent is circular and deep, with steep walls. Eight months later, it is shallower — and appears to have filled in.

“What might have happened is that an eruption occurred and a lava lake nearly filled it up to the rim,” Herrick said.

In the next decade, Venus will be a major target for space exploration. NASA has an orbiter mission called VERITAS on the docket, as well as an orbiter and probe called DAVINCI+. The European Space Agency plans to launch an orbiter called EnVision.

Scientists hope that these missions will help answer questions about just how volcanically active Venus is, and studies like this one will help decide where to focus high-resolution measurements.

Scientists believe that the smattering of observations of Venus’s volcanic activity so far are just the beginning.

“When I say it’s the tip of the iceberg, I really do mean that,” said Stephen Kane, a professor of planetary astrophysics at the University of California at Riverside. “I suspect there’s a lot more going on we haven’t yet detected.”