Venus may still have active volcanoes on its surface, deep beneath its suffocating clouds of sulphuric acid.
The planet has a number of volcanoes, but it was unclear if any of these were still active.
However, US scientists say they've found evidence of a volcanic eruption in data captured 30 years ago by the Magellan spacecraft.
Analysis of the Magellan data, published in Science and presented today at the 54th Lunar Planetary Science Conference in Houston, appears to show a volcanic vent that grew in size and changed shape over eight months.
"It's the first evidence of any kind of change on the surface [of Venus]," said the study's lead author Robert Herrick of the University of Alaska.
"It could be that we saw the only thing that's happened on Venus in the past million years and we just happened to get lucky and see it.
"But realistically, we now know that it's active enough that some changes occurred over a several-month time frame."
He said the discovery boded well for future missions to Venus such as NASA's VERITAS and the European Space Agency's EnVision missions, both of which he is involved with.
"Now we're confident we're going to see something exciting," he said.
Detail in the data
Professor Herrick worked as a graduate student on the Magellan mission, which mapped Venus using radar back in the early '90s.
"Magellan launched before the Berlin Wall came down — that was the last time NASA launched a mission to Venus," he said.
The spacecraft sent realms of data back to Earth, but only a small percentage stored on CDs could be analysed with technology available at the time.
Recently, Professor Herrick and his colleague Scott Hensley of NASA's Jet Propulsion Laboratory decided to revisit the data looking for signs of changes on the planet's surface that might be targets for future missions.
"It was a search for a needle in a haystack without any guarantee that there was a needle there."
But if they were going to find anything, they figured they'd find it near the tallest volcano on Venus, Maat Mons.
"If you were to bet on where the most likely place for an eruption to occur on Venus is, this would be it," Professor Herrick said.
Maat Mons is a large shield volcano similar to those found in Hawaii, which erupt every few years.
"There are a few dozen volcanoes on Venus that seem comparable to Hawaii," Professor Herrick said.
The Magellan spacecraft mapped parts of Venus over three cycles between 1991 and 1992, with a total of about 42 per cent of the planet imaged at least twice.
The scientists detected changes near the summit of Maat Mons in the first and third cycles, which viewed the planet from the same direction.
In the first images, the 2.2-square-kilometre-wide area appeared circular. Eight months later, it had nearly doubled in size and was kidney-shaped.
"The most reasonable interpretation of that data is there has been an eruption that has come up and changed the shape of the vent and filled it, so it looks like a lava lake that has filled up to near the rim," he said.
Professor Herrick said he was confident the shapes detected in the images were not due to the viewing angle, or caused by other processes such as a collapse.
"On Earth there has never been, as far as I know, a volcanic event that is changed by multiple kilometres without an eruption occurring somewhere nearby."
Downhill from the vent is an area that appears brighter in images obtained from the second cycle taken from a different angle.
"This suggests that perhaps a new flow [of lava] has formed," Professor Herrick said.
But, he said, unlike the other images, they can't rule out the feature in this second cycle could be an artefact of the viewing angle.
'Tip of the iceberg'
Stephen Kane, an Australian planetary scientist at the University of California, Riverside and not involved in the study, said the data was convincing.
Professor Kane said many scientists believed volcanism was happening on Venus, but there had only been indirect evidence such as clouds of sulphuric acid in the atmosphere and hotspots detected by infrared sensors by a European spacecraft in the early 2000s.
"This [study] is further evidence that indeed Venus still has this volcanic activity going on," said Professor Kane, who is the lead of another future mission to Venus called DAVINCI.
And, he said, it was likely to be the tip of the iceberg.
The new paper only looked at about 1.5 per cent of the Venusian surface, and Magellan only had the capacity to take low-resolution images.
"That means there's a lot of surface that's missed and a lot of smaller changes in size that are missed," Professor Kane said.
"So if this is real, this could mean that Venus is not just a little bit active, but substantially active, which is exciting."
Why do we want to find out if there are active volcanoes?
Finding out whether Venus still has active volcanoes is important to understand its evolution, as well as the habitability of similar planets in other solar systems.
One of the big questions about Venus, which is nearly the same size and mass of Earth, is whether or not it once had water.
The presence of large amounts of volcanic rock like granite suggests it did, Professor Herrick said.
"It's hard to make huge volumes of granite without some water," he said.
But, at some point in time, it became the hellish world it is today with a runaway greenhouse atmosphere dominated by carbon dioxide and sulphuric acid.
"I think one of the reasons that it failed as a habitable planet could be because it was still volcanically outgassing, but lost the ability to put carbon [in the atmosphere] back into the interior," Professor Kane said.
"So what we could be looking at now is the last remnants of that."
The only way to tell if there are still active volcanoes on Venus is to send spacecraft there to map it in high resolution
But already the launch of NASA's VERITAS mission has been pushed back from 2028 until at least 2031, which is the same time ESA's EnVision mission is due to launch, instead of staggering the launches.
"We would very much like to try and claw back some of the delay in the launch date of VERITAS," Professor Herrick said.
"That would really help the time-lapse aspect of studying volcanic activity."
