It's hard to picture two more similar yet different worlds than Earth and Venus.
Our own world is lush, wet, and vibrant, teeming with life and geological activity.
Venus, by contrast, is like an evil twin.
It shares DNA with Earth – the two worlds have more or less the same size, density, and mineral composition – but it has no liquid surface water, ambient temperatures hot enough to melt lead, crushing atmospheric pressure, and choking clouds of carbon dioxide that rain sulfuric acid.
Scientists have long wondered whether Venus was always this hostile.
While some models suggest our neighboring planet may once have been covered by oceans, others argue any water disappeared so early that no trace should remain today.
Now, a new Earth & Planetary Science Letters study argues the surface of Venus may still bear the geological scars of those long-lost seas.
"While the interpretation of these features is not unambiguous," writes a team led by geologist Richard Ghail of the University of London in the UK, "there is a compelling case that Venus once supported oceans, and perhaps life, across most of its surface."
The surface of Venus is not easy to see through its thick layer of cloud. Most of the available data today comes from NASA's Magellan probe, which spent several years in the early 1990s using radar to map Venus's surface from planetary orbit.
Those images revealed a rugged landscape shaped by volcanism – that Venus is a volcanic world is without question, with over 85,000 volcanic features identified to date.
What Ghail and his colleagues wondered was whether some of those putatively volcanic features might actually be marine in origin.
If so, the oceans they left behind would have been immense. The researchers estimate water may once have covered around 90 percent of Venus's surface, holding roughly 40 percent as much water as Earth's oceans.
They argue three types of geological features make more sense in the context of those vanished seas.
The first are vast terrains crisscrossed by giant polygonal patterns.
Previous studies had suggested these fractures formed as volcanic rock cooled and contracted.
Ghail and his colleagues argue they more closely resemble polygonal fault systems found in clay-rich marine sediments on Earth.
These fracture networks form as thick layers of seafloor mud are buried and compacted, squeezing out water like a sponge. As the sediments shrink and settle, they can develop polygonal patterns stretching for kilometers.
The second clue lies in enormous channels known as canali.
For decades, these have generally been interpreted as lava channels. But this explanation is not without problems. Some of the channels stretch for thousands of kilometers, farther than many researchers think lava could realistically flow.
The researchers posit that many of these channels instead resemble submarine channels on Earth, carved by dense currents flowing beneath the ocean's surface.
They note that nearly a third of the polygonal terrains are associated with canali that appear to emerge from the plains without an obvious volcanic source, before terminating in deeper basins – just as underwater channels would be expected to do.
Finally, the third clue comes from wrinkle ridges that pattern Venus's lowlands.
These ridges, the team says, may overlie thick layers of salt left behind as ancient oceans evaporated.
They note that Earth's Mediterranean Sea deposited vast salt layers when it largely dried up during the Messinian Salinity Crisis around 5.5 million years ago. Venus, they estimate, could have produced a salt layer at least 64 meters (210 feet) thick – perhaps even thicker – which would have been sufficient to deform the overlying crust.
It's a fascinating hypothesis, but one that is likely to remain controversial until new spacecraft arrive to study Venus, which has recently been somewhat neglected by space exploration programs.
However, ESA's upcoming EnVision mission and NASA's VERITAS orbiter, both expected to map Venus in far greater detail than Magellan, could help test whether these features really do preserve evidence of ancient oceans.
Related: Giant Concentric Rings Discovered in The Atmosphere of Venus
None of the three lines of evidence is conclusive on its own. Each could, in principle, be explained by volcanic activity, tectonic processes, or other aspects of Venus's geological history.
Taken together, however, the researchers argue they paint a compelling picture of a world that once hosted oceans before succumbing to a runaway greenhouse effect less than a billion years ago.
"On Earth, life was well-established by that time; the possibility that a runaway greenhouse might have eradicated it on Venus has profound implications for our own future and for life outside the Solar System," the researchers write.
The paper has been published in Earth & Planetary Science Letters.
This article was fact-checked and edited by Fiona MacDonald. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.