A vast shallow sea shimmered beneath oxygen-rich skies, with the rocky crests of cliffs and hills reflected in the still water below.
The landscape would have been familiar, except for its eerie desolation; nothing on the planet moved but the sands shifting in the wind.
This was Mars, circa maybe four billion years ago. Or at least, it is one vision of Mars by Dr Nina Lanza, a researcher at Los Alamos National Laboratory in New Mexico.
In a study published in the journal Geophysical Research Letters, she and her colleagues argue that the discovery of manganese oxide - which forms in wet, oxygen-rich conditions - on the Martian surface suggests that the planet was once much more Earth-like.
Now, of course, it is a frozen, barren wasteland, covered in dull red rock that has been bombarded and twisted into strange formations.
Another study of the geology of Mars, published in the journal Science, suggests that a particular kind of formation - a ripple in the sand that forms only in Mars' current thin, arid atmosphere - can be found in fossilised form going back 3.7 billion years.
These two studies seem to bracket an important and confusing period in Mars' history, when it started losing its atmosphere, and thus its ability to hold onto liquid water.
"Really the question again comes back to: How fast did it get thin, when did it get thin, and why?" said planetary scientist Christopher Edwards of the US Geological Survey.
These studies "are placing a constraint on that, which is great".
For years now, scientists have been gathering evidence that Mars was very different in its distant past. There are features on its surface that can form only in the presence of large bodies of water: traces of ancient tsunamis and clays at the bottom of vanished lakes.
A 2015 study analysing the ratios of two types of water molecule on Mars concluded that, at one point, it had an ocean covering 20 per cent of its surface. Dr Lanza's study adds to that body of work.
She and her colleagues were analysing rock data collected by the Curiosity rover when they saw signs of manganese oxide. On Earth, it is found only in rock layers younger than the first photosynthetic organisms, as it cannot be created without oxygen.
The manganese oxide appeared in a context very similar to how it is usually found on Earth: in subterranean deposits - left by groundwater - that were later exposed as the rock around it eroded.
That makes sense, Dr Lanza said, because manganese deposits are "always going to be the result of dissolved igneous rocks in water".
She believes that the manganese was dissolved in water that interacted with an oxygen-rich atmosphere, and was then deposited as manganese oxide.
She said: "It's sort of mysterious. But it's a very strong indicator of two things: liquid water, lots of it, and a strong oxidant somewhere.
"We know that doesn't happen on Mars today, so that opens up the question of, was there actually more oxygen in the atmosphere in Mars' past?
"We don't know how that could have been true. But these deposits are suggesting that it could have been."
That is a bold claim, as scientists have no evidence of how Mars could have had such an oxygen-rich atmosphere. On Earth, the oxygen came from photosynthesis, and that clearly was not happening on Mars.
Other geologists suggested Dr Lanza may have overlooked an alternative explanation: Another oxidant, perchlorate, could be behind the manganese oxide. But she said that the oxygen might have been released from Mars' ancient oceans.
As its atmosphere became thinner, the increased solar radiation that blasted the planet could have broken apart water molecules into their constituent elements: hydrogen, which would have drifted away, and oxygen.
Exactly why the atmosphere was lost is a mystery. Perhaps it was eroded by solar wind or blown away in a catastrophic asteroid impact. Or maybe Mars' low gravity was unable to hold onto it. Whatever the explanation, the fossilised ripples examined in the Science study offer a potential deadline for that transition.
Mr Mathieu Lapotre, a planetary geologist at the California Institute of Technology who was the lead author, was studying pictures of modern Mars' dunes taken by Curiosity when he noticed they took a form rarely seen on Earth.
In our atmosphere, sand is shaped by wind into centimetre-scale ripples and hundred metre-scale dunes. But Mars also hosted an intermediate bedform - ripples with wavelengths roughly a metre long.
Analysis of the ripples showed that they can form on the Martian surface because of its thin air; the light atmosphere causes winds to move across the sand differently than they would on Earth.
Mr Lapotre said: "That in itself is a cool implication because there is this new alien bedform that does not exist on Earth."
But it could also inform the discussion about Mars' transition to a dry, airless world. The intermediate- length ripples were found hardened in a rock formation believed to be as old as 3.7 billion years, suggesting that Mars' atmosphere was already thin at that point.
"We're looking at rocks younger than the rocks the manganese study looked at. Those two outcrops probably bracket the decline in atmospheric density," said Mr Lapotre.
Dr Edwards, who was not involved in either study, said: "This is the period that everyone is trying to understand." What happened on Mars has no parallel on Earth, so it's going to require a lot of analysing ancient Martian rocks to reconstruct.
"I don't think we have fully worked out the details," Dr Edwards said. "But we're on a path to understanding."