Two new studies are challenging the notion that the desolate Martian plains once brimmed with salty pools of water that could have supported some form of life.
Instead, the studies argue, the layered rock outcrops probed by NASA's robot rover Opportunity and interpreted as signs of ancient water could have been left by explosive volcanic ash or a meteorite impact eons ago.
That would suggest a far more violent and dry history than proposed by the scientists operating Opportunity and its twin rover, Spirit, on the other side of the planet.
The new scenarios, published in Thursday's journal Nature, paint a rather pessimistic view of whether the ancient Martian environment could have supported life.
Soon after NASA's robotic rover Opportunity began exploring Mars, it found minerals and rocks that its handlers said were evidence of a warm, wet history. But two groups of scientists have now questioned this interpretation.
Most scientists agree that water flowed on Mars's surface at some point. The planet has deep valleys similar to Earth's own Grand Canyon, which are thought to have been carved by rushing water.
But it is unclear whether all the water on Mars came in sudden bursts a long time ago, when meteorites battered the ice deposits of the young planet, or whether some stood about in warmish puddles later in Mars' life, which might have given life time to evolve.
When Opportunity touched down on a part of Mars called Meridiani Planum, it came across geology that looked tantalizingly like the product of standing water. The researchers running the mission wrote that "surface conditions at Meridiani may have been habitable for some period of time in martian history".
Opportunity found a scattering of tiny round pebbles that looked as if they had formed in water, and rectangular holes in the crater wall that could have been left by dissolved mineral crystals. The team also saw ripple patterns in the rock, and a layering that looked like sediments that had settled out of water.
But in this week's Nature, other researchers suggest an alternative, dry, explanation.