The Chasma Borealis is a 530-mile long canyon that cuts into Mars' northern polar ice cap, which was once covered in water. This image is a 3-D simulation of the chasm, created from THEMIS image data. Image: R. Luk / NASA / JPL / Arizona State University

In the beginning, Mars was a water world. But at some point in Mars' distant past, much of that water disappeared, leaving behind polar ice caps and a complex geology. Figuring out just where it went has been a major priority for scientists — life as we know it can't exist without water, and any future settlers would need a steady supply.

What's new: A new study, published Wednesday in Nature, suggests that much of what remains might in inaccessible. Some went into space, but even more of it may have sunk into the ground like a sponge, only to become bound up in minerals deep within the planet. "Mars, by virtue of its chemistry, was doomed from the start," study author Jon Wade, of Oxford University, tells Axios.

What they did: Wade and his team used data about the composition of Mars and its gravity to estimate how quickly the planet could have sucked up the water.

Their conclusion: The reactions "could have consumed a 3km thick ocean covering the entire Martian surface," says Wade. That is a lot of water.

The rocky history of water on Mars: The new study is the latest in a series of findings that have reshaped our story of the fourth planet's H20.

  • In 2015, scientists thought streaks of dark-colored earth on the sides of Martian mountains may have been caused by liquid flowing water.
  • But last month, hopes for liquid water on the Red Planet dried up: another group of researchers concluded the lines were caused by falling sand.
  • Another study found evidence that small, buried patches of ice might exist near the planet's equator. And researchers used radar to find a chunk of ice roughly the size of Lake Superior buried near Mars' Utopia Planitia region.
  • On Tuesday, some scientists suggested the best place to look for life on Mars would be in the planet's crust, where geothermal activity could have kept some of that ice water liquid. But if Wade and his colleagues are right, there may not be much subsurface ice available to melt.

Is there life on Mars? Or, more accurately: could Mars support life? Wade is unsure. "Running out of water, which is what happened on Mars, would have been catastrophic for life," says Wade. "I guess it's possible that there could still be very deep, ancient aquifers on Mars, but it's more likely that any liquid water would have simply reacted with the rocks forming hydrous minerals and so 'locking-up' the water."

The bottom line: Mars is the closest planet we have to explore, and in some ways it's a lot like Earth: It had water, is the right distance from the Sun to support life and has similar chemistry. When we look for planets that might support life, those are usually the indicators we look for.

But this shows that there are other factors to consider, says Wade: "It's also important to explore the subtleties, like a planet's accretionary history and mantle rock chemistry. These subtleties may play a significant role on whether the planet's surface can 'hang on' to water for lengths of time that are relevant to the evolution of complex life."

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