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Icy Asteroids Dumped Huge Amounts of Water on Ancient Mars, Scientists Say

A new estimate of water on early Mars suggests it had enough of the wet stuff to cover the planet in a 1,000-foot-deep ocean.

Illustration of reddish Mars with a big splash of a blue ocean across the top of the planet.
NASA created this illustration of an ocean on Mars to accompany a 2015 study on water in the planet's early history.
NASA/GSFC

This story is part of Welcome to Mars, our series exploring the red planet.

It's hard to imagine that the dry, dusty and inhospitable Mars we're familiar with today could have been a giant swimming pool. But go ahead and picture a blue marble shining in space. 

Scientists are pretty sure there was a decent amount of water on Mars in its earlier days. An international team has worked out a new estimate for just how much water the planet might have had 4.5 billion years ago. Their conclusion? A lot. So much it could have covered the planet in a global ocean 1,000 feet (300 meters) deep if it was all spread out. 

The study published in Science Advances on Wednesday points to asteroids as a delivery mechanism for water on early Mars. "At this time, Mars was bombarded with asteroids filled with ice. It happened in the first 100 million years of the planet's evolution," said study co-author Martin Bizzarro in a University of Copenhagen statement on Thursday. The researchers saw clues to this activity by studying the composition of Martian meteorites found on Earth. They worked out how much of the precious liquid the asteroids might have packed when they dive-bombed the planet long ago. 

This isn't just a story of water, it's also a story of how early Mars could have been habitable. "Another interesting angle is that the asteroids also carried organic molecules that are biologically important for life," Bizzarro said. Mars may have been welcoming to life long before Earth was. 

Researchers have seen lots of evidence for past oceans and water flow on Mars. NASA's Perseverance rover is currently investigating an ancient river delta and lakebed in the Jezero Crater as it looks for signs of ancient life. A 2015 study suggested the planet hosted a primitive ocean bigger than Earth's Arctic Ocean. That paper estimated there was enough water for a global ocean 450 feet (137 meters) deep. The new work bumps that up considerably.

Over time, Mars lost much of its water, though researchers are still working out where it went. Some was likely lost to space, but some might have been trapped in minerals in the planet's crust. And there's an ongoing debate about possible reservoirs of liquid water hiding out under the Martian polar cap.

Scientists will continue to stay busy teasing out the history of Mars water. We might not be able to swim there today, but the ancient past was a different story.