Mars was probably more like Earth billions of years ago. What happened?

Mars may not have always been the dry, cold, barren husk that it is today. Some scientists paint a picture of a shallow sea covering much of the Red Planet, making Earth's neighbor appear more like a version of our own planet. 

But that was probably about 4 billion years ago. A lot has changed since then, and scientists point to the planet's depleted atmosphere as the culprit.

Back then, Mars's atmosphere may have had a different composition and was likely thick enough to maintain an ocean of liquid water.

So scientists must ask "'How fast did it get thin, when did it get thin, and why?'" planetary scientist Christopher Edwards of the US Geological Survey told The Washington Post.

Sorting out this story is a bit like picking up pieces of a puzzle without knowing the picture they make.

Scientists are sampling various parts of the atmosphere in search of these interlocking clues. Some are looking at rocks, while others are looking at the atmosphere itself.

One study published last week found a hint that Mars's ancient atmosphere may have been rich with oxygen: Manganese oxide.

The compound forms in wet, oxygen-rich environments on Earth, so the study authors suggest that liquid water and an oxygen-rich atmosphere played a role in the formation of these deposits.

Another team of researchers spotted strange ripples in the sand on Mars. These ripples were unlike any seen on Earth and seem to be unique to the Red Planet. 

But what can ripples say about an ancient atmosphere? The scientists' analysis suggests that a thin atmosphere changed the way winds move across the sand, forming these distinct features. And these features have been fossilized in rocks as old as 3.7 billion years.

So these ripples could help to indicate when Mars might have lost much of its atmosphere.  But where did that atmosphere go?

Because Mars is smaller than Earth, it was likely more difficult for the planet to hold onto its protective blanket. Without a strong enough pull of gravity holding the atmosphere to the planet, radiation from the sun likely stripped away Mars's air. 

And the Martian atmosphere is actually still escaping. NASA's MAVEN mission is helping scientists study that process. 

The MAVEN spacecraft is currently orbiting Mars and is monitoring changes in the planet's atmosphere, particularly as solar activity changes. 

The scientists want to see how solar flares, radiation, and coronal mass ejections (CMEs) interact with Mars's atmosphere. But the sun hasn't been very cooperative. 

"We've been getting nothing because of the stupid solar cycle," Shannon Curry, a research scientist at the University of California, Berkeley and part of the MAVEN team, said at the 228th American Astronomical Society meeting in San Diego in June, according to Space.com. "We're really wishing for big storms at Mars and haven't really gotten any of them."

Still, MAVEN's measurements have helped scientists establish the baseline estimates for Mars's atmospheric loss. If the sun's activity was as minimal as it is now throughout its history, Mars would have lost about 63,000 gigatons of atmosphere, which would allow a layer of water about 15 feet deep to exist on the planet's surface.

Better understanding the Red Planet's escaping atmosphere could help scientists better understand exoplanets too.

"The solar environment we're talking about is specific to our sun today, but all of these are physical processes that occur on planetary bodies throughout the universe," Curry said.

And scientists have already spotted an exoplanet with an atmospheric tail streaming away from it, like Mars. A trail of hydrogen that stretches 9 million miles behind it is escaping from GJ 436b, a Neptune-sized planet.

GJ 436b is too far from its star to lose all of its atmosphere, but understanding Mars's atmospheric loss could help scientists better understand the formation of rocky super-Earths like this exoplanet.

"We're at the brink of starting to understand the habitability of terrestrial planets," Curry said. "Mars right now is our ground truth for planets of this size."