It’s become clear in recent years that Mars had lots of water in its distant past. But that raises the question—when did Mars stop being so Earth-like? And what happened to cause the change?

One way to address such questions is by analyzing rocks from different times in the planet's history. While no samples from the red planet have ever been brought back, meteorites have landed on Earth that scientists identified as originating on Mars.

One such meteorite is called Northwest Africa (NWA) 7533. This dark, glossy meteorite originally formed some 4.43 billion years ago, not long after Mars itself finished accreting. And, crucially, it contains zircon grains, which provide clues to the planet’s past.

Zircons

Zircons, or more properly zirconium silicates, are minerals that form when lava cools both on Mars and on Earth. They are useful for geological study because they can be used for dating.

“When you find a zircon, it’s like finding a watch,” Munir Humayun, professor of geoscience at Florida State University and one of the paper’s lead authors, told his university's news site. “A zircon begins keeping track of time from the moment it’s born.”

In this case, the zircons are significant because they contain a distinctive ratio of oxygen isotopes. On Earth, when zircons are in a metamict state—that is, when their crystal structure has gradually decayed, leaving an amorphous mineral—they can exchange oxygen isotopes with a fluid. This fluid can be water vapor from the atmosphere, mixed with other gasses like carbon dioxide and ozone. These sources typically contain different isotopes of oxygen.

As a result, the oxygen isotope ratios present in the zircon can provide some indication of when the mineral was exposed to water.

Because of the properties of zircon, scientists were able to date when this likely happened in the case of NWA 7533. They found that oxygen was probably exchanged between 1.7 and 1.4 billion years ago. That’s when the zircon had become metamict, and thus susceptible to oxygenation.

How was the zircon's crystal structure altered to make it susceptible? Unlike Earth, Mars has no plate tectonics, which would allow oxygen isotopes from the oceans and atmosphere to get mixed in with its rocky lithosphere. If it did have plate tectonics, the hydrosphere and the lithosphere would eventually reach an equilibrium, with oxygen isotopes spread evenly between them. Instead, they remain largely separate, except for cases where the surface is violently stirred up—say, by asteroid impacts.

That’s how scientists know that the variety of oxygen seen in the zircons must have gotten into the lithosphere during such an event.

Primary atmosphere

The new study also sheds light on another piece of Martian history: the loss of its primary atmosphere.

Early in their histories, the terrestrial planets all had thick atmospheres. Unlike the present-day (secondary) atmospheres, the primary atmospheres had roughly the same makeup as the original nebula that formed the Solar System, which is mostly hydrogen and helium. Jupiter still has this makeup, as does the Sun, but Earth, Mars, and the other rocky planets have long since lost their primary atmospheres to space.

Because NWA 7533 formed so soon after Mars itself, it should show evidence of interaction with the primary atmosphere, provided that the atmosphere survived for long after the meteorite’s formation. But as there's no sign of this interaction, the authors conclude that the primary atmosphere was gone before the meteorite formed, less than 120 million years after Mars finished accreting.

“We now have an isotopic record of how the atmosphere changed, with dates on it,” said Humayun.

Dry Mars

The record in the zircon tells us more than just the timing of the primary atmosphere’s loss; it also reveals how long Mars has been in its present dry state. The event that took place between 1.7 and 1.4 billion years ago, in which oxygen isotopes were exchanged with the atmosphere and hydrosphere. It's described in the paper as a “major heating event," one that may even be the cause of the planet’s current conditions.

“Now we can conclude that the conditions that we see today on Mars, this dry Martian desert, must have persisted for at least the past 1.7 billion years,” said Humayun. “We know now that Mars has been dry for a very long time.” That still leaves plenty of time in the Martian past for a wet planet, one we might have found hospitable.

While this study sheds significant light on the Red Planet’s mysterious past, mysteries remain—mysteries that the team hopes to address as they continue their research with NWA 7533.

Nature Geoscience, 2014. DOI: 10.1038/NGEO2231 (About DOIs).