Mars could have entire oceans' worth of water locked in rocks deep underground, scientists say.

The finding suggests that ancient volcanic eruptions may have been major sources of water on early Mars—and could have created habitable environments.

According to a new study, Martian meteorites contain a surprising amount of hydrated minerals, which have water incorporated in their crystalline structures.

In fact, the study authors estimate that the Martian mantle currently contains between 70 and 300 parts per million of water—enough to cover the planet in liquid 660 to 3,300 feet (200 to 1,000 meters) deep.

"Basically the amount of water we're talking about is equal to or more than the amount in the upper mantle of the Earth," which contains 50 to 300 parts per million of water, said study leader Francis McCubbin, a planetary scientist at the University of New Mexico in Albuquerque.

And if water exists today in the Martian mantle, that means the red planet likely had a lot of water in its interior all the way back to the moment the planet formed.

"We don't have to rely on sources like comets and asteroids to bring in water afterward," McCubbin said. (See "What Created Earth's Oceans? Comet Offers New Clue.")

And if that's the case for Mars, it's probably the case for the other rocky planets—Mercury, Venus, and Earth—as well as for some large asteroids.

"Earth is not unique," McCubbin said. "We should be finding water nearly everywhere in the solar system."

Water Boiled Out of Mars Lava

McCubbin's team found water while analyzing meteorites that had been blasted off the Martian surface by asteroid impacts and sent careening to Earth.

The meteorites are basaltic, which means the rocks must have formed from deep magmas brought to the surface during volcanic eruptions.

By carefully examining a mineral called apatite, McCubbin's team found hydroxyl ions—a form of water that contains an oxygen atom bound to a hydrogen atom.

The presence of hydroxyl means that standard water—oxygen bound to two hydrogens—was also present in Martian magma. But because the hydroxyl is more tightly bound to rock than ordinary water, the ions remained behind when the rest of the water boiled out of the cooling lava.

From the amount of hydroxyl in the meteorites, it's possible to reconstruct how much water is in Mars's interior, McCubbin added.

"We're using apatite as a hydrometer to record how much water was in the rock before it degassed," he said. For instance, similar studies of lunar apatite in 2010 found that the moon's interior is a hundred times wetter than previously thought.

Furthermore, he said, the Mars meteorites examined in the new study came from extremely young basalts, only 150 to 350 million years old.

That means all large Martian eruptions throughout the planet's history probably carried substantial water to the surface—including eruptions that happened during the Noachian, the period when ancient Mars was warm enough to have possibly hosted liquid water on the surface.

It's also possible more recent eruptions might have created zones that were temporarily favorable to life as we know it.

"That makes these volcanic regions the most promising regions in which to look for past life on Mars," McCubbin said.