Meteorites reveal that Martian water came from different sources

(CNN)One of the biggest mysteries in piecing together the story of Mars' past is a key question: Where did the water come from? Researchers may have found a large clue in tiny slices from Martian meteorites that fell to Earth, according to a new study. Mars was likely a warm, wet planet billions of years ago before its atmosphere was slowly stripped down and whisked out into space -- leaving behind the thin atmosphere and frozen desert planet we know today. But how did the water get to Mars in the first place?

To understand that, researchers have to look at the layers of Mars. Like any planet, it has a core, mantle, crust and atmosphere. Fortuitously, Martian meteorites contain samples of the planet's crust. The crust is also where the largest reservoir is estimated to be on Mars, containing 35% of the total estimated water beneath the surface.

The two well-known meteorites are known as Black Beauty and Allan Hills, and researchers studied thin slices of them to look into Mars' past, including how the planet formed and when water entered into the equation. Their study published Monday in the journal Nature Geoscience. The Black Beauty meteorite, which is estimated to be two million years old, formed and broke off of the planet when a massive impact hit Mars and laminated pieces of Martian crust together. This effectively also captured material from different points in the Martian timeline.

"This allowed us to form an idea of what Mars' crust looked like over several billions of years," said Jessica Barnes, study author and assistant professor of planetary sciences in the University of Arizona Lunar and Planetary Laboratory. When looking at the two meteorites, the researchers conducted a chemical analysis seeking out two types of hydrogen isotopes. Isotopes are the atoms that make up chemical elements. They were specifically looking for "light hydrogen" and "heavy hydrogen," because the ratio of these two isotopes can be used to understand the origin of water traces found in rocks.

For example, on Earth, scientists can study rocks and determine a similar ratio of hydrogen isotopes in all of them that translates to ocean water. But those values differ wildly in Martian meteorites, and none of them have been similar, the researchers said. The Black Beauty and Allan Hills meteorites suggested two different sources of water on Mars, based on their isotopes.