The bombardment of Mars by a special type of meteorite carried a large volume of water to the planet early in its history, according to new research in Science Advances.
"The amount of water delivered to Mars was enough to make a planet-wide ocean 300 meters thick," said Martin Bizzarro, corresponding author of the study and professor at the University of Copenhagen.
He and his colleagues derived this estimate using a measurement called a global equivalent layer (GEL), which is frequently used to conceptualize quantities of water on Mars. It means that if all the water brought by meteorites was in liquid form — resting on the planet's surface, rather than absorbed in its crust or tied up in ice — it would constitute a global ocean roughly 300 meters deep.
The team also traced the water-bearing meteorites, known as chondrites, to asteroids in the region of the outer solar system known to contain prebiotic elements essential for the emergence of life.
"Our study is the first to firmly establish that organic molecules relevant for life must have been present in the early evolution of the planet together with water," Bizzarro added.
Evidence from Mars orbiters indicates that there were likely expansive oceans on the planet at some point. But the debate about the origins and abundance of the ancient Martian water reservoir is ongoing. While much of the planet's water is thought to have formed from gas emissions by its cooling mantle, it also underwent a period where chondrites crashed into its surface. Planetary scientists suspected that a significant amount of water may have come from those chondrites.
"One important question in planetary science is understanding the source of the material that bombarded the terrestrial planets in their infancy," said Bizzarro. "This is important because this material could be water-rich and, hence, provide a source of volatile [molecules] to nascent planets."
To learn more about the chondrites' origins, the team examined chromium isotopes in 31 Martian meteorites, a diverse group representing most of the planet's geologic history.
"We know that water-rich asteroids that formed in the outer solar system have a different isotopic composition than dry asteroids formed in the inner part of the solar system," said Bizzarro. "We can use this variation in the isotopic make-up of material as a fingerprint of their formation region."
The analysis showed that bombarding chondrites that melted into and mixed with the planet's crust billions of years ago came from the outer solar system and therefore held water and carbon.
"These formed in a region of the solar system rich in volatiles and water, in the region beyond the orbit of Jupiter," said Bizzarro.
Estimating Water Volumes
After identifying the chondrites' origins, Bizzarro and his colleagues then investigated the volume of water that those chondrites could have brought to young Mars. They began by comparing the ratios of chromium likely sourced from both exotic chondrites and primordial mantle material in the meteorite samples.
"The magnitude of the isotopic shift allowed us to determine the mass of asteroids that had to be delivered to the surface of Mars. This, in turn, further allowed us to estimate how much water and organics [were] delivered to Mars by these asteroids," said Bizzarro.
Their analysis suggests that even if the chondrites held only 10% water, the rocks still would have transported liquid equal to more than 300 meters GEL on Mars. They also would have brought other elements that could support the creation of life.
"Two key ingredients required for life to emerge, namely water and prebiotic chemistry, were delivered to Mars during the first 100 [million years] of the planet's history. Remember that carbonaceous chondrites also contained complex organic molecules. If they delivered water, they must also have delivered organics," said Bizzarro.