Mars' water may have come from ancient asteroid impacts

Asteroids bombarding Mars nearly 4.5 billion years ago may have delivered enough water to create a global ocean 1,000 feet (300 meters) deep.

Scientists based this conclusion on analysis of 31 meteorites from Mars that have been discovered on Earth. The tantalizing results could point to a hidden reservoir of water still present on the Red Planet today. In addition, the work may have implications for understanding not only the early history of the Red Planet, but also Earth's own past.

"The observation that water-rich asteroids bombarded Mars means that there may also have been a contribution to Earth, but this is difficult to quantify," Martin Bizzarro, a cosmochemist at the University of Copenhagen in Denmark and a co-author on the new research, told Space.com. "Unlike Mars, Earth has plate tectonics and the early record of our planet's history has been erased."

Related: 12 amazing photos from the Perseverance rover's 1st year on Mars

So the researchers turned to Mars, and particular rocks blasted off Mars by giant impacts that have fallen to Earth. These meteorites serve as little pieces of Mars on Earth for scientists to study, and they carry a record of the history of water on the Red Planet in the form of isotopes — slightly different flavors of an element, each with a different number of neutrons in its core.

The scientists, led by Ke Zhu of Paris University and the University of Bristol in the U.K., measured the relative abundances of chromium-54 and chromium-53 in the meteorites, finding the high proportion of chromium-54 to be close to that of a type of asteroid called a carbonaceous chondrite. More specifically, the isotopic analysis points to a subset of carbonaceous chondrites that are related to the Renazzo meteorite that fell in 1824. Scientists believe this meteorite hails from a population of water-rich bodies that formed beyond the giant planets of our solar system. These asteroids can contain as much as 10% water by mass.

Not all of Mars' water would have originated from the impacts of carbonaceous chondrites during the first 100 million years of the solar system's history. A lot of water would have also reached Mars' surface by outgassing from the Red Planet's molten mantle. Just how much water outgassed remains a mystery, but together outgassing and impacts could have put enough water onto the Martian surface to create a global ocean up to 0.9 miles (1.5 kilometers) deep. 

Scientists hotly debate where Mars and Earth got their water. Studies of rocks brought back from Earth's moon by the Apollo missions contain traces of water, suggesting that Earth contained at least some water at the time of the giant impact that formed the moon. 

Like on Mars, Earth's water could have outgassed and then been supplemented by impacts. Scientists have suggested a variety of potential impactors, with research focusing on comets or asteroids. Intriguingly, the water inside carbonaceous chondrites resembles that of Earth's oceans in terms of it's deuterium to hydrogen (D/H) ratio (deuterium is a heavy isotope of hydrogen with an added neutron). However, proving that this is where much of Earth's water came from is difficult because our planet has destroyed much of its ancient crust.

Mars has a geological advantage because it has been largely unchanged for billions of years. Although impacts and flowing water have affected the surface, Mars doesn't have plate tectonics to churn up the planet's crust and recycle it in the deep mantle. Consequently, the surface we see on Mars today is the same surface it had 4.5 billion years ago. This makes it far simpler to determine Mars' geological record and the origin of its water.

However, the Red Planet's old surface also complicates the history of that water. Mars' once plentiful water has, over billions of years, mostly leaked into space. NASA's MAVEN (Mars Atmosphere and Volatile Evolution) ventured to Mars in 2014 to measure the current rate of atmospheric loss. 

But estimates of historical water loss to space have to date been based on the D/H ratio of water in Mars' mantle. In Mars' atmosphere, water molecules are blasted by ultraviolet radiation from the sun, which breaks them apart into their component atoms of oxygen and hydrogen or deuterium. Because deuterium is heavier than regular hydrogen, it doesn't escape into space as quickly, so the proportion of deuterium on Mars relative to regular hydrogen increases over history. If one knows the D/H ratio that Mars' water started off with, then one can calculate the amount of water lost to space.

However, carbonaceous chondrites have a different, higher D/H ratio compared to Mars' mantle, so therefore using only the mantle D/H measurement to calculate water loss will skew the result, making it seem as though more water has escaped than really has. 

This leads to a problem, since scientists have an estimate for how much water all told has made it to Mars. If less water has escaped over history, then there must be more water still lurking somewhere on Mars besides that which is locked up in the polar ice deposits.

"There must be a reservoir of water that we don't see," Bizzarro said. "People have hypothesized that this reservoir could reside in the crust in the form of hydrated minerals — i.e., clays — or buried ice deposits."

The current known amount of water left on Mars, if it all turned into liquid at the surface, would form a global ocean 66 feet (20 m) deep. The unseen reservoir could be much larger, enough to create a global ocean between 330 feet (100 m) and 3,300 feet (1,000 m).

Zhu and Bizzarro's team estimate that carbonaceous chondrites totaling between 4.5 x 10^20 kilograms and 6 x 10^21 kilograms impacted Mars with their water, based on evidence of the most ancient impact craters on the Red Planet, including the immense collision that created Mars' north–south dichotomy (the lowlands to the north and the highlands to the south). 

If the lower estimate is correct, then simply mixing the asteroidal material with the upper 2.5 miles (4 km) of Martian crust would create the composition detected in the Martian meteorites. On the other hand, if the upper limit is correct, then the entire crust, which averages 28 miles (45 km) deep, would have needed to mix with asteroidal material and water to produce the results scientists found.

Besides water, the asteroidal impacts would have also brought various kinds of organic carbon to Mars. This carbon is the very stuff necessary for the basic chemistry of life. This connects to the hypothesis that isotopes of carbon found by NASA's Curiosity rover in ancient lake sediments within Gale Crater were brought to Mars by impacts.

Indeed, the work of Zhu's team is the first study to ascertain with some confidence that organic carbon molecules important for life were brought to Mars at the same time as water. The confluence of both these ingredients vital for life supports the case that ancient Mars may have been habitable.

The findings were published Wednesday (Nov. 16) in the journal Science Advances.

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