Water on Mars could have formed as a result of the impact of an ancient asteroid

Asteroids that bombarded Mars almost 4.5 billion years ago may have brought enough water to form a global ocean 1,000 feet (300 meters) deep.

Scientists base this conclusion on the analysis of 31 meteorites from Mars, which were found on Earth. The tantalizing results may 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.

“Seeing water-rich asteroids bombarding Mars means they may have also contributed to Earth, but it’s hard to quantify,” says Martin Bizzarro, a space chemist at the University of Copenhagen in Denmark and co-author of the new study. . research, reported. “Unlike Mars, Earth has plate tectonics and the early records of our planet’s history have been erased.”

On the subject: 12 amazing photos of the first year of the Perseverance rover on Mars

Therefore, the researchers turned to Mars, and some of the rocks thrown from Mars by giant impacts fell to Earth. These meteorites serve as little pieces of Mars on Earth for scientists to study, and they record the history of water on the Red Planet in the form of isotopes—slightly different flavors of the element, each with a different number of neutrons per molecule. its core.

Scientists led by Ke Zhu of the University of Paris and the University of Bristol in the UK have measured the relative abundances of chromium-54 and chromium-53 in meteorites and found that the high proportion of chromium-54 is close to that of chromium-54. a type of asteroid called a carbonaceous chondrite. In particular, isotope analysis points to a subset of carbonaceous chondrites associated with the Renazzo meteorite that fell in 1824. Scientists believe this meteorite comes from a water-rich population that formed outside the giant planets of our solar system. These asteroids can contain up to 10% water by mass.

Not all Martian water was formed by impacts of carbonaceous chondrites during the first 100 million years of the solar system’s history. Much water has also reached the surface of Mars as a result of the release of gas from the molten mantle of the Red Planet. How much water was released remains a mystery, but along with the release of gas and impacts, enough water could have entered the surface of Mars to form a global ocean up to 0.9 miles (1.5 km) deep.

Scientists are hotly debating where Mars and Earth get their water from. Studies of rocks brought back from the Moon by the Apollo missions contain traces of water, suggesting that the Earth contained at least some water during the giant impact that formed the Moon.

As on Mars, water on Earth could have evaporated and then been replenished by impacts. Scientists have proposed many potential impactors, with research focusing on comets or asteroids. Interestingly, the water inside carbonaceous chondrites resembles that of Earth’s oceans in terms of the ratio of deuterium to hydrogen (D/H) (deuterium is a heavy isotope of hydrogen with an added neutron). However, it is difficult to prove that most of the water on Earth appeared here, because our planet has destroyed much of its ancient crust.

Mars has a geological advantage because it hasn’t changed much in billions of years. Although impacts and water currents have affected the surface, there are no plate tectonics on Mars that churn up the planet’s crust and recycle it in the deep layers of the mantle. Therefore, the surface we see on Mars today is the same as it was 4.5 billion years ago. This greatly simplifies the determination of the geological record of Mars and the origin of its water.

However, the old surface of the Red Planet also complicates the history of this water. The once abundant water on Mars has mostly seeped into space over billions of years. NASA MAVEN (Mars Atmosphere and Volatile Evolution) traveled to Mars in 2014 to measure the current rate of atmospheric loss.

But estimates of historical water loss to space to date have been based on the D/H ratio of water in the Martian mantle. In the Martian atmosphere, water molecules are exploded by ultraviolet radiation from the sun, which breaks them down into their constituent atoms of oxygen and hydrogen or deuterium. Since deuterium is heavier than ordinary hydrogen, it does not escape into space as quickly, so the proportion of deuterium on Mars compared to ordinary hydrogen increases over time. If the D/H ratio that started Martian water is known, then the amount of water lost in space can be calculated.

However, carbonaceous chondrites have a different, higher D/H ratio than the Martian mantle, so using only mantle D/H measurements to calculate water loss will skew the result, giving the impression that more water has escaped than is actually the case. .

This leads to a problem, as scientists have an estimate of how much water has ended up on Mars. If less water has leaked in history, then there must be more water lurking somewhere on Mars, in addition to what is trapped in polar ice deposits.

“There must be a water tank that we can’t see,” Bizzarro said. “People have hypothesized that this reservoir may be in the Earth’s crust as hydrated minerals – i.e. clay – or buried ice deposits.”

The current known amount of water left on Mars, if all of it were liquidized on the surface, would form a global ocean 66 feet (20 m) deep. The invisible reservoir could be much larger, enough to create a global ocean of 330 feet (100 m) to 3300 feet (1000 m).

Zhu and Bizzarro’s team calculated that carbon-bearing chondrites weighing between 4.5 x 10 kg and 6 x 10 kg collided with water on Mars, based on evidence from the oldest impact craters on the Red Planet, including the huge impact that created Mars. north-south dichotomy (lowlands in the north and highlands in the south).

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

In addition to water, asteroid impacts could also have brought various types of organic carbon to Mars. This carbon is the very substance that is necessary for the basic chemistry of life. This is due to the hypothesis that carbon isotopes detected by NASA’s Curiosity rover in the sediments of an ancient lake in Gale Crater were brought to Mars by impacts.

Indeed, Zhu’s team’s work is the first study to establish with some certainty that organic carbon molecules essential to life were brought to Mars at the same time as water. The merger of both of these components, vital for life, confirms the fact that ancient Mars could be habitable.

The results were published Wednesday (November 16) in the journal Science Advances.

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