If you asked someone who was reasonably scientifically literate how Earth got its water, they’d likely tell you it came from asteroids—or maybe comets and planetesimals, too—that crashed into our planet in its early days. There’s detail, nuance, and uncertainty around that idea, but it’s widely believed to be the most likely reason that Earth has so much water.
But a new explanation for Earth’s water is emerging. It says that the water comes along for the ride when Earth formed out of the solar nebula.
If that’s correct, it means that most rocky planets might have water for at least a portion of their lives.
A new paper presents evidence suggesting that water isn’t delivered to rocky planets but forms as part of the planet-forming process itself. The title of the paper is “Early oxidation of the martian crust triggered by impacts.” The lead author is Zhengbin Deng, an Assistant Professor at the Centre for Star and Planet Formation, Globe Institute, University of Copenhagen. The study is published in the journal Science Advances.
“There are two hypotheses about the emergence of water. One is that it arrives on planets by accident, when asteroids containing water collide with the planet in question,” said co-author Professor Martin Bizzarro in a press release. Bizarro is also from the Centre for Star and Planet Formation at the University of Copenhagen.
“The other hypothesis is that water emerges in connection with the formation of the planet. Our study suggests that this hypothesis is correct, and if that is true, it is extremely exciting, because it means that the presence of water is a bioproduct of the planet formation process,” Martin Bizzarro explains.
The evidence for this hypothesis comes from a small meteorite named Black Beauty. Black Beauty (aka Northwest Africa 7034) is a chunk of Mars that fell to Earth and was discovered in the Sahara Desert in 2011. It was mysterious because it defied categorization. Eventually, scientists determined that it represented a new classification of Martian meteorites they named “Martian (basaltic breccia)”.
Black Beauty is really old; components of it are 4.45 billion years old according to this study. It’s the second oldest Martian meteorite ever found. It’s so old that it comes from Mars’ original crust. But Black Beauty also has the highest water content of any Martian meteorite.
According to this research, Mars had water for the first 90 million years of its existence. That’s way before there was enough time for asteroids to bombard the planet and deliver water. The water has to have another source.
Black Beauty forced scientists to ask a question: if Mars’ water—and by extension Earth’s water—was delivered by collisions with water-bearing bodies like asteroids, how did the planets have water in their first 90 million years? There simply wasn’t enough time for asteroids to deliver the water.
“It suggests that water emerged with the formation of Mars. And it tells us that water may be naturally occurring on planets and does not require an external source like water-rich asteroids,” he says.
The researchers obtained about 50 grams of Black Beauty for this study, and they developed a new method to unlock the meteorite’s secrets. They took 15 grams of it and crushed it, dissolved it, then analyzed it.
The analysis revealed something shocking. Although impactors didn’t deliver that water, they delivered the evidence for the source of that water.
“We have developed a new technique that tells us that Mars in its infancy suffered one or more severe asteroid impacts. The impact, Black Beauty reveals, created kinetic energy that released a lot of oxygen. And the only mechanism that could likely have caused the release of such large amounts of oxygen is the presence of water,” Zhengbin Deng said.
Much of the evidence in this study relates to oxygen. Oxygen is a swinger; it likes to combine with almost anything. As it combines with other elements in Mars, the resulting minerals carry traces of their origins as isotopes. By tracing the origins of compounds containing elements like iron and titanium, the researchers developed a sort of timeline of the evolution of Martian rocks as they melted and solidified.
This research focused on 15 igneous clasts from Black Beauty. The team performed a detailed analysis of these clasts, using multiple types of spectroscopy.
“These clasts have been proposed to be the products of an early remelting, likely by impacts, of the primary crust derived from the martian mantle. Hence, these igneous clasts can provide insights into the ancient surface of Mars, allowing us to investigate the physicochemical conditions that existed at the surface of the planet, including the oxygen fugacity at the time of crustal reworking. This information is critical to constrain the timing of establishment of Mars’ hydrosphere and atmosphere and, hence, the potential for early habitability,” the authors explain.
Titanium isotopes played a key role in the work. “Thus, the combination of chemical and Ti isotopic compositions can be used to determine the magmatic thermal and/or redox histories of igneous rocks, in other words, the T–fO2 paths during magma evolution,” the authors write in their paper.
That’s fine as far as it goes. But how did a cold planet like Mars keep that water at a time when the Sun was much younger and fainter? How was that water deposited into the ancient lakes and rivers—and even oceans—that we find evidence of today?
According to the researchers, the same impact that released all that oxygen also released greenhouse gases. Those gases warmed the atmosphere enough for liquid water to persist. According to Zhengbin Deng, ‘this means that the CO2-rich atmosphere may have caused temperatures to rise and thus allowed liquid water to exist at the surface of Mars’.
But there’s a cautionary note to these results, and it comes from the authors themselves. “The high-?17O water component on early Mars may represent either water delivered by impacting material such as water-rich asteroidal bodies or, alternatively, water equilibrating with photochemical products from the early martian atmosphere. Our data cannot discriminate between these two possibilities.”
But that doesn’t mean their data is altogether weak. “Nonetheless, an impact origin for the NWA 7533/7034 basaltic clasts is established from their enrichment in highly siderophile <iron-loving> elements.” They also point out that their interpretation of the data lines up with other evidence “…indicating that the first 8 to 11 km of the martian crust is intensely fractured. It has been proposed that such early bombardment episodes may have induced elevated surface temperatures on Mars, resulting in a warm and wet early climate that is implied by the ancient records of fluvial activity.”
Seldom is a theory confirmed or disproved on the backs of a single study. This one is no exception. But it does bring up another recent study examining the origins of Earth’s water.
That recent research suggests that Earth’s water actually came from the solar nebula shortly after the planet formed. Not water itself, but hydrogen and oxygen that became locked inside the planet’s mantle. Over time, those elements combined into water. If this study is right, hydrogen and oxygen present in Mars’ mantle may have also combined to form water by violent impacts long before asteroids and other bodies could’ve delivered it.
Or it may be that Earth’s water, and Mars’ water, had multiple sources. It may have come from both asteroid impacts and from the solar nebula.
It looks like there’s a possibility that rocky planets may have water early more often than not, and that delivery by asteroids is not required. In any case, the conversation about the source of water on rocky planets just got more interesting.
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