(CN) — While scientists have long assumed that water arrived on Earth through meteorites, new research suggests the planet’s substantial amount of water could have only come from unmelted material.
A study published in Nature on Wednesday describes how a team of researchers analyzed a collection of meteorites to understand the "how" and "when" of Earth’s massive water content.
The meteorites — formed just a few million years after the solar system’s formation 4.5 billion years ago — consisted of seven achondrite meteorites, meaning melted meteorites from the Moon, Mars and asteroids originating anywhere from the inner and outer, icier, reaches of the solar system. However, meteorites also come in the form of “chondrites” — meteorites that have never melted — and “irons,” some of which are thought to have come from the cores of ancient asteroids.
Dr. Megan Newcombe of the University of Maryland elucidated how meteorites are further differentiated by whether they are “carbonaceous” or not. This isotopic dichotomy, she wrote, “supports the idea that the early solar system consisted of two distinct chemical reservoirs that were physically separated, either by the growth of Jupiter or as a consequence of planetesimal formation at notable condensation fronts in the protoplanetary disk.”
Newcombe explains that non-carbonaceous meteorites — even including Earth, Mars, the moon, enstatite chondrites and regular chondrites — are all thought to have formed in the warmer inner solar system, while carbonaceous meteorites, including carbonaceous chondrites, formed in the cooler outer solar system.
As such, scientists have hypothesized that non-carbonaceous meteorites accreted very little water due to the proximity to the sun, while carbonaceous meteorites were more likely water sources. But Newcombe also points out that studies have found non-carbonaceous materials accreted water ice and that recent work suggests that water contents of enstatite chondrites may have even accounted for “the entire water budget of Earth.”
Could it be true that water-bearing objects existed near the sun within the first few million years of the solar system? Newcombe found the question important, particularly as many objects and planetesimals — objects that collided to form planets in the early solar system — underwent the process of melting and differentiation from the heat of radioactive decay.
“We wanted to understand how our planet managed to get water because it’s not completely obvious,” Newcombe said in a statement. “Getting water and having surface oceans on a planet that is small and relatively near the sun is a challenge.”
University of Maryland geology graduate student Liam Peterson used an electron microprobe to measure the meteorites’ levels of magnesium, iron, calcium and silicon before joining Newcombe at the Carnegie Institution for Science’s Earth and Planets Laboratory to measure the rocks’ water contents with a secondary ion mass spectrometry instrument.
Once at Carnegie, researchers baked the meteorites in a low-temperature vacuum oven to remove surface water before drying the samples once more.
“The challenge of analyzing water in extremely dry materials is that any terrestrial water on the sample’s surface or inside the measuring instrument can easily be detected, tainting the results,” study co-author Conel Alexander, a scientist at the Carnegie Institution for Science, said in a statement.
The team knew some of their meteorite samples came from the inner solar system, while other rarer specimens came from the colder outer reaches of the planetary system. Newcombe told Courthouse News that the meteorites that formed in the outer solar system were the most exciting to study because the team thinks they formed in the presence of water ice.
“When they first accreted into their rocky rubble piles, they would have been very icy and water rich, and so, we know they started off wet,” Newcombe said. “Then the question was, since we know they melted, does the melting process remove all of that water, or is it possible to melt a baby planet or a planetesimal and retain the water that it started off with? So that was kind of our question going in.”
But while Newcombe expected that the materials formed in the outer solar system might be wetter than those formed in the inner solar system, the study results indicated otherwise.
Overall, the professor described both sample sets as dry with no difference in between, even reporting the water concentrations as “among the lowest ever measured for extraterrestrial materials.”
“So even though the ones that formed a long way from the sun started off with more water, the melting process very efficiently dries them out,” Newcombe said.
What this effectively means is that not all outer objects in the solar system have as much water as scientists initially believed. Additionally, Newcombe’s finding means that however Earth acquired water, it likely came from unmelted or chondritic meteorites.
“Our results demonstrate that differentiated planetesimals efficiently degassed before or during melting,” Newcombe wrote. “This finding implies that substantial amounts of water could only have been delivered to Earth by means of unmelted material.”
Newcomb said the meteorites she analyzed, which fell to the Earth within the last 20 years primarily around the region of Northwest Africa "are actually quite rare."
“They're called ungrouped achondrites, and they're called ungrouped because most meteorites fall in groups. They can be classified," Newcombe said. "But these meteorites are unlike other things that have been found previously. So, they were likely from smaller bodies, probably, that aren't well represented in our meteorite record.”
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