What are the origins of water on Earth?

Although water makes up 71 percent of our planet’s surface, scientists are not yet sure where and how this water originated. Many experts have previously argued that water most likely arrived to Earth from meteorites. 

However, by analyzing melted meteorites which have been floating around in space since the formation of our solar system (four and a half billion years ago), a team of researchers led by the University of Maryland has discovered that these meteorites had extremely low water content. These findings rule out such meteorites as the primary source of Earth’s water, and could have major implications for the search for water – and implicitly life – on other planets.

“We wanted to understand how our planet managed to get water because it’s not completely obvious. Getting water and having surface oceans on a planet that is small and relatively near the sun is a challenge,” said study lead author Megan Newcombe, a professor of Geology at Maryland.

The scientists analyzed seven melted, or achondrite, meteorites that crashed into our planet billions of years after splintering from at least five astronomical objects which collided to form planets in our solar systems, known as “planetesimals.” During melting, these planetesimals were heated up by the decay of radioactive elements in the early solar system’s history, causing them to separate into layers with a crust, mantle, and core.

The experts used an electron microprobe to measure these objects’ levels of magnesium, iron, calcium, and silicon, as well as a secondary ion mass spectrometry instrument to measure their water contents. While some of the meteorite samples came from the inner solar system, where conditions are assumed to be warm and dry, others originated form colder and icier outer areas of our solar system.

“We knew that plenty of outer solar system objects were differentiated, but it was sort of implicitly assumed that because they were from the outer solar system, they must also contain a lot of water,” said study co-author Sune Nielsen, a geologist at the Woods Hole Oceanographic Institution. “Our paper shows this is definitely not the case. As soon as meteorites melt, there is no remaining water.” 

In fact, the analysis revealed that in these achondrite meteor samples – regardless of their origin – water comprised less than two millionth of their mass, suggesting that the heating and melting of planetesimals causes near-total water loss. However, another type of meteorites called “carbonaceous chondrites” were found to contain up to 20 percent of water, making them the most likely source for the origin of water on Earth. “This finding implies that substantial amounts of water could only have been delivered to Earth by means of unmelted material,” the authors wrote.

Due to the inherent relation between water and life, these findings have important implications beyond Geology, particularly in the ongoing search for extraterrestrial life. 

“Water is considered to be an ingredient for life to be able to flourish, so as we’re looking out into the universe and finding all of these exoplanets, we’re starting to work out which of those planetary systems could be potential hosts for life. In order to be able to understand these other solar systems, we want to understand our own,” Newcombe concluded.

The study is published in the journal Nature.

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By Andrei Ionescu, Earth.com Staff Writer

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