If you remember even basic elementary school geography, you know that Earth’s surface is mostly water. Scientists have disagreed about how all that water ended up on Earth. Was it all here when the planet formed, or was Earth a dry husk until asteroids and comets delivered water? A new analysis of meteorites published in the journal Science points to a watery Earth from the start.
It’s not unthinkable that a planet could form with water or ice, but Earth formed in a much warmer part of the solar system than chilly planets like Jupiter or the uncountable icy Kuiper Belt objects. The current thinking is that no water ice would have remained frozen amidst the swirling cloud that became Earth, and that would mean Earth accumulated water later on to become the wet world it is today. To know for sure, we’d have to look at the material that formed Earth. That’s not possible 4.5 billion years after the fact, but we have something almost as good.
The latest clues to Earth’s beginnings came from a rare type of space rock known as an “enstatite chondrite meteorite,” also known as E-type chondrites. Only about two percent of meteorites are in this class, which have chemical compositions that date them to the earliest era of the solar system. Since these objects are essentially the same material that coalesced to form the planets, the amount of hydrogen locked up inside is of great interest to scientists.
Researchers from the Centre de Recherches Pétrographiques et Géochimiques in France took a close look at 13 of these uncommon meteorites. They measured the amount of hydrogen present in the rocks because hydrogen plus oxygen gets you water, and we know Earth had plenty of oxygen at the beginning.
The team found less hydrogen in the enstatite chondrites than in other types of space rock, but it was still more than enough. According to the study, the hydrogen present in enstatite chondrites could account for several times more water than is currently in Earth’s oceans. That supports the idea Earth formed with most or all of the water we have today. Backing up this claim, the team analyzed the ratios of hydrogen isotopes in the meteorites, finding they are very similar to the Earth’s interior.
This conclusion is appealing because it’s much simpler than the alternative — that Earth picked up oceans of water from other objects. We probably did accumulate some water from the occasional comet, but this study offers strong evidence Earth has always been a watery planet.
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