科学美国人60秒:Young Earth May Have Been All Wet

Water. Earth has loads of H₂O. But did it always? This question has plagued scientists for decades. Conventional wisdom has it that the Earth formed without any water. Then meteorites carrying water smashed into the planet, dumping their precious cargo. But now, a team of geologists has challenged this idea. They say water may have been here from the start.
 
The scientists discovered that the chemical signature of water on Earth matches the signature of water in an ancient group of asteroids called eucrites. These bodies formed just a few million years after the birth of the solar system. And because Earth and eucrites seem to share the same source of water, the reasoning goes, Earth must have gotten its water around the same time the eucrites did—which has big implications.
 
“Because Earth got its water early—really the whole inner solar system got its water early—it means that planets, once they fully formed and got cool enough that water would be stable on the surface, they could be habitable.”
 
The Woods Hole Oceanographic Institution’s Adam Sarafian, who led the research. [Adam Sarafian et al, Early accretion of water in the inner solar system from a carbonaceous chondrite–like source, in Science]
 
“It wasn’t like these planets are just waiting around, looking at their watches, saying, ‘Where’s the water? Life’s ready to go, we just need water here.’ So it pushes the date at which planets can become habitable back. And we need to go back to the drawing board on how planets are built and how water survives large impacts.”
 
—Julia Rosen
 
(The above text is a transcript of this podcast)