THE notion that Earth’s water was delivered by asteroids has taken a hit. Analysis of lunar rocks suggests that our planet was born wet, and was even able to give some water to the moon.
Alberto Saal of Brown University in Providence, Rhode Island, and colleagues studied moon rocks brought back from the Apollo 17 mission in 1972. They looked at volcanic glasses inside the rocks, which were protected from weathering, to examine the ratio of hydrogen to its heavier isotope, deuterium.
They found that the deuterium ratios were almost identical to those seen in earthly rock, suggesting the two worlds got their water from the same source (Science, doi.org/mg5).
The moon probably formed after a massive collision between the infant Earth and another proto-planet about 4.5 billion years ago. Water could have been delivered to both bodies by asteroids, but it could have become mixed into lunar rock only if they hit while the moon was still molten after the impact that created it.
Based on known impact rates, there would not have been time for enough asteroids to hit the moon before it cooled and solidified. That hints lunar water must have come from Earth, and that our planet had its water from the start, says Saal.
This article appeared in print under the headline “Moon stole water from baby Earth”