But Earth’s ratio should have changed over time. Like most planets, Earth probably lost some of its atmosphere to space, and the lighter hydrogen would be easier to strip from the planet than its heavier counterpart. Geological processes, such as the evaporation of water from reservoirs such as lakes and oceans, can also change the ratio, as can biological reactions, because lighter isotopes are used differently than heavier ones in metabolic processes. All of these processes would give the modern Earth a higher D/H ratio compared with when the planet was newly formed.

When Meech heard that primordial water could be spouting from the surface in Iceland, she grew excited at the chance to study the earliest flavor of water. But after chatting with a geologist, she learned that the plumes actually came from more recent activity — they weren’t primordial after all. However, the geologist revealed that some rocky material brought up from Earth’s mantle does contain small traces of water. That material may never have mixed with the stuff on the surface and could represent Earth’s early water. No one had investigated the D/H ratio in those samples because the technology to do so was new. But the University of Hawaii, where Meech is based, had just purchased a new ion microprobe that might be able to do the job.

“I thought, wow, here’s a way we can actually measure the original fingerprints,” Meech says. “At that point, I got very excited.”



In search of the culprit

Earth and the rest of the planets formed inside a nest of gas left over from the birth of the Sun. This material, known as the solar nebula, contained all the elements that built the planets, and the compositions varied with distance from the Sun. The region near the star was too warm for some material to coalesce as ices, which instead formed in the outer part of the solar system. Around Earth, hydrogen and other elements could stick around only as a gas. Because the nebula was short-lived, most scientists suspect that Earth didn’t have enough time to collect these gases before they escaped into space. That idea, along with the planet’s high D/H ratio, led many to believe that Earth’s water must have arrived after Earth had cooled.

When the European Giotto spacecraft visited Halley’s Comet in 1986, researchers noticed its heavy water content was higher than the gas in Earth’s part of the early solar system. A new theory emerged: Comets could have carried water to early Earth. After the planets formed, the enormous bodies would continue to stir things up, with giant planets like Jupiter hurling some material toward the inner solar system. Icy objects that formed in the outer solar system could have been tossed at Earth to rain down as giant water-laden impacts.