A groundbreaking new study led by scientists at the Woods Hole Oceanographic Institution (WHOI) has revealed that the first evidence of water on Earth and in the inner solar system dates the arrival of water to a much earlier time than previously believed. Thanks to new data from NASA’s 4-Vesta project, the team demonstrated that the most primitive type of known meteorite could have brought water to Earth very early on in the planet’s formation. The implications for an earlier “wet Earth” include the possibility that life on our planet began much earlier than previously thought and could once have existed on the other inner planets.
A commonly held theory is that water arrived on Earth hundreds of millions of years after the planet’s formation. The process of planet forming is so volatile and explosive that any water that had been present was suspected to have evaporated or been blown into space. Water is then thought to have arrived via icy comets or “wet” asteroids once the planet had begun to stabilize a little more. However, Adam Sarafian, lead author of the new study published today in Science, says, “The answer to one of the basic questions is that our oceans were always here. We didn’t get them from a late process, as was previously thought.”
To arrive at this finding, the team studied another potential source of Earth’s water — the most primitive known meteorites, known as carbonaceous chondrites. These were formed at the same time as the sun, around 4.6 billion years ago and well before the planets were formed. WHOI geologist and study coauthor Sune Nielsen states, “These primitive meteorites resemble the bulk solar system composition. They have quite a lot of water in them, and have been thought of before as candidates for the origin of Earth’s water.”
To determine the source of water in planetary bodies, scientists measure the ratio between the two stable isotopes of hydrogen: deuterium and hydrogen. Highly variable ratios of these isotopes are found in and differentiate various regions of the solar system. Because the study’s authors knew the ratio for carbonaceous chondrites, they compared them with those of 4-Vesta, an asteroid that was known to have formed around the same time as Earth. This then gave them clues to the timing of water accretion and an indication of when water appeared on Earth.
4-Vesta formed in the same region of the solar system as Earth. Basaltic meteorites that derive from 4-Vesta carry a unique signature of one of the oldest hydrogen reserves in the solar system. They date to approximately 14 million years after the solar system formed and the team considered them ideal for determining the source of water in the inner solar system while Earth was in its main building phase. NASA has been collecting samples from 4-Vesta and so was able to provide these to the team so their isotopes could be studies for the first time. The measurements show that 4-Vesta contains the same hydrogen isotopic composition as carbonaceous chondrites, just as Earth does. That, combined with nitrogen isotope data, points to carbonaceous chondrites as the most likely common source of water between the two bodies.
WHOI geologist and coauthor Horst Marschall said, “The study shows that Earth’s water most likely accreted at the same time as the rock. The planet formed as a wet planet with water on the surface. While the findings don’t preclude a late addition of water on Earth, it shows that it wasn’t necessary since the right amount and composition of water was present at a very early stage. Adds Nielsen, “An implication of that is that life on our planet could have started to begin very early. Knowing that water came early to the inner solar system also means that the other inner planets could have been wet early and evolved life before they became the harsh environments they are today.”