Earth's water does not come from molten meteorites

The white dotted line in the diagram shows the boundary between the inner and outer solar systems, with the asteroid belt roughly located between Mars and Jupiter. The bubbles near the top of the image show water molecules attached to rock fragments, indicating the type of object that may have brought water to Earth.

Photo credit: Jack Cook/Woods Hole Oceanographic Institution

Science and Technology Daily, Beijing, March 16 (Reporter Zhang Jiaxin) Water accounts for 71% of the earth's surface area, but no one knows how or when such a large amount of water arrived and existed on the earth. A new study published in the journal Nature on the 15th brings scientists one step closer to answering this question. Researchers have found that the water on Earth does not come from melting meteorites.

A research team from the University of Maryland analyzed molten meteorites that have been floating in space since the formation of the solar system 4.5 billion years ago. They found that these meteorites had extremely low water content. In fact, they are among the driest substances ever measured. These results led researchers to rule out that they are the main source of water on Earth, which is important for the search for water and life on other planets.

The researchers analyzed seven molten meteorites, known as chondrites, that fell to Earth billions of years after splitting from at least five "planetesimals" (planetesimates). These "planetesimates" collide to form the planets in the solar system. During melting, many of these planetesimal bodies are heated by the decay of radioactive elements from the early solar system, causing them to split into layers with crust, mantle and core.

Since these meteorites have only recently fallen to Earth, this experiment measured their volatiles for the first time. The researchers measured their levels of magnesium, iron, calcium, and silicon using electron probes, and then measured their water content with a secondary ion mass spectrometer.

To reduce contamination, the researchers first placed their samples in a low-temperature vacuum oven to remove any surface water. The sample must be dried again before it can be analyzed in a secondary ion mass spectrometer.

After analyzing samples of chondrites, the researchers found that water accounts for less than two parts per million of its mass. In contrast, the most wettest meteorites, carbonaceous chondrites, contain up to 20% water (by weight), 100,000 times more than the meteorite samples studied by the team. This means that no matter where these planetesimal bodies originate in the solar system or how much water they have in the first place, the heating and melting of these planetesimal bodies will cause almost all water loss.

The researchers found that, contrary to popular belief, not all extrasolar bodies are rich in water. This led them to conclude that water was most likely transported to Earth through unmelted meteorites or chondrites.