Where did the water on Earth come from? About 71% of Earth’s surface area is oceans, which contain 97% of Earth’s water. Human bodies are 55-60% water. Its origin is a missing link in scientists’ understanding of our planet’s history and life upon it.
A new paper published earlier this month in Icarus argues that a new category of space rock called “dark comets” are much more common than previously thought and may have delivered water to Earth long ago. A newly defined type of near-Earth object—comets and asteroids whose orbits of the sun bring them close to Earth—dark comets are asteroids that speed up unexpectedly, suggesting they may be firing gas jets, as comets do. They’re a new class of object somewhere between an asteroid and a comet.
Icy Bodies
The study, which examined seven dark comets, estimates that between 0.5 and 60% of all near-Earth objects could be dark comets from the asteroid belt. These dark comets appear to contain ice—something asteroids don’t—though there is evidence that ice exists on some asteroids in the main asteroid belt between Jupiter and Mars. “We think these objects came from the inner and/or outer main asteroid belt, and the implication of that is that this is another mechanism for getting some ice into the inner solar system,” said Aster Taylor, lead author of the paper and a graduate student in astronomy at the University of Michigan. “There may be more ice in the inner main belt than we thought. There may be more objects like this out there.”
Asteroid Belt
Whether Earth’s water came from dark comets is still being determined because the timescales make evidence hard to find. “Near-Earth objects don’t stay on their current orbits very long because the near-Earth environment is messy,” said Taylor. “They only stay in the near-Earth environment for around 10 million years.” The solar system is about 4.5 billion years old.
The research indicates that the main asteroid belt is the most likely source of near-Earth objects.
“We don’t know if these dark comets delivered water to Earth. We can’t say that. But we can say that there is still debate over how exactly the Earth’s water got here,” said Taylor. “The work we’ve done has shown that this is another pathway to get ice from somewhere in the rest of the solar system to the Earth’s environment.
Age Of Water
Research in 2023 suggested that all the water on Earth is older than the Sun. Scientists found at least 1,200 times the amount of water in all Earth’s oceans around a very young star in the early stages of its evolution about 1,305 light-years from the solar system in the constellation Orion. That supports a theory that water comes from the interstellar medium, the gaseous, dusty regions of space between stars. It indicates that water in our solar system formed billions of years before the sun.
Separate research indicated that the water on Earth didn’t come from meteorites—debris from a comet, asteroid or meteoroid that strikes Earth’s surface—which were found to be among the driest extraterrestrial materials ever measured.
Another paper describing the discovery of salt crystals on samples of the asteroid Itokawa—brought to Earth in 2010 by Japan’s Hayabusa mission—supports the theory that all of the water on the surface of Earth originally came from asteroid strikes.
