A recent study suggests that “dark comets,” near-Earth asteroids that behave like comets and may contain water ice, could make up as much as 60% of all near-Earth objects. These dark comets were first identified in 2023 based on their anomalous motions, showing orbital acceleration that cannot be explained by gravity alone. While they do not have visible outgassing, their non-gravitational acceleration indicates the presence of subsurface ice, earning them the nickname “dark comets.”
New research led by Aster Taylor of the University of Michigan has found that these dark comets likely originate from the inner region of the asteroid belt between Mars and Jupiter. This discovery aligns with a theory dating back to the 1980s, suggesting that there is a significant amount of water ice buried beneath the surface of objects in the asteroid belt. These findings are supported by observations of other objects, such as “active asteroids,” which exhibit comet-like behaviors in the asteroid belt.
Taylor’s team’s calculations and modeling indicate that up to 60% of the near-Earth object population could be dark comets. These small, fast-rotating objects are thought to be formed when larger asteroids break apart due to outgassing activity. This process also explains the fast rotations and modest sizes of dark comets. The discovery of dark comets has raised questions about the origins of Earth’s water, as these objects could potentially be a source of water delivery to our planet.
Furthermore, the study suggests that the asteroid Bennu, recently visited by NASA’s OSIRIS-REx mission, displayed subtle outgassing activity when viewed up close. This implies that asteroids with minor outgassing levels, undetectable from Earth, could be common in near-Earth orbits. The constant replenishment of dark comets from the asteroid belt is essential to maintain their numbers near Earth, as these objects have a relatively short lifetime of about 10 million years before gravitational forces scatter them.
The findings regarding the origins of dark comets were published in the journal Icarus on July 6. The research sheds light on the complex dynamics of near-Earth objects and their potential role in delivering water to our planet. As scientists continue to study these intriguing celestial bodies, new insights may emerge about the formation and evolution of our solar system.
