Ann Arbor Times

A recent study from the University of Michigan proposes that up to 60% of near-Earth objects could be dark comets, mysterious asteroids orbiting the sun in our solar system. These bodies likely contain or previously contained ice and may have played a role in delivering water to Earth.

The research suggests that asteroids within the asteroid belt, located between Jupiter and Mars, have subsurface ice. This theory has been suspected since the 1980s, according to Aster Taylor, a graduate student in astronomy at the University of Michigan and lead author of the study.

Taylor explains that the study reveals a potential pathway for transporting ice into the near-Earth solar system. "We don’t know if these dark comets delivered water to Earth. We can’t say that," Taylor said. "But we can say that there is still debate over how exactly Earth's water got here."

Published in the journal Icarus, the study also indicates that one large object may originate from Jupiter-family comets—comets whose orbits bring them close to Jupiter. Dark comets are enigmatic because they exhibit characteristics of both asteroids and comets. Unlike typical asteroids, which lack ice and orbit closer to the sun, dark comets do not have visible comae but do show nongravitational accelerations caused by sublimating ice.

The research examined seven dark comets and estimated that between 0.5% and 60% of all near-Earth objects might be dark comets originating from the asteroid belt. "We think these objects came from the inner and/or outer main asteroid belt," Taylor said. This finding implies a mechanism for bringing ice into the inner solar system.

Previous work by Taylor's team identified nongravitational accelerations on certain near-Earth objects, dubbing them “dark comets.” The current study aimed to determine their origins using dynamical models assigning nongravitational accelerations over 100,000 years.

One such dark comet is 2003 RM, which follows an elliptical orbit close to Earth before extending out towards Jupiter—a path consistent with a Jupiter family comet. However, most other dark comets likely originated from the inner band of the asteroid belt.

Taylor suggests that as these objects get bumped within the solar system’s ice line, their ice starts to off-gas causing rapid rotation until they break apart into smaller pieces containing more ice—leading to further fragmentation.

The researchers believe while larger dark comet 2003 RM was likely ejected from the outer main belt, other studied objects probably came from smaller fragments originating in the inner main belt.

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