The study, led by Aster Taylor, a graduate student in astronomy at the University of Michigan, suggests that asteroids in the asteroid belt-located between Mars and Jupiter-have subsurface ice. This theory has been considered since the 1980s.

Taylor explained, "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. 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."

The research indicates that one significant object may originate from the Jupiter-family comets, whose orbits bring them near Jupiter. The results of this study are published in the journal *Icarus*.

Dark comets, combining traits of both asteroids and comets, present a unique mystery. Asteroids are typically rocky bodies without ice, orbiting close to the sun within the "ice line"-the distance at which ice can sublimate. Comets, however, are icy bodies that develop a coma, a surrounding cloud of sublimating ice carrying dust.

The study focused on seven dark comets, estimating that 0.5 to 60% of all NEOs could be dark comets. These comets lack comae but exhibit nongravitational accelerations, implying the presence of subsurface ice. The findings suggest these dark comets likely originate from the asteroid belt.

Taylor stated, "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. There may be more ice in the inner main belt than we thought. There may be more objects like this out there. This could be a significant fraction of the nearest population. We don't really know, but we have many more questions because of these findings."

Previous research involving Taylor identified nongravitational accelerations in a set of NEOs, dubbing them "dark comets." This acceleration likely results from small amounts of sublimating ice.

In the current study, Taylor and colleagues aimed to trace the origins of dark comets. "Near-Earth objects don't stay on their current orbits very long because the near-Earth environment is messy," Taylor noted. "They only stay in the near-Earth environment for around 10 million years. Because the solar system is much older than that, that means near-Earth objects are coming from somewhere-that we're constantly being fed near-Earth objects from another, much larger source."

To uncover the source of dark comets, the researchers developed dynamical models incorporating nongravitational accelerations. They simulated the trajectories of these objects over 100,000 years, concluding that the main asteroid belt is the most likely origin.

One notable dark comet, 2003 RM, follows an elliptical orbit that brings it near Earth and Jupiter, a path consistent with a Jupiter-family comet. The study suggests that other dark comets likely originate from the inner asteroid belt, indicating the presence of ice in this region.

The team also applied an existing theory to explain why these objects are small and rapidly rotating. As comets lose their ice near the sun, they spin faster, potentially breaking into smaller pieces. Taylor explained, "These pieces will also have ice on them, so they will also spin out faster and faster until they break into more pieces. You can just keep doing this as you get smaller and smaller and smaller. What we suggest is that the way you get these small, fast rotating objects is you take a few bigger objects and break them into pieces."

The researchers believe the larger dark comet, 2003 RM, originated from the outer main belt, while the other objects likely came from the inner belt, breaking apart as they moved inward.

Research Report:The dynamical origins of the dark comets and a proposed evolutionary track

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