Astronomers suggest that up to 60% of near-Earth objects could be dark comets

According to a University of Michigan study, up to 60 percent of near-Earth objects may be dark comets, mysterious asteroids that orbit the Sun in our solar system and likely contain or once contained ice and could have been a pathway for water to reach Earth.

The findings suggest that asteroids in the asteroid belt, a region of the solar system roughly between Jupiter and Mars that contains much of the system’s rocky asteroids, have subsurface ice, something that has been suspected since the 1980s, said Aster Taylor, a U-M astronomy graduate student and lead author of the study.

The study also shows a potential pathway for ice to reach Earth’s near-solar system, Taylor said. How Earth got its water has long been a question.

“We don’t know if these dark comets brought water to Earth. We can’t say. But we can say that there’s still some debate about how Earth’s water got here,” Taylor said. “The work we’ve done has shown that this is another pathway for bringing ice from the rest of the solar system to the Earth’s environment.”

The research also suggests that a large object could come from Jupiter-family comets, whose orbits bring them close to the planet Jupiter. The team’s findings are published in the journal Icarus.

Dark comets are a mystery because they combine the characteristics of both asteroids and comets. Asteroids are ice-free rocky bodies that orbit the Sun, usually within what is called the ice line. This means that they are close enough to the Sun that the ice they were carrying sublimates, that is, goes directly from solid ice to gas.

Comets are icy bodies that have a fuzzy coma, a cloud that often surrounds a comet. The sublimating ice carries dust with it, creating the cloud. Additionally, comets typically have slight accelerations driven not by gravity, but by the sublimation of ice, called nongravitational accelerations.

The study looked at seven dark comets and estimates that between 0.5 and 60 percent of all near-Earth objects could be dark comets, which are not comets but have non-gravitational accelerations. The researchers also suggest that these dark comets likely originated in the asteroid belt, and since these dark comets have non-gravitational accelerations, the study’s results suggest that asteroids in the asteroid belt contain ice.

“We think these objects came from the inner and/or outer main asteroid belt, which suggests that this is another mechanism for introducing ice into the inner solar system,” Taylor said. “There may be more ice in the inner main belt than we thought. There may be more objects like this out there. It could be a significant fraction of the nearest population. We don’t really know, but these findings raise a lot of questions.”

In previous work, a team of researchers including Taylor had identified non-gravitational accelerations on a set of near-Earth objects, calling them “dark comets.” They determined that the non-gravitational accelerations on dark comets were likely the result of small amounts of sublimating ice.

As part of their current work, Taylor and his colleagues wanted to find out where dark comets came from.

“NEOs don’t stay in their current orbits for very long because the environment near Earth is chaotic,” they explained. “They only stay in that environment for about 10 million years. Since the solar system is much older, that means that NEOs come from somewhere, that we’re constantly getting NEOs from another, much larger source.”

To determine the origin of this population of dark comets, Taylor and his co-authors created dynamical models that assigned non-gravitational accelerations to objects from different populations. They then modeled the trajectory that these objects would follow based on the assigned non-gravitational accelerations over a period of 100,000 years.

The researchers observed that many of these objects ended up where dark comets are today and found that, of all the potential sources, the main asteroid belt is the most likely place of origin.

One of the dark comets, called 2003 RM, which is passing in an elliptical orbit near Earth, then toward Jupiter and back past Earth, is following the same path that one would expect for a Jupiter-family comet, Taylor said, meaning its position is consistent with that of a comet that has been ejected from its orbit.

At the same time, the study reveals that the rest of the dark comets likely originate from the inner belt of the asteroid belt. Since dark comets likely contain ice, this shows that ice is present in the inner main belt.

The researchers then applied a previously suggested theory to their population of dark comets to figure out why these objects are so small and spinning so fast. Comets are rocky structures bound together by ice — think of a dirty ice cube, Taylor explains. Once they collide with the solar system’s ice line, that ice starts to release gas. This causes the object to accelerate, but it can also make it spin quite fast — fast enough that it breaks apart.

“These pieces will also be covered in ice, so they will spin faster and faster until they break into new pieces,” Taylor said. “You can keep doing this as you get smaller and smaller. What we’re suggesting is that to get these small, fast-spinning objects, you take a few larger objects and break them into pieces.”

Meanwhile, the objects continue to lose their ice, become even smaller and rotate even faster.

The researchers believe that while the largest dark comet, 2003 RM, was likely a larger object that was ejected from the outer main belt of the asteroid belt, the other six objects they examined likely came from the inner main belt and consisted of an object that was flung inward and then broke apart.