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

According to a study by the University of Michigan, up to 60% of objects near Earth could be dark comets. These are mysterious asteroids that orbit the sun in our solar system and probably contain or once contained ice. These asteroids could have been one of the routes for delivering water to Earth.

According to Aster Taylor, a U-M astronomy graduate student and lead author of the study, the findings suggest that asteroids in the asteroid belt, a region of the solar system roughly between Jupiter and Mars that contains a large fraction of the system’s rocky asteroids, have subsurface ice beneath their surfaces. This has been suspected since the 1980s.

The study also shows a possible route for delivering ice to Earth’s nearby solar system, Taylor said. How Earth got its water is a long-standing 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 exactly the water from Earth got here,” Taylor said. “The work we’ve done has shown that this is another way to get ice from somewhere in the rest of the solar system to the Earth’s neighborhood.”

The research further suggests that one large object may have originated from the Jupiter family of comets, comets whose orbits take them close to the planet Jupiter. The team’s results were published in the journal Icarus.

Dark comets are a bit of a mystery because they combine features of both asteroids and comets. Asteroids are rocky bodies without ice that orbit closer to the sun, usually within what is called the ice line. This means that they are close enough to the sun for any ice the asteroid may have carried to sublimate, or change directly from solid ice to gas.

Comets are icy bodies that exhibit a hazy coma, a cloud that often surrounds a comet. Sublimating ice carries dust with it, creating the cloud. In addition, comets typically exhibit slight accelerations that are not driven by gravity but by the sublimation of ice, so-called non-gravitational accelerations.

The study examined seven dark comets and estimates that between 0.5 and 60 percent of all near-Earth objects could be dark comets, which lack a coma but do have non-gravitational accelerations. The researchers also suggest that these dark comets likely originate from the asteroid belt, and because these dark comets have non-gravitational accelerations, the study’s findings suggest that asteroids in the asteroid belt contain ice.

“We think these objects are coming from the inner and/or outer asteroid belt, and the implication of that is that this is another mechanism for getting some 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. This could be a significant fraction of the nearby population. We don’t really know, but we have a lot more questions because of this finding.”

In previous work, a research team including Taylor identified nongravitational accelerations on a number of objects close to Earth, which they called “dark comets.” They determined that the nongravitational accelerations of the dark comets are likely the result of small amounts of subliming ice.

In their current study, Taylor and their colleagues wanted to discover where the dark comets came from.

“Earth objects don’t stay in their current orbits for very long because the Earth’s environment is messy,” they said. “They only stay in the Earth’s neighborhood for about 10 million years. Because the solar system is much older, that means that Earth objects come from somewhere — that we’re constantly getting Earth objects from some other, much bigger source.”

To determine the origins of this dark comet population, Taylor and her co-authors created dynamical models that assigned nongravitational accelerations to objects from different populations. They then modeled a path that these objects would follow given the assigned nongravitational accelerations over a period of 100,000 years.

The researchers saw that many of these objects landed where dark comets are now, and they also discovered that of all possible sources, the asteroid belt is the most likely origin.

One of the dark comets, called 2003 RM, which is passing close to Earth on an elliptical orbit, then to Jupiter and back past Earth, is following the same path you would expect a Jupiter-family comet to follow, Taylor says. That is, its position is consistent with a comet that has been knocked inward from its orbit.

Meanwhile, the study concludes that the rest of the dark comets likely originate from the innermost belt of the asteroid belt. Since the dark comets likely have ice, this shows that there is ice in the innermost belt.

The researchers then applied a previously proposed theory to their population of dark comets to determine why the objects are so small and spinning so quickly. Comets are rocky structures bound together by ice—picture a dirty ice cube, Taylor says. Once they’re knocked into the solar system’s ice line, that ice starts to shed gas. This is what causes the object to accelerate, but it can also cause the object to spin quite quickly—fast enough to break it apart.

“These pieces will also have ice in them, so they’ll also be spinning faster and faster until they break into more pieces,” Taylor said. “You can just keep doing this as you get smaller and smaller and smaller. What we’re proposing is that the way to get these small, fast-spinning objects is to take a bunch of larger objects and break them into pieces.”

As this happens, the objects lose more and more ice, becoming even smaller and spinning even faster.

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