Up to 60% of near-Earth objects could be dark comets, mysterious asteroids that orbit the Sun and likely contain or once contained ice. These objects could have been one of the paths for delivering water to Earth, according to a study from the University of Michigan. The research results suggest that asteroids in the asteroid belt, a region of the Solar System between Jupiter and Mars that contains many rocky asteroids, have underground ice. This has been suspected since the 1980s, according to Aster Taylor, a doctoral student in astronomy and the lead author of the study.

The study also shows a possible route for delivering ice to near-Earth space. How Earth got its water is a question that has long intrigued scientists.

"We can't say that these dark comets brought water to Earth, but we can say that there is another possibility of how ice could have traveled from the rest of the Solar System to Earth," said Taylor. "Our research shows that this is yet another way for ice from other parts of the Solar System to come to Earth's vicinity."

Dark comets are an enigma because they combine characteristics of asteroids and comets. Asteroids are rocky bodies without ice that orbit closer to the Sun, within the so-called frost line, where it is warm enough for ice to sublimate. Comets, on the other hand, are icy bodies that display a fuzzy coma, a cloud of gas and dust surrounding the comet as the ice sublimates.

The study examined seven dark comets and estimated that between 0.5% and 60% of all near-Earth objects could be dark comets, which lack a coma but have non-gravitational accelerations. Researchers suggest that these dark comets likely come from the asteroid belt, and since these comets have non-gravitational accelerations, it implies that asteroids in the asteroid belt contain ice.

"We believe these objects come from the inner or outer main asteroid belt, which means this is yet another way for ice to arrive in the inner Solar System," said Taylor. "There may be more ice in the inner main belt than we thought. There may be more of these objects. This could be a significant part of the near-Earth object population. We don't know for sure, but we have many more questions because of these findings."

In a previous work, the research team, including Taylor, identified non-gravitational accelerations in a set of near-Earth objects, calling them "dark comets." They determined that these accelerations were likely the result of small amounts of sublimating ice.

In the current study, Taylor and colleagues wanted to find out where the dark comets come from. "Near-Earth objects don't stay in their current orbits for long because the Earth's environment is chaotic," they said. "They stay near Earth for about 10 million years. Since the Solar System is much older than that, it means that near-Earth objects come from somewhere - that we are constantly resupplied with near-Earth objects from another, much larger source."

To determine the origin of the dark comets, Taylor and co-authors created dynamic models that assigned non-gravitational accelerations to objects from different populations. They modeled the paths these objects would take given the assigned accelerations over a period of 100,000 years. Researchers observed that many of these objects ended up where dark comets are today, and found that the main asteroid belt is the most likely place of origin.

One of the dark comets, named 2003 RM, follows an elliptical orbit near Earth, then near Jupiter, and back near Earth, following a path that would be expected of a Jupiter family comet. This suggests it was pushed from its original orbit inward.

The study also reveals that other dark comets likely came from the inner asteroid belt. Since dark comets likely have ice, this shows that ice exists in the inner main belt.

Researchers applied a previously proposed theory to the population of dark comets to explain why these objects are so small and rotate quickly. Comets are like dirty ice cubes, made of rock and ice. When they get close to the Sun, the ice begins to sublimate, causing the object to accelerate and rotate faster. This rotation can cause the comet to break into smaller pieces that continue to accelerate and rotate faster until they break into even smaller pieces.

Researchers believe that the larger dark comet, 2003 RM, was likely a larger object ejected from the outer asteroid belt, while the other dark comets came from the inner asteroid belt and formed by breaking up after being pushed inward.

According to Taylor, "these pieces still have ice, so they accelerate and rotate faster until they break into even smaller pieces. This process can continue, creating small, fast-rotating objects."

Dark comets are mysterious objects that could have delivered ice, and perhaps water, to Earth. They come from the asteroid belt and have unique characteristics that set them apart from ordinary asteroids and comets. This study opens new questions about the role of dark comets in our Solar System and their possible impact on Earth.

Source: University of Michigan