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

According to a University of Michigan study, up to 60% of near-Earth objects may be dark comets, mysterious sun-orbiting asteroids in our solar system that likely contain or formerly contained ice and may have been a route for sending water to Earth. .

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

The study also points to a possible route for the delivery of ice to the near-Earth solar system, according to Taylor. How the Earth got its water is an age-old question.

“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 about exactly how Earth’s water got here,” Taylor said. “The work we’ve done has shown that this is another route to get ice from somewhere in the rest of the solar system to Earth’s environment.”

The research further suggests that a large object may come from comets of the Jupiter family, comets whose orbits bring them close to the planet Jupiter. The team’s results are published in the journal Icarus.

Dark comets are a bit of a mystery because they combine the characteristics of asteroids and comets. Asteroids are ice-free rocky bodies that orbit closest to the sun, usually within what is called the ice line. This means they are close enough to the sun for any ice the asteroid may have held to sublimate, or change from solid ice directly to gas.

Comets are icy bodies that show a faint coma, a cloud that often surrounds a comet. The sublimation of ice carries dust along with it, creating the cloud. Furthermore, comets usually have slight accelerations driven not by gravity but by ice upwelling, called non-gravitational accelerations.

The study examined seven dark comets and estimates that between 0.5 and 60% of all near-Earth objects may be dark comets, which do not have comas but have non-gravitational accelerations. The researchers also suggest that these dark comets likely come from the asteroid belt, and because these dark comets have non-gravitational accelerations, the study’s findings suggest that the asteroids in the asteroid belt contain ice.

“We think these objects came from the main belt of inner and/or outer asteroids, and the implication of that is that this is another mechanism for getting some ice into the inner solar system,” Taylor said. “There could be more ice in the inner main belt than we thought. There could be more objects like this out there. That could be a significant fraction of the nearest population. We don’t really know, but we have a lot more question because of these findings”.

In previous work, a team of researchers including Taylor identified non-gravitational accelerations in a group of near-Earth objects, naming them “dark comets”. They determined that the non-gravitational accelerations of dark comets are likely the result of small amounts of ice sublimation.

In the current work, Taylor and their colleagues wanted to find out where dark comets came from.

“Near-Earth objects don’t stay in their current orbits very long because the near-Earth environment is chaotic,” they said. “They stay in the near-Earth environment for only about 10 million years. Because the solar system is much older than that, that means that near-Earth objects are coming from somewhere—that we’re constantly being fed near-Earth objects from a other. , much larger source.”

To determine the origin of this dark comet population, Taylor and their coauthors created dynamical models that assigned non-gravitational accelerations to objects from different populations. They then modeled a path these objects would follow given certain non-gravitational accelerations over a period of 100,000 years.

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

One of the dark comets, called 2003 RM, which passes in an elliptical orbit near Earth, then toward Jupiter and back to Earth, follows the same path that would be expected of a Jupiter-family comet, Taylor says—that is, its position is consistent with a comet that was knocked inward from its orbit.

Meanwhile, the study finds that the rest of the dark comets likely came from the inner belt of the asteroid belt. Since dark comets are likely to have ice, this indicates that ice is present in the inner main belt.

The researchers then applied a previously suggested theory to their population of dark comets to determine why the objects are so small and rapidly rotating. Comets are rocky structures held together by ice—imagine a dirty ice cube, Taylor says. Once they collide within the solar system’s ice line, that ice begins to break away from the gas. This causes the object to accelerate, but it can also cause the object to spin fast enough – fast enough that the object breaks away.

“These pieces will also have ice on them, so they will also spin faster and faster until they break into more pieces,” Taylor said. “You can just keep doing that as you get smaller and smaller. What we’re suggesting is that the way you get these small, fast-spinning objects is to get some larger objects and tear them to pieces.”

As this happens, the objects continue to lose their ice, become even smaller and spin even faster.

The researchers believe that while the largest dark comet, 2003 RM, was likely a larger object that fell from the main outer belt of the asteroid belt, the other six objects they were examining probably came from the belt. the main interior and were made from an object that had crashed inwards and then broken up.