Are We Drinking Water Delivered by Dark Comets?

A University of Michigan study suggests that up to 60 percent of near-Earth objects could be dark comets, some of which contain ice that could have contributed to Earth’s water supply.

These objects have characteristics of both asteroids and comets, and are probably Jupiter and MarsThe study highlights the possibility that ice may exist underground in asteroids, suggesting that these objects may play a role in transporting ice into the inner solar system.

A University of Michigan study has found that up to 60 percent of near-Earth objects could be dark comets, mysterious asteroids orbiting the Sun in our solar system that may contain or have contained ice in the past and could have been one of the pathways that delivered water to Earth.

The discovery suggests that asteroids in the asteroid belt — the region roughly between Jupiter and Mars that contains many of the solar system’s rocky asteroids — may have subsurface ice, something that’s been suspected since the 1980s, said Aster Taylor, a graduate student in astronomy at the University of Michigan and lead author of the study.

Ice delivery route

Taylor said the study also points to a possible pathway that could have delivered ice to the near-Earth solar system. How Earth got its water is a long-standing 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 how exactly Earth’s water got here,” Taylor said. “Our study shows that this is another pathway that could deliver ice from elsewhere in the solar system to the Earth’s environment.”

The study also suggests that one large object may have come from a Jovian comet, a type of comet that orbits close to Jupiter. The team’s findings are published in the journal ICARUS.

Combining asteroid and comet features

Dark comets are a bit of a mystery because they combine characteristics of both asteroids and comets: asteroids are rocky, ice-free bodies that orbit close to the Sun and usually lie within the ice line, meaning they are close enough to the Sun that any ice they carry can sublimate, i.e. turn directly from solid ice into gas.

Comets are icy objects that are often surrounded by a fuzzy cloud called a coma. The sublimating ice carries dust with it, forming a cloud. In addition, comets usually accelerate slightly due to the sublimation of the ice, rather than due to gravity. This is called non-gravitational acceleration.

Estimating the proportion of dark comets

The study looked at seven dark comets and estimated that between 0.5 and 60% of near-Earth objects could be dark comets. Dark comets do not have a coma, but they do have non-gravitational acceleration. The researchers also suggested that these dark comets likely come from the asteroid belt, and because these dark comets have non-gravitational acceleration, the study suggests that asteroids in the asteroid belt contain ice.

“These objects are thought to be from the inner and/or outer main asteroid belt, suggesting that this is another mechanism for bringing ice into the inner solar system,” Taylor said. “There may be more ice in the inner main asteroid belt than we thought. There may be other objects like this out there. This could be a significant portion of the nearest population. We don’t really know, but these discoveries have raised many more questions.”

Origin and dynamics of dark comets

In a previous study, Taylor and a team of researchers identified non-gravitational acceleration in near-Earth objects that they named “dark comets. ” They determined that the non-gravitational acceleration in dark comets was likely due to the sublimation of small amounts of ice.

In the current study, Taylor and his colleagues wanted to understand where the dark comets come from.

“Near-Earth objects do not remain in their current orbits for long due to the messy environment near Earth,” the researchers said. “They only remain in the near-Earth environment for around 10 million years. Since the Solar System is much older than that, near-Earth objects must come from somewhere; that is, we are constantly receiving our supply of near-Earth objects from another, much larger source.”

Modeling the orbits of dark comets

To determine the origin of this dark comet, Taylor and his co-authors created a dynamical model that assigned non-gravitational accelerations to different populations of objects. They then modeled the paths these objects would have taken over a 100,000-year period given their assigned non-gravitational accelerations. The researchers observed that many of these objects ended up in locations where dark comets are currently located, and found that of all potential sources, the main asteroid belt was the most likely place of origin.

One such dark comet, 2003 RM, is traveling in an elliptical orbit close to Earth, heading toward Jupiter, then passing by Earth again, and its orbit is the same as would be expected for a Jovian comet, Taylor said, meaning its position is consistent with a comet being pushed inward from its orbit.

The role of ice in the dynamics of dark comets

Meanwhile, the study found that the remaining dark comets likely came from the inner belt of the asteroid belt, which is an indication of the presence of ice in the inner main belt, as dark comets are more likely to contain ice.

Next, the researchers applied a previously proposed theory to the dark comet population to determine why these objects are so small and spinning so quickly. Comets are rocky structures held together by ice — think of them as dirty chunks of ice, Taylor says. When they collide with the solar system’s icy layers, the ice starts to release gases. This causes the object to accelerate, but it also causes it to spin so fast, sometimes fast enough to break apart.

“These pieces also have ice attached to them, so they spin faster and faster until they break off,” Taylor said. “They just keep doing that until they get smaller and smaller. What we’re proposing is that to create these small, fast-spinning objects, you break up a few larger objects.”

As this happens, the object continues to lose ice, getting smaller and spinning even faster.

The researchers believe that the larger dark comet, 2003 RM, is probably a larger object that was ejected from the outer main belt of the asteroid belt, while the other six objects studied are more likely to have come from the inner main belt and were created by an object that was pushed inward and collapsed.

Reference: “Proposed Dynamical Origin and Evolutionary Trajectories of Dark Comets” by Aster G. Taylor, Jordan K. Steckloff, Daryl Z. Seligman, Davide Farnocchia, Luke Dones, David Vochoroflicki, David Nesvorny, and Marco Micheli, 6 July 2024, Icarus.
DOI: 10.1016/j.icarus.2024.116207