More than half of near-Earth objects may be 'dark comets'

The next time you visit the beach or a large lake, or just drink a cold glass of water, think about where it came from. There are many ways water could have reached the nascent Earth: via comets, “wet asteroids,” and the ejecta of gases from early volcanism. Astaire Taylor, a graduate student at the University of Michigan, has another idea: dark comets. They are a cross between asteroids and comets, and they may have played a role in delivering water to our planet.

Dark comets are small celestial bodies in the solar system. They have a short rotation period due to non-gravitational pushes from sublimation, which creates jets. These mysterious objects probably make up more than half of all near-Earth objects.

Dark comets and asteroids

Planetologists consider dark comets to be a population of active asteroids. However, they are not in the same category as regular asteroids and comets. They orbit the Earth, so when one passes close to the Sun, it does not develop a coma. Because of this lack of a coma, they are called “dark comets.” However, their sublimation jets seem to be a reaction to radiation from the Sun. They are likely rich in water ice, which raises an interesting question. Could they also have been a source of water for Earth in the distant past?

“We don’t know whether these dark comets delivered water to Earth,” says Taylor. “But we can say that there’s still a debate about how exactly the water got to Earth,” says Taylor. “The work we’ve done has shown that this is another way for ice to be delivered from the rest of the solar system to the Earth’s environment.”

Water delivery from small bodies

The story of how Earth got its water is still not clear. One theory is that the young Earth formed with the molecular precursors of water already in place. Another is that asteroids and comets brought water to Earth during or shortly after its formation. This is interesting because most asteroids are near the so-called “ice line,” a region far beyond Earth where liquids freeze. Something prompted them to travel to the inner solar system. As they approached the Sun, their ice sublimated. The same thing happens with a comet. So perhaps both comets and planetesimals were carriers of water during Earth’s formation. Volcanic activity could have released the water they captured as vapor.

What about wet asteroids? Where did they come from? We know that comets form in the cooler part of the protosolar nebula. Somehow (through gravitational perturbations and dynamical effects) they fell into the inner solar system. There they collided with Earth (just as Comet Shoemaker-Levy 9 collided with Jupiter in 1994).

That leaves the water-ice-rich asteroids, or “dark comets.” Most water-rich asteroids, or “dark comets,” are found in the Asteroid Belt. However, there are also quite a few orbiting in the inner solar system. These near-Earth objects likely found their way to the Sun through gravitational interactions with Jupiter or other worlds. Those with some water ice on or below their surfaces may have been the mechanism for delivering water to the early Earth.

Searching for water-rich dark comets

The same could be said for dark comets, Taylor said. “We think these objects are coming from the inner and/or outer main asteroid belt, and that suggests that this is another mechanism for delivering ice to the inner solar system,” he said. “There may be more ice in the inner main belt than we thought. There may be more of these objects out there. It may be a significant part of the nearby population. We don’t really know, but these findings raise a lot of new questions for us.”

To test their ideas about dark comets, Taylor and team members created dynamic models that looked at different populations of these objects and simulated possible paths they might take to get to Earth. Many of these objects in the model ended up in places where dark comets exist today — on orbits that take them into the inner solar system. The model showed the team that many of these objects ended up in places where dark comets exist today, and that their source was the main asteroid belt.

One object implies many

The team's work also suggests that one large object may originate from a Jupiter-family comet that orbits close to Jupiter. This object is called 2003 RM and is on an elliptical orbit that takes it close to Earth, then to Jupiter, and back past Earth. Its orbit is fairly typical of Jupiter-family comets that have been knocked out of their orbits.

Taylor's team focused on seven dark comets. Their findings suggest that between 0.5 and 60 percent of all near-Earth objects may be dark comets, which are not accelerated by gravity. Instead, these objects are accelerated non-gravitationally, meaning they are propelled by the “reactive action” of ice as it sublimes. The researchers suggest that these dark comets likely originated from the asteroid belt, but were propelled by these non-gravitational accelerations. They also believe that other asteroids in the belt also contain ice.

More about dark comets

The dark comet population includes small, rapidly rotating objects, especially compared to larger asteroids. Comets are known to rotate quite rapidly because as they approach the Sun, they begin to lose their ice through sublimation. As we saw when the Rosetta spacecraft studied comet 67P/Churyumov-Gerasimenko, the comet's nucleus ejects small jets during the sublimation process. These jets push the comet's nucleus forward. They also accelerate it, giving the object that non-gravitational acceleration discussed above. Sublimation can also cause an object to rotate quite rapidly. If it rotates fast enough, the object (the comet's nucleus or asteroid) breaks apart.

“These pieces will also have ice on them, so they'll spin faster and faster until they break apart into new pieces,” Taylor says. “You can just keep doing that, getting smaller and smaller and smaller. We suspect that the way you get these small, fast-spinning objects is that you take a bunch of big objects and break them apart.”

As these dark objects lose their ice, they become even smaller and spin faster. Taylor's team believes that while the large dark comet 2003 RM was likely a larger object ejected from the outer main belt of the asteroid belt, the six other objects studied likely came from the inner main belt. They were likely part of a larger object that was ejected inward and broke apart. Further study of this and similar dark comets should help determine how much these objects contributed to bringing water to Earth.