More Than Half of Near Earth Objects Could Be “Dark Comets”

Next time you’re visiting the seaside or a large lake, or even sipping a frosty glass of water, think about where it all originated. There are many pathways that water could have taken to the infant Earth: via comets, “wet asteroids”, and outgassing from early volcanism. Aster Taylor, a University of Michigan graduate student has another idea: dark comets. They’re something of a cross between asteroids and comets and could have played a role in water delivery to our planet.

Dark comets are small Solar System bodies. They have short rotational periods thanks to non-gravitational pushes by sublimation that creates jets. These mysterious objects probably make up more than half of all near-Earth objects.

Dark Comets and Asteroids

Planetary scientists consider dark comets as a population of active asteroids. Yet, they aren’t in the same category as regular asteroids and comets. They’re on near-Earth orbits, so when one passes close to the Sun, it doesn’t grow a coma. That lack of a coma is why they’re called “dark comets.” Yet, their sublimation jets appear to be a response to radiation from the Sun. They’re likely rich in water ice so that raises an interesting question. Could these also have been a source of water for Earth in the distant past?

“We don’t know if these dark comets delivered water to Earth,” said Taylor. “But we can say that there is still debate over how exactly the Earth’s water got here,” Taylor said. “The work we’ve done has shown that this is another pathway to get ice from somewhere in 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 unfolding. One theory says infant Earth formed with molecular precursors to water. Another one says that water-laden asteroids and comets brought water to Earth during or just after formation. That’s interesting because most asteroids exist near the so-called “ice line”—a region well beyond Earth where liquids freeze. Something propelled them to the inner solar system. When they got close to the Sun, their ice sublimated. That’s actually what happens with a comet, too. So, maybe both comets and planetesimals were water-bearers during Earth’s formation. Volcanic activity could have released their trapped water as vapor.

How about the wet asteroids, though? Where did they come from? We know that comets formed out in the cooler reaches of the protosolar nebula. Somehow they make their way (through gravitational perturbations and dynamical action) to the inner solar system. There, they have collided with Earth (just like Comet Shoemaker-Levy 9 encountered Jupiter in 1994).

That leaves the water ice-rich asteroids or “dark comets”. Most water-rich asteroids or “dark comets” exist in the Asteroid Belt. However, plenty of them orbit in the inner solar system, too. Those near-Earth objects probably made their way sunward due to gravitational interactions with Jupiter or other worlds. Those with some amount of water ice trapped on or below their surfaces could have been a delivery mechanism for water to early Earth.

Searching for Water-rich Dark Comets

The same would be true of the dark comets, according to Taylor. “We think these objects came from the inner and/or outer main Asteroid Belt, and the implication of that is that this is another mechanism for getting some ice into the inner solar system,” he 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 nearest population. We don’t really know, but we have many more questions because of these findings.”

To test their ideas about dark comets, Taylor and fellow team members created dynamical models that looked at different populations of these objects and modeled possible paths they could have taken to get to Earth. Many of these objects in the model ended up where today’s dark comets exist—on orbits that bring them into the inner solar system. Their model showed the team that many of these objects ended up where dark comets are today and that the main Asteroid Belt is their source.

One Object Implicates Many

The team’s work also suggests that one large object may come from the Jupiter-family comets, with orbits that take them close to Jupiter. It’s called 2003 RM, and follows an elliptical orbit that brings it close to Earth, then out to Jupiter and back past Earth. Its orbit is pretty typical of a Jupiter family comet that was knocked inward from its orbit.

Taylor’s team studies focused on seven dark comets. The result of their work suggests that between 0.5% and 60% of all near-Earth objects could be dark comets that aren’t accelerated by gravitational interactions. Instead, these objects experience non-gravitational accelerations—that is, they are moved along by the “jet action” of ice as it sublimates. The researchers suggest that these dark comets likely came from the asteroid belt but that they moved due to those nongravitational accelerations. They also think that other asteroids in the Belt also contain ice.

More About Dark Comets

The population of dark comets includes small, fast-rotating objects, especially when compared to larger asteroids. Comets are known to rotate fairly fast because they start to lose their ice to sublimation as they near the Sun. As we saw when the Rosetta spacecraft studied Comet 67P/Churyumov-Gerasimenko, a comet nucleus sprouts little jets as part of the sublimation process. Those jets have the effect of pushing the comet nucleus along. It also accelerates it, giving the object that non-gravitational acceleration mentioned above. Sublimation can also cause the object to spin quite fast. If it rotates quickly enough, the object (comet nucleus or rubble pile asteroid) breaks apart.

“These pieces will also have ice on them, so they will also spin out 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 suggest is that the way you get these small, fast rotating objects is you take a few bigger objects and break them into pieces.”

As these dark objects lose their ice, they get even smaller and rotate more rapidly. Taylor’s team thinks that while the larger dark comet, 2003 RM, was likely a larger object that got kicked out of the outer main belt of the Asteroid Belt, the six other objects they studied likely came from the inner main belt. They probably were part of a larger object that got knocked inward and broke apart. Further study of this and similar dark comets should help determine what contribution these objects played in the delivery of Earth’s water.

For More Information

The Origins of Dark Comets
The Dynamical Origins of the Dark Comets and a Proposed Evolutionary Track