The decades-long debate over the origin of water on planet earth just got a little more complicated. A University of Michigan-led study published in the journal Icarus makes the case that a large part of earth’s water may have come from so-called ‘dark comets.’

Dark comets are near-earth objects that exhibit nongravitational accelerations (likely due to outgassing) but have no comae, Aster Taylor, the paper’s lead author and a Hertz fellow and graduate student in astronomy at the University of Michigan in Ann Arbor, told me via email. This means that they are evaporating ices from their surface but are not losing enough dust for there to be a visible cloud, says Taylor.

In fact, up to 60 percent of near-earth objects could be dark comets, the University of Michigan reports. They likely contain (or previously contained) ice and could represent one way that water could have been delivered to earth, the university notes.

The existence of dark comets implies a larger population of volatiles (substances readily vaporize) and organics in the inner Main Asteroid Belt than previously known, says Taylor. Even though their orbits are what we would expect for asteroids, their nongravitational accelerations imply that they’re losing ices from their surface, which is an important characteristic of comets, says Taylor.

How important were dark comets in delivering earth’s water?

They could be important in delivering earth’s water, since they probably have water and are near earth, but we don’t know how common these objects are, or how common they used to be, says Taylor.

As for the difference between a dark comet and an ordinary comet?

Asteroids are rocky bodies with no ice that orbit closer to the sun, typically within what’s called the ice line, notes the University of Michigan. This means they are close enough to the sun for any ice the asteroid may have been carrying to sublimate (or change from solid ice directly into gas), the university explains.

As Taylor notes, the primary difference is that a dark comet doesn’t have a visible coma.

Dark comets are also on orbits that are more typical for asteroids and will spend much or all of their time in a region of space that ices will be sublimating, says Taylor.

The Michigan study examined seven dark comets and estimates that between 50 and 6000 additional near-earth objects could be dark comets, says the University of Michigan. In other words, these comets do not appear to have comae but do have nongravitational accelerations that must come from some sort of degassing, the university notes.

In their paper, Taylor and co-authors created dynamical models that assigned nongravitational accelerations to objects from different populations, notes the University of Michigan. The team then modeled a path these objects would follow given nongravitational accelerations over a period of 100,000 years, the university notes. The researchers found that the Main Asteroid Belt was their most likely place of origin, the university reports.

Strange Near-Earth Objects

Dark comets are small near-earth objects that do not have any dust clouds around them, which makes them look like an asteroid, says Taylor. But they have nongravitational accelerations that must be from ices, they say. While you might expect that such quickly spinning objects would tear themselves apart, we find that even a little ‘stickiness’ is enough to hold them together, says Taylor.

These dark comets are generally rapidly rotating and small —- on the order of ten to 100 meters in size.

We suggest that these objects are produced by larger objects spinning so fast that they tear themselves apart, says Taylor. If this happens many times, one object produces many objects like dark comets, they say. Using simulations of their motion, we find that most of these bodies come from the inner edge of the asteroid belt, Taylor says.

How many are known?

We know of seven objects right now, says Taylor. But there’s six of them at orbital distances around the earth and one has a longer orbit that crosses the asteroid belt, they say.

Near-earth space may be littered with numerous dead fully devolatilized (or degassed) comet fragments which may be exceedingly difficult to detect, the authors note.

What’s Next?

We need to find more dark comets, says Taylor. Looking for activity on these objects, particularly when they’re the closest to the sun, will also help constrain the composition of these objects, they say.

To that end, in 2031, as the authors note, Japan’s Hayabusa2 lander will arrive at the asteroid 1998 KY26. Once there, this Japanese mission should provide researchers with a much better idea about this particular dark comet’s origins and composition.