“Dark comets” may have distributed much of Earth’s water

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

Dark comets are near-Earth objects that exhibit non-gravitational acceleration (likely due to outgassing) but have no coma, Aster Taylor, the paper’s lead author and a Hertz colleague and graduate student in astronomy at University of Michigan in Ann Arbor. email. That means they’re evaporating ice from their surface, but not losing enough dust to have a visible cloud, Taylor says.

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

The existence of dark comets implies a larger population of volatiles (substances that evaporate easily) and organics in the asteroid’s Inner Main Belt than previously known, Taylor says. Although their orbits are what we would expect for asteroids, their non-gravitational accelerations mean they are losing ice from their surface, which is an important characteristic of comets, Taylor says.

How important were dark comets in the distribution of Earth’s water?

They may be important in the distribution of Earth’s water, since they probably have water and are close to the ground, but we don’t know how common these objects are or how common they once were, Taylor says.

What about the difference between a dark comet and an ordinary comet?

Asteroids are ice-free rocky bodies that orbit closest to the sun, usually 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 held to sublimate (or change from solid ice directly to gas), the university explains.

As Taylor notes, the main difference is that a dark comet does not have a visible coma.

Dark comets are also in orbits that are more typical of asteroids and will spend much or all of their time in a region of space where the ice will sublimate, Taylor says.

The Michigan study examined seven dark comets and estimates that between 50 and 6,000 additional near-Earth objects may be dark comets, the University of Michigan says. In other words, these comets do not appear to have comas, but have non-gravitational accelerations that must come from some sort of degassing, the university notes.

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

Strange near-Earth objects

Dark comets are small near-Earth objects that don’t have any dust clouds around them, which makes them look like an asteroid, Taylor says. But they have non-gravitational accelerations that must be from the ice, they say. While you might expect such rapidly spinning objects to fall apart, we find that even a little ‘sticking’ is enough to hold them together, Taylor says.

These dark comets are generally fast-rotating and small — ten to 100 meters in size.

We suggest that these objects are produced by larger objects that spin so fast that they break apart, Taylor says. 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 now, says Taylor. But there are six of them at orbital distances around Earth, and one has a longer orbit that crosses the asteroid belt, they say.

Near-Earth space may be filled with large numbers of completely desorbed (or degassed) fragments of dead comets, which may be extremely difficult to detect, the authors point out.

What is expected next?

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

To that end, in 2031, as the authors note, the Japanese lander Hayabusa2 will reach asteroid 1998 KY26. Once there, this Japanese mission should provide researchers with a much better idea of ​​the origin and composition of this particular dark comet.