Mysterious 'Dark Comets' Could Be The Majority of Near-Earth Objects
If you peer out into the space sharing Earth's general orbital vicinity around the Sun, you'll find a lot of rocks. We have counted, to date, more than 35,000 near-Earth objects: asteroids and comets that can sometimes enter our corner of the Solar System before swooping away again.
It's thought that there might be many more out there, too. And a new study has found that up to 60 percent of them could be something fascinating – what are known as "dark comets" that once contained or still contained ice, locked up in their enigmatic rocky bodies.
This, according to a team of astronomers led by Aster Taylor of the University of Michigan, could be an important clue about where Earth got its water, back when the Solar System was but a mewling cosmic babe.
"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 over how exactly the Earth's water got here," Taylor explains.
"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."
Free-roaming rocks within the Solar System can take several different forms. Asteroids and comets are the two most well-known, with asteroids being dry rocks, and comets being icy rocks that start to eject material when they move near the Sun and heat up. (Meteors are rocks that enter a planetary atmosphere, and meteorites are the chunks that fall to a planetary surface.)
But within the two space rock categories, there's a lot of variation, and even some crossover between them. Dark comets are thought to be one type of hybrid, but we don't know a lot about them.
A fuzzy 'coma' or atmosphere of gas and a tail produced by the sublimation of ice are not the only two characteristics that are diagnostic of cometary properties. Another characteristic is its acceleration.
As a comet ejects material, that ejection gives the comet an additional push of propulsion beyond what you'd expect to see if the rock was just an asteroid experiencing only gravitational forces. In addition, this outgassing can speed up the spin of the comet.
A dark comet is one that doesn't have a visible coma or tail, but when measured, has this additional non-gravitational acceleration. Taylor and their colleagues studied the seven known dark comets to try to get a better understanding of how many more could be out there, lurking in near-Earth space.
According to the team's estimates, somewhere between 0.5 and 60 percent of all near-Earth objects could be dark comets.
That's a lot of wiggle room, and will need to be narrowed down; but the results as they currently stand suggest that there could be a lot more frozen material out there in the Solar System, not just in Earth's vicinity, but in the belt of asteroids that orbit the Sun between the orbits of Mars and Jupiter.
"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," Taylor says.
"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."
Near-Earth objects, they explain, don't tend to hang around our home planet for very long, cosmically speaking. The inner Solar System is pretty gravitationally hectic, which means the longevity of a near-Earth object is only around 10 million years or so.
Since the Solar System is around 4.6 billion years old and we haven't run out of near-Earth objects, the supply must be constantly replenished.
The research team conducted simulations to see where the dark comets might originate, assigning non-gravitational accelerations to objects from different populations within the Solar System. Then, they set them running to see where they ended up. This suggests that most of the dark comets near Earth originated from the main asteroid belt.
But remember that accelerated spin? A comet can end up spinning so fast that it breaks apart under the sheer centrifugal force of its rotation. The resultant pieces of the comet are also icy and gassy, and therefore also spinning and moving under non-gravitational acceleration. So you get a whole bunch of rocks for the price of one.
Of the seven dark comets the team analyzed, one of them, named 2003 RM, appears to be a larger rock ejected from the main belt – but the other six are the product of the centrifugal fragmentation of a large rock that left the main belt and broke apart when it grew closer to the Sun.
"Ground- and space-based follow-up observations of dark comets may enable measurement of the outgassing rates and compositions, potentially constraining their dynamical origins," the researchers write in their paper.
"Future survey missions … may also identify more dark comets than are currently known, which will refine our understanding of this evolutionary pathway and their source populations."
The research has been published in Icarus.
