If you look around the space that shares Earth’s general orbit around the Sun, you’ll find lots of rocks. To date, we’ve counted more than 35,000 near-Earth objects: asteroids and comets that can occasionally swoop into our corner of the solar system before swooping back down.
It is thought that there could be many more. A new study has found that up to 60% of them could be fascinating: so-called “dark comets,” which once contained or still contain ice, locked away in their mysterious rocky bodies.
According to a team of astronomers led by Aster Taylor of the University of Michigan, this could be an important clue to where water came from on Earth, back when the solar system was just a cosmic baby.
“We don’t know if these dark comets brought water to Earth. We can’t say. But we can say that there is still some debate about exactly how Earth’s water got here,” Taylor says.
“The work we’ve done has shown that this is another pathway for delivering ice from the rest of the solar system to the Earth’s environment.”
Rocks that move freely through the solar system can take many different forms. Asteroids and comets are the two best known, with asteroids being dry rocks and comets being icy rocks that begin to eject material as they move close to the Sun and heat up. (Meteors are rocks that enter a planet’s atmosphere, and meteorites are the pieces that fall onto a planet’s surface.)
But within these two categories of space rocks, there are many variations, and even crosses between them. Dark comets are thought to be a type of hybrid, but we don’t know much about them.
A fuzzy “coma” or gas atmosphere and a tail produced by sublimation of ice are not the only features that can diagnose the properties of a comet. Another feature is its acceleration.
When a comet ejects material, this ejection gives it an additional propulsive thrust, beyond what would be expected from an asteroid subjected only to gravitational forces. In addition, this outgassing can accelerate the comet’s rotation.
A black comet is a comet that does not have a visible coma or tail, but when measured, exhibits additional non-gravitational acceleration. Taylor and his colleagues studied the seven known black comets to try to better understand how many more might be out in near-Earth space.
According to the team’s estimates, between 0.5 and 60 percent of all near-Earth objects could be dark comets.
There’s a lot of room for improvement, and they’ll need to be refined; but the results as they stand now suggest that there may be a lot more frozen material in the solar system, not just near Earth, but in the asteroid belt that orbits the Sun between the orbits of Mars and Jupiter.
“We think these objects originated from the inner and/or outer main asteroid belt, implying that this is another mechanism for introducing 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. It could be a significant fraction of the nearest population. We don’t really know, but these findings raise a lot more questions for us.”
Near-Earth objects, they explain, don’t usually hang around our planet for very long, from a cosmic perspective. The inner solar system is pretty gravitationally choppy, meaning the lifespan of a near-Earth object is only about 10 million years.
Since the solar system is about 4.6 billion years old and we are not running out of near-Earth objects, the reserves must be constantly replenished.
The research team ran simulations to determine where dark comets originated, assigning non-gravitational accelerations to objects from different populations in the solar system. They then set them on a course to see where they ended up. This suggests that most dark comets near Earth originate in the main asteroid belt.
But remember that acceleration of rotation? A comet can eventually spin so fast that it breaks apart under the centrifugal force of its rotation alone. The resulting pieces of the comet are also icy and gaseous, and so they also spin and move 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, named 2003 RM, appears to be a large boulder ejected from the main belt — but the other six are the product of the centrifugal fragmentation of a large boulder that left the main belt and broke up as it approached the Sun.
“Follow-up observations of dark comets from the ground and in space can help measure 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, refining our understanding of this evolutionary pathway and their source populations.”
The research was published in Icarus.
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