In the dark space surrounding Earth, a swarm of small, icy objects is swarming after being expelled from farther out in the solar system. Neither quite comets nor quite asteroids, these mysterious objects could be a way for water to end up on Earth billions of years ago.
A new study suggests that up to 60 percent of near-Earth objects are dark comets, a nearly invisible hybrid that behaves like a comet and an asteroid at the same time. These dark comets may have been part of larger objects in the main asteroid belt (the region of the solar system between Mars and Jupiter) that were then expelled and broken into smaller pieces and are now closer to Earth. The research results are available on the preprint server arxiv and have been accepted for publication in Icarus Magazine
“We don’t know if these dark comets delivered water to Earth. We can’t say that. But what we can say is that there is still debate about how exactly Earth’s water got here,” said U-M astronomy graduate student Dr. Astor Taylor, lead author of the new study, said in a statement. “The work we’ve done suggests that this is another pathway for transporting ice from elsewhere in the solar system into the Earth environment.”
About 71% of the earth is covered by water, and water is the key to life on earth. As vital as water is to Earth, scientists still aren’t sure how it got here. There are different theories about how the Earth obtained its water. Perhaps the Earth was born with it, or the water moved here through space rocks like asteroids or comets.
Asteroids are rocky objects orbiting the sun in a ring between Mars and Jupiter that are close enough to the star for ice to turn into gas. Comets, on the other hand, are made of ice and dust. As they approach the sun, their material begins to evaporate, forming a fuzzy tail called a coma.
In between are dark comets, which are small, nearly invisible objects that have no tails but whose orbits are not determined by the Sun’s gravity like asteroids. Instead, the seven dark comets examined by the researchers behind the new study showed orbital accelerations that couldn’t be explained by the Sun’s gravity alone. However, comets do exhibit this erratic behavior, speeding up in their orbits as they heat up and the ice sublimates.
“We think these objects came from the inner and/or outer main asteroid belt, which means this is another mechanism for bringing some ice into the inner solar system,” Taylor said.
To determine the origin of dark comets, researchers created models to simulate gravity-independent accelerations on different objects, tracing the paths these objects followed over a period of 100,000 years. They found that the main asteroid belt is the most likely source of objects experiencing non-gravitational acceleration, with most of the objects ending up where dark comets are found today.
Dark Comet 2003 RM, one of the objects examined in the new study, follows an elliptical orbit close to Earth before flying toward Jupiter and then passing Earth again. Research suggests it was likely pushed inward from a more distant orbit around the gas giant.
Dark comets are simply fragments of larger asteroids that have survived the cosmic thrust, but they may still contain ice, which causes their spins to accelerate. “The pieces will also have ice on them, so they will also spin faster and faster until they break into more pieces… The way to get these small, fast-spinning objects is to take some larger objects and break them into pieces, “Taylor said.
The new study barely scratches the surface of the unstable surface of these mysterious objects. There’s still much to learn, including how dark comets contribute to the story of Earth’s origins. “There may be more ice in the inner main zone than we thought. There may be more such objects out there,” Taylor said. “This could be a large portion of the recent population. We really don’t know, but because of these findings we have more questions.
more: Beyond the Planets: The Solar System’s Weird Underdogs
