In the darkness of space that surrounds Earth, a group of small, icy bodies are swarming after being expelled from the farther reaches of the solar system. The mysterious objects, which are not quite comets nor are they asteroids, could be one way that water ended up on Earth billions of years ago.
A new study suggests that up to 60% of near-Earth objects are dark comets, a nearly invisible hybrid object that behaves like a comet and an asteroid at the same time. These dark comets may have been a part of much larger bodies in the main asteroid belt, a region of the solar system that lies between Mars and Jupiter, before being expelled and broken-up into smaller fragments that now travel closer to Earth. The findings are available on the pre-print server arxiv and have been accepted for publication in the Icarus journal
“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,” Aster Taylor, astronomy graduate student at the University of Michigan, and lead author of the new study, said in a statement. “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.”
Around 71% of our planet is covered in water, the key potion to life on Earth. As essential as water is to Earth, scientists are still not sure how it got here. There are different theories of how Earth got its water; maybe the planet was born with it, or water made its way here by way of traveling space rocks like asteroids or comets.
Asteroids are rocky bodies that orbit the Sun in the ring between Mars and Jupiter, they are close enough to the star for ice to turn into gas. Comets, on the other hand, are made of ice and dust. When they travel close to the Sun, their material starts to vaporize, forming a fuzzy tail known as a coma.
Somewhere in between are dark comets, small, barely visible objects that lack the tail of a comet, and yet their orbits aren’t dictated by the Sun’s gravity like asteroids. Instead, the seven dark comets examined by the researchers behind the new study displayed orbit acceleration that cannot be explained by the gravity of the Sun alone. Comets, however, do display this erratic behavior, speeding up in their orbits when they heat up and their ice sublimates.
“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 said.
To determine the origin of dark comets, the researchers created models to simulate acceleration not related to gravity on different objects, tracing the path these objects would follow over a period of 100,000 years. They found that the main asteroid belt is the most likely source of objects that experience non-gravitational acceleration, and that most of the objects end 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, then out to Jupiter and back past Earth again. It was likely pushed inward from its farther orbit around the gas giant, according to the research.
The dark comets are mere fragments of larger asteroids that survived the cosmic shove, but they likely still contain ice, which causes their spin to accelerate. “These pieces will also have ice on them, so they will also spin out faster and faster until they break into more pieces…the way you get these small, fast rotating objects is you take a few bigger objects and break them into pieces,” Taylor said.
The new study barely scratches the volatile surface of these shadowy celestial beings. There’s still so much to be learned, including how dark comets may have contributed to the origin story of Earth. “There may be more ice in the inner main belt than we thought. There may be more objects like this out there,” Taylor said. “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.”
More: Beyond the Planets: The Quirky Underdogs of the Solar System
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