How life began on Earth is one of the most profound questions of our time, and a key part of that is understanding the origins of the Solar System. Comets, in that regard, are hugely important.
But astronomers hadn’t been sure if comets were remnants of the early Solar System or fragments of collisions later on. Now, thanks to a new study in Astronomy and Astrophysics, we are pretty sure it’s the former.
The study, led by Björn Davidsson of NASA’s Jet Propulsion Laboratory (JPL), looked into the history of objects in the Solar System, specifically comets and trans-Neptunian objects (TNOs), those originating beyond Neptune.
Using data from ESA's Rosetta spacecraft, we knew that the nucleus of Comet 67P/Churyumov-Gerasimenko is highly porous, or in other words, it has a lot of empty space. If this comet had been formed by a collision, the forces involved would likely have squashed the material down, reducing the porosity.
Davidsson’s study suggests that TNOs formed earlier in the first million years of the Solar System, gradually accreting material over the next 400 million years. Comets then grew out of the remains left behind by the TNOs in cold, outer parts of the solar nebula. They accreted material slowly, giving rise to the highly porous but low-density objects we see today. In doing so, they also retained some of the oldest material from the nebula.
"Comets do not appear to display the characteristics expected for collisional rubble piles, which result from the smash-up of large objects like TNOs," said Davidsson in a statement. "Rather, we think they grew gently in the shadow of the TNOs, surviving essentially undamaged for 4.6 billion years."
The importance of this research is that it confirms that comets like 67P are giving us a glimpse into the early Solar System.
At one stage, it had been thought that comets may have been responsible for bringing water to the early Earth. However, Rosetta found that the type of water on 67P was different to that on Earth, potentially ruling out this theory. Some theories instead suggest asteroids may have been the cause of our seas and oceans.
But, more recently, 67P has been found to be rich in some of the key ingredients for life. This provides a tantalizing hint that comets may have kick-started the formation of life on Earth.
"Comets really are the treasure-troves of the Solar System," said ESA Rosetta Project Scientist Matt Taylor in the statement. "They give us unparalleled insight into the processes that were important in the planetary construction yard at these early times and how they relate to the Solar System architecture that we see today."