Comets are the ancient leftovers of early solar system formation, and not younger fragments resulting from subsequent collisions between other larger bodies, according to a new study of data collected by the Rosetta space probe.
Understanding how and when comets took shape may help determine how exactly they can be used to interpret the formation and early evolution of our solar system.
If comets are primordial, then they could help unveil the properties of the solar nebula from which the Sun, planets and small bodies condensed 4.6 billion years ago, and the processes that transformed our planetary system into the architecture we see today, researchers said.
The alternative hypothesis is that they are younger fragments resulting from collisions between older 'parent' bodies such as icy trans-Neptunian objects (TNOs).
They would then provide insight into the interior of such larger bodies, the collisions that disrupted them, and the process of building new bodies from the remains of older ones.
"Either way, comets have been witness to important solar system evolution events, and this is why we have made these detailed measurements with Rosetta - along with observations of other comets - to find out which scenario is more likely," said Matt Taylor, project scientist European Space Agency's Rosetta which is performing a detailed study of the comet 67P/Churyumov-Gerasimenko (67P).
The low-density, high-porosity, double-lobed body of 67P/Churyumov-Gerasimenkoas with its extensive layering, suggests that the lobes accumulated material over time before they merged.
The unusually high porosity of the interior of the nucleus provides the first indication that this growth cannot have been via violent collisions, as these would have compacted the fragile material, researchers said.
Earlier work showed that the head and body were originally separate objects, but the collision that merged them must have been at low speed in order not to destroy both of them.
The fact that both parts have similar layering also tells us that they must have undergone similar evolutionary histories and that survival rates against catastrophic collision must have been high for a significant period of time.
"While larger TNOs in the outer reaches of the solar system appear to have been heated by short-lived radioactive substances, comets don't seem to show similar signs of thermal processing," said Bjorn Davidsson of the Jet Propulsion Laboratory, California Institute of Technology in the US.
"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.
"Rather, we think they grew gently in the shadow of the TNOs, surviving essentially undamaged for 4.6 billion years," he said.
The study was published in the journal Astronomy and Astrophysics.
