The Rosetta mission is dispelling the notion that comets are leftover bits from crushing cosmic collisions. Instead, analysis of data from the European Space Agency mission to Comet 67P/Churyumov–Gerasimenko suggests that the icy bodies could actually be leftover from the primordial process of Solar System formation.
A new study using Rosetta data and led by Björn Davidsson of NASA's Jet Propulsion Laboratory has been published in the journal Astronomy & Astrophysics laying out the evidence that comets are older remnants of the cosmos rather than younger bits of debris from violent space impacts.
"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," says Matt Taylor, ESA's Rosetta project scientist.
Rosetta's extensive study of Comet 67P has shown it to be highly porous and of low density. The butterfly-shaped body also is made up of numerous layers that scientists believe points to the accumulation of material on each of the lobes of the shape until they eventually merged. Because the nucleus of the comet is so highly porous, it suggests that it can not be the creation of violent impacts because they would have compacted the fragile material.
Instead, Davidsson says comets formed from leftover grains and pebbles of icy material in the outer parts of the solar nebula as it was in the process of forming what came to be our Solar System.
These leftovers were what remained after larger Trans-Neptunian Objects (TNOs) began forming early in the Solar System's history when turbulent gas streams were present to aide their growth into objects as large as 400 km (249 mi) across. Eventually, some TNOs would grow to become even larger objects like dwarf planets eons later.
Davidsson and his team believe that comets, on the other hand, can be traced to a few million years later when much of the gas had disappeared from the solar nebula and only solid materials were left behind. This leftover material came together at a low velocity to form comets measuring about 5 km (about 3 mi) in size with fragile porous nuclei and low density.
TNOs also affected their smaller neighbors later on by stirring up the orbits around them, speeding up material that would eventually be accreted by comets to form their outer layers and allowing small kilometer-sized objects to bump into each other and even merge.
"Comets do not appear to display the characteristics expected for collisional rubble piles, which result from the smash-up of large objects like TNOs. Rather, we think they grew gently in the shadow of the TNOs, surviving essentially undamaged for 4.6 billion years," said Davidsson. "Our new model explains what we see in Rosetta's detailed observations of its comet, and what had been hinted at by previous comet flyby missions."
Those observations include numerous "goosebumps" and "clod" features seen on the comet that suggest an overall "lumpiness" to the comet that can be explained by an accumulation of small cometesimals (small comets) into the body of the comet over millions of years early in the history of the Solar Aystem.
Rosetta's spectral analysis also revealed the comet's surface has been altered very little by liquid water and that it is rich in carbon monoxide, oxygen, nitrogen, argon and other supervolatiles. For such molecules to survive so long means the comet must have formed in extremely cold conditions and has not been impacted by much internal radiation. (Rosetta has also found evidence of other key building blocks of life at the comet.)
"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. We had to resolve this paradox by taking a detailed look at the time line of our current Solar System models, and consider new ideas," explains Davidsson.
The gist of those new ideas is that comets are likely to be time capsules that have captured a primordial moment from the genesis of the solar system. By forming slowly over time in extreme cold, rather than from violent collisions as previously suggested, comets preserve ancient material from the solar nebula, the cosmic womb of the solar system.
"Comets really are the treasure-troves of the Solar System," adds Matt Taylor. "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."
Rosetta arrived in orbit around Comet 67/P almost two years ago, in August of 2014. After a troubled attempt to communicate with the Philae lander it deployed to the surface of the comet, it has continued to orbit the comet as it passed the sun and was slung back out towards deep space. ESA now plans to end Rosetta's mission by crashing it into the comet as soon as September 30.
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