Comets — those small, icy objects orbiting the sun — act like floating time capsules, offering a window into the early days of our solar system's formation and holding clues about the origins of life on Earth.
In a recent study, scientists developed a groundbreaking model to trace how the chemistry of one comet, known as "Hale-Bopp," has evolved over time. Ultimately, this endeavor managed to reveal insights that could enhance our understanding of planetary formation as a whole — and it may even have shed light on the potential for life beyond our planet.
"You would be hard pressed to find two comets that are completely alike," Drew Christianson from the University of Virginia, told Space.com. "Hale-Bopp has been a favorite of comet studies in part because it was one of the brightest comets seen [...] even dubbed the Great Comet of 1997. As a result, there is a wealth of observations made for which to compare our model's results with."
Hale-Bopp originated in the Oort Cloud, a theoretical shell of icy planetesimals surrounding the sun in the outer reaches of our solar system, and is believed to contain some of the most primitive and well-preserved remnants from the primordial nebula that formed our solar system.
"The Oort Cloud is about as far away from the sun as you can get while still being in the solar system," said Christianson.
But then, at some point in the distant past, Hale-Bopp was knocked out of the Oort Cloud and began its approach toward the sun. Now, it takes roughly 2,400 years to complete its orbit around our star. In other words, we now have an object relatively close to us that not only contains information about the outer reaches of our solar system, but also about the early days of our cosmic neighborhood.
Thus, using observational data collected over the years, scientists have studied Hale-Bopp — particularly its chemical composition and how it might have evolved over time — to provide valuable insights into the state of the ancient solar system. This data is also supplemented by computational models that help us understand the dynamic interactions and transformations that take place within the comet, simulating how complex organic molecules may have formed, survived, or broken down over time. Knowing about those internal cometary interactions is important because it reveals what kinds of molecules might've been present during the beginning of our solar system — and may therefore suggest how life began.
However, such analyses are challenging when studying fully formed comets like Hale-Bopp, and become even trickier as the comet approaches the sun, where it heats up and spews dust and gases into a giant glowing "coma."
One of the main questions the new study's researchers have concerns how to tell apart the chemical species created in the comet's ice (the "parent" species) from those formed through energetic processes in the coma (the "daughter" species).
Scientists have developed models of gas-phase chemistry to investigate the processes occurring in cometary comas, but current iterations of these models focus only on active comets and do not consider the chemistry occurring in icy comet nuclei. As a result, those models may not adequately capture the full range of conditions and processes experienced by a comet throughout its lifetime.
"There have been plenty of chemical models of the gas and surfaces around comets in the past," said Christianson. "Similarly, there have been studies of the physical conditions of the comet ice and dust itself. However, prior to [our model], MAGICKAL, there had been no chemical models of the body of the comet, making it the first of its kind."
The team's new model tracks the orbit of Hale-Bopp, starting from its cold-storage phase in the Oort Cloud followed by five of its orbits around the sun. The model divides the comet body into 25 distinct layers of ice and dust, with complex chemistry happening at various depths, allowing them to better represent the true reality of the comet's chemistry.
"When a comet is heated as it approaches the sun, it is not heated uniformly," said Christianson. "The outside becomes warmer first before reaching deeper into the comet. Similarly, UV rays and cosmic rays only reach to certain depths, and affect each depth differently.
Hale-Bopp's nucleus, composed of frozen water, gases, dust and rocky material, contains complex organic molecules such as formamide, methyl formate, ethylene glycol, methanol, and acetonitrile — substances that scientists indeed suspect may be linked to the origins of life on Earth.
Interestingly, the research suggests that most of the complex organic molecules in Hale-Bopp are likely inherited from its primordial origins rather than formed in its current journey. "Comets are aggregates of the icy dust grains that grow during the star and planet formation process," said Christianson. "While dust grains are ubiquitous in the interstellar medium, in dense star-forming regions, they build up icy mantles from material in the surrounding gas while temperatures are still low."
Scientists believe that it is within these ices that complex organic molecules form.
"JWST observations are also beginning to confirm this view that [these molecules] are there in the cold ices," added Christianson. "Eventually, those icy grains aggregate to form larger and larger bodies that we know as comets. So many of the complex organic molecules in comets seem to have an interstellar origin."
Before we can jump to any conclusions, Christianson cautions that comets being a source for organic molecules on Earth is a very contentious topic. "We don't know for sure what influence, if any, early comets had on the early Earth. Our model's findings don't much affect how the solar system formed, but it does suggest that it was indeed a very complex environment with all manner of chemicals, organic or otherwise. But organic molecules delivered to an early Earth from comets could have originated from well before the Sun was ever formed. It's a fascinating question that we and others will continue to study."
He says the team plans to further refine its model to more accurately represent real comets, aiming to make more reliable predictions about new comets and the insights they may offer into evolutionary histories. To this end, they are currently preparing a model study of comet 67P–a Jupiter-family comet that made history as the first comet to be orbited and landed on by a spacecraft from Earth.
"When it comes to observations, we haven't even observed a fraction of a percent of the comets in the solar system," Christianson said. "Most are simply out of reach. If we observe more comets and find them to be less complex than we initially thought, then perhaps the composition could be formed entirely within the solar system. Or, if we find more chemically complex comets, that would further solidify the idea that complex molecules must have been formed prior to the formation of the solar system.
"Only time will tell."
