Uncovering a 'centaur's' tracks: Scientists examine unique asteroid-comet hybrid

Chiron belongs to the class of objects that astronomers call "Centaurs." Centaurs are space objects that orbit the Sun between Jupiter and Neptune. They are akin to the mythological creature they borrow their name from in that they are hybrid, possessing characteristics of both asteroids and comets.

Using the James Webb Space Telescope, UCF Florida Space Institute (FSI) scientists recently led a team that found, for the first time, that Chiron has surface chemistry unlike other centaur. Its surface it has both carbon dioxide and carbon monoxide ice along with carbon dioxide and methane gases in its coma, the cloud-like envelope of dust and gas surrounding it.

The researchers' results were recently published in the journal Astronomy & Astrophysics.

UCF FSI Associate Scientist Noemí Pinilla-Alonso, who now works at the University of Oviedo in Spain, and Assistant Scientist Charles Schambeau led the research. The new findings build upon prior discoveries from Pinilla-Alonso and colleagues that detected carbon monoxide and carbon dioxide ice on trans-Neptunian objects (TNOs) for the first time earlier this year.

Those observations, paired with ones of Chiron, are creating foundational knowledge for understanding the creation of our Solar System, as these objects have largely remained unchanged since the Solar System was formed, Pinilla-Alonso says.

"All the small bodies in the Solar System talk to us about how it was back in time, which is a period of time we can't really observe anymore," she says. "But active centaurs tell us much more. They are undergoing transformation driven by solar heating and they provide a unique opportunity to learn about the surface and subsurface layers."

Since Chiron possesses characteristics of both an asteroid and a comet, it makes it rich for studying many processes that could assist in understanding them, she says.

"What is unique about Chiron is that we can observe both the surface, where most of the ices can be found, and the coma, where we see gases that are originating from the surface or just below it," Pinilla-Alonso says. "TNOs don't have this kind of activity because they're too far and too cold. Asteroids don't have this kind of activity because they don't have ice on them. Comets, on the other hand, show activity like centaurs, but they are typically observed closer to the Sun, and their comas are so thick that they complicate the interpretations of observations of the ices on the surface. Discovering which gases are part of the coma and their different relationships with the ices on the surface help us learn the physical and chemical properties, such as the thickness and the porosity of the ice layer, its composition, and how irradiation is affecting it."

The discovery of these ices and gases on an object as distant as Chiron -- observed near its farthest point from the Sun -- is exciting because it could help contextualize other centaurs and provide insight into the earliest era of our Solar System, Schambeau says.

"These results are like nothing we've seen before," he says. "Detecting gas comae around objects as far away from the Sun as Chiron is very challenging, but JWST has made it accessible. These detections enhance our understanding of Chiron's interior composition and how that material produces the unique behaviors as we observe Chiron."

Schambeau specializes in studying centaurs, comets and other space objects. He analyzed the methane gas coma and determined that the outflowing gas detected was consistent with it being sourced from a surface area that was exposed to the most heating from the Sun.

Chiron, first discovered in 1977, is characterized much better than most centaurs and comparatively is unique, Schambeau says. The newly analyzed information helps scientists better understand the thermophysical process going on in Chiron that produces methane gas, he says.

"It's an oddball when compared to the majority of other Centaurs," Schambeau says. "It has periods where it behaves like a comet, it has rings of material around it, and potentially a debris field of small dust or rocky material orbiting around it. So, many questions arise about Chiron's properties that allow these unique behaviors."

The researchers concluded that the coexistence of the molecules in various states adds another layer of intrigue for studying comets and centaurs. The study also highlighted the presence of irradiated byproducts of methane, carbon monoxide and carbon dioxide that will require further research and could help scientists further reveal the unique processes producing Chiron's surface composition.

Chiron originated from the TNO region and has traveled around our Solar System since its creation, says Pinilla-Alonso. The orbits of Chiron and many other large non-planetary objects occasionally experience close encounters with one of the giant planets where the gravitational pull from the planet changes the smaller object's orbit, taking them all over our Solar System and exposing them to many different environments, she says.

"We know it has been ejected from the TNO population and is only now transiting through the region of the giant planets, where it will not stay for too long," Pinilla-Alonso says. "After about 1 million years, centaurs like Chiron typically are ejected from the giant planets region, where they may end their lives as Jupiter Family comets or they may return to the TNOs region."

Pinilla-Alonso notes that the JWST's spectra showed for the first time Chiron's plethora of ices with different volatilities and their formation processes, she says.

Some of these ices, such as methane, carbon dioxide, and water ice, may be primordial components of Chiron inherited from the pre-solar nebula. Others, such as acetylene, propane, ethane, and carbon oxide, could have formed on the surface because of reduction and oxidation processes, she says.

"Based on our new JWST data, I'm not so sure we have a standard centaur," Pinilla-Alonso says. "Every active centaur that we are observing with JWST shows some peculiarity. But they cannot be all outliers. There must be something that explains why they appear to all behave differently or something that is common between them all that we cannot yet see."

The analysis of Chiron's gases and ices opens new frontiers and opportunities for exciting research, she says.

"We're going to follow up with Chiron," Pinilla-Alonso says. "It will come closer to us, and if we can study it at nearer distances and get better reads on the quantities and nature of the ices, silicates, and organics, we will be able to better understand how seasonal insolation variations and different illumination patterns can affect its behavior and its ice reservoir."

The JWST is the world's premier space science observatory, and it is solving mysteries in our solar system, looking beyond to distant worlds around other stars, and probing the mysterious structures and origins of our universe. The JWST is an international collaboration led by NASA with its partners the European Space Agency and the Canadian Space Agency.

Researchers' Credentials

Pinilla-Alonso was a professor at FSI who joined UCF in 2015. Most of her work on this project was conducted while she was at UCF. Pinilla-Alonso also holds a joint appointment as a research professor in UCF's Department of Physics and has led numerous international observational campaigns in support of NASA missions, such as New Horizons, OSIRIS-REx and Lucy. Pinilla-Alonso is a distinguished professor at the Institute for Space Sciences and Technologies in Asturias, within the Universidad de Oviedo. She received her doctoral degree in astrophysics and planetary sciences from the Universidad de La Laguna in Spain.

Schambeau is an assistant scientist who received his doctoral degree in physics with a concentration in planetary sciences in 2018 from UCF. He subsequently joined FSI where he expanded upon his work examining comets and centaurs as part of UCF's Preeminent Postdoctoral Program.