A UCF-led research team has found significant amounts of ancient carbon dioxide and carbon monoxide ices in trans-Neptunian objects, indicating that carbon dioxide may have been present during the formation of our solar system. For the first time, carbon dioxide and carbon monoxide ices have been detected in trans-Neptunian objects (TNOs) in the outermost regions of our solar system.

A research team led by planetary scientists Mario Nascimento De Pra and Noemi Pinilla-Alonso of the Florida Space Institute (FSI) at the University of Central Florida made the findings by using the infrared spectral capabilities of the James Webb Space Telescope (JWST) to analyze the chemical composition of 59 trans-Neptunian objects and centaurs.

Published pioneering research Natural Astronomy, It suggests that carbon dioxide ice was abundant in the cold outer regions of the protoplanetary disk, the vast rotating disk of gas and dust from which the Solar System formed. More research is needed to understand the origin of carbon monoxide ice, as it is also prevalent in the TNOs in the study.

The researchers reported that, out of a sample of 59 objects observed by JWST, carbon dioxide was detected in 56 TNOs and carbon monoxide in 28 (plus six with questionable or minor detections). Carbon dioxide was widely distributed across the surfaces of the trans-Neptunian population, regardless of dynamical class and body size, while carbon monoxide was only detected in objects with high carbon dioxide content.

This work is part of the UCF-led Exploring Surface Compositions of Trans-Neptunian Objects (DiSCo-TNOs) program, one of JWST’s programs aimed at analyzing our Solar System.

“This is the first time we’ve observed this region of the spectrum for a large collection of TNOs, so everything we saw was exciting and unique in a way,” says de Pra, a co-author of the study. “We didn’t expect to find that carbon dioxide is so prevalent in the TNO region, or even that carbon monoxide is present in many TNOs.”

He says the discovery of the ice could help us further understand the formation of our solar system and how celestial bodies may have migrated.

“Trans-Neptunian objects are remnants of the planet formation process,” De Pra said. “These findings could provide important constraints on where these objects formed, how they got to where they are now, and how their surfaces have changed since their formation. Because they formed at greater distances from the Sun and are smaller than the planets, they provide pure information about the initial composition of the protoplanetary disk.”

History of ancient ice

The New Horizons probe observed carbon monoxide ice on Pluto, but until JWST there was no observatory powerful enough to identify and detect traces of carbon monoxide ice or carbon dioxide ice on the largest TNO population.

Carbon dioxide is common in many objects in our solar system, so the DiSCo team was curious about whether it might be too abundant for Neptune to reach.

Possible reasons why carbon dioxide ice has not been detected in TNO before include smaller amounts of non-volatile carbon dioxide buried under layers of other less volatile ices and refractory materials over time, transformation into other molecules due to radiation, and simple observation. There are limitations, according to the study.

The discovery of carbon dioxide and carbon monoxide in TNO provides context but also raises many questions, De Pra says.

“While carbon dioxide probably accumulated from the protoplanetary disk, the origin of carbon monoxide is less clear,” he says. “The latter is volatile ice, even on the cold surfaces of TNO. We cannot rule out that carbon monoxide accumulated first and has somehow been preserved until now. However, the data suggest that it could have been produced by irradiation of carbonaceous ice.”

Responses like an avalanche

Pinilla-Alonso, one of the study’s authors and who leads the DiSCo-TNOs program, says confirming the presence of carbon dioxide and carbon monoxide in TNOs opens up many opportunities to further investigate and measure how and why they are present.

“The discovery of carbon dioxide in trans-Neptunian objects was exciting, but its properties were even more exciting,” he says. “The carbon dioxide spectral signature revealed two different surface compositions in our sample. In some TNOs, carbon dioxide is mixed with other materials such as methanol, water ice, and silicates. But in another group, where carbon dioxide and carbon monoxide are the main surface components, the spectral signature was strikingly unique. This sharp carbon dioxide signature is unlike anything observed on other objects in the Solar System or even produced in the laboratory.”

When carbon dioxide is in excess, it appears to be isolated from other materials, but that alone doesn’t explain the shape of the band, Pinilla-Alonso says. Understanding these carbon dioxide bands is another mystery, he says, likely related to their unique optical properties and how they reflect or absorb certain colors of light.

A common theory is that carbon dioxide could be present in TNO because it is found as a gas in comets, Pinilla-Alonso says.

“In comets, we observe that carbon dioxide is a gas released by the sublimation of ice at or just below the surface,” he says. “But since carbon dioxide has never been observed on the surface of TNO, the general consensus was that it was trapped below the surface. Our latest findings refute this idea. We now know that carbon dioxide is not only present on the surface of TNO, but is also more abundant than water ice, which we previously thought was the most abundant surface material. This discovery significantly changes our understanding of the composition of TNOs and suggests that the processes affecting their surfaces are more complex than we thought.”

Decoding of data

Co-authors of the study were Elsa Hainault, a postdoctoral researcher at the Institute for Space Astrophysics of the University of Paris-Saclay and the French National Center for Scientific Research, and Rosario Brunetto, Hainault’s scientific advisor, who brought laboratory and chemical perspectives to the interpretation of JWST’s observations.

Hénot analyzed and compared the absorption bands of carbon dioxide and carbon monoxide for all the objects. Hainault says that while there is sufficient evidence for the presence of ice, there is great variability in its amount and distribution.

“Although we found that CO2 is ubiquitous in TNO, it is certainly not distributed evenly,” he says. “Some objects are carbon dioxide poor, while others are very rich in carbon dioxide and detect carbon monoxide. Some objects show pure carbon dioxide, while others contain it mixed with other compounds. Correlating the properties of carbon dioxide with orbital and physical parameters allowed us to conclude that changes in carbon dioxide likely represent different regions of object formation and early evolution.”

Hainault says that based on the analysis, it is very likely that carbon dioxide is present in the protoplanetary disk, but it is unlikely that carbon monoxide is primary.

“Carbon monoxide could be produced efficiently by continuous ion bombardment from our sun or other sources,” he says. “We are now investigating this hypothesis by comparing the observations with ion irradiation experiments that can reproduce the freezing and ionization conditions of TNO surfaces.”

The research provided some answers to long-standing questions that have existed since the discovery of TNO nearly 30 years ago, but Eno says researchers still have a long way to go.

“There are now other questions,” he says, “especially when considering the origin and evolution of carbon monoxide. The observations across the entire spectral range are so rich that they will undoubtedly interest scientists for many years to come.”

While the DiSCo program’s observations are nearing completion, the analysis and discussion of the results have yet to take place. De Pra says the fundamental information gained from the research will be an important contribution to future planetary and astronomical research.

“We have only scratched the surface of what these objects are made of and how they came into being,” he says. “We now need to understand the relationship between these ices and other compounds found on their surfaces, and the interaction between their formation scenario, dynamical evolution, volatile retention and irradiation mechanisms throughout the history of the Solar System.”