How scientists use radiative transfer modeling and laboratory measurements to study the comet's surface composition
When the European Space Agency's Rosetta spacecraft embarked on its historic mission to comet 67P/Churyumov-Gerasimenko, it revealed a world of astonishing complexity—a rubber-duck-shaped celestial body with dramatic cliffs, dusty plains, and crackling with activity. This comet, which orbits the Sun every 6.44 years, has become one of the most extensively studied small bodies in our Solar System 1 .
But beyond the stunning imagery lies a deeper scientific quest: understanding what this cosmic iceberg is made of and what it can tell us about the origins of our Solar System.
Today, an international team of scientists coordinated by the International Space Science Institute (ISSI) is using this fascinating comet as a natural laboratory. They're combining cutting-edge radiative transfer modeling with precise laboratory measurements to unravel the secrets locked within 67P's surface composition—a scientific endeavor that could reshape our understanding of how planets form and life's building blocks are distributed throughout the cosmos.
Comet 67P/Churyumov-Gerasimenko is a Jupiter-family comet with an orbital period of approximately 6.44 years 1 . During its 2021/2022 apparition—its closest approach to Earth since 1982—astronomers recorded remarkable activity, including significant outbursts that were an order of magnitude stronger than any observed by the Rosetta spacecraft during its 2014-2016 mission 1 .
Recent observations have revealed surprising asymmetries in the comet's behavior. Gas production rates were 2 to 6 times higher after perihelion (the closest point to the Sun) compared to before, while dust production rates were 1.5-2.5 times higher before and around perihelion compared to previous apparitions 1 . This suggests complex processes occurring within the comet's interior and on its surface that scientists are still working to understand.
| Characteristic | Description |
|---|---|
| Orbital Period | 6.44 years |
| Last Perihelion | 2 November 2021 |
| Closest Earth Approach (2021) | 0.418 au (astronomical units) |
| Size & Shape | Approximately 4 km across, "rubber duck" shape with two distinct lobes |
| Notable Features | Hapi region (neck), dramatic cliffs, active pits, and varying surface composition |
The ISSI has long facilitated groundbreaking research through its International Teams program, which brings together scientists with complementary expertise from different institutions and countries worldwide 4 . These teams hold a series of meetings over 24 months to collaborate on research projects leading to publications in scientific journals 4 .
In 2025, a record number of 113 proposals were submitted to ISSI, with 31 selected for implementation—4 of which focused specifically on planets and small bodies 8 . While the exact team studying 67P's surface composition isn't named in the available sources, ISSI's framework ensures that such projects benefit from diverse scientific perspectives and interdisciplinary tools essential for tackling complex questions in space science 4 .
These collaborations typically include up to 14 researchers, including early-career scientists, who work together in a series of one-week meetings spread over two years 4 . This sustained interaction allows for deep dives into complex problems that would be difficult to solve through standard short-term collaborations.
International teams with scientists from multiple countries working together
Radiative transfer modeling is a powerful scientific tool that helps researchers understand how light interacts with materials. In the context of comet 67P, these models allow scientists to decode the chemical and physical properties of the comet's surface by analyzing how it reflects and emits light.
Fundamental Principle: When sunlight hits 67P's surface, different compounds and grain sizes absorb, reflect, and scatter light in distinctive ways. By building sophisticated mathematical models that account for these interactions, researchers can work backward from the observed light to determine what the surface must be made of to produce such signatures.
Creating accurate models for cometary surfaces presents unique challenges. The surface of 67P is anything but uniform—it features a complex topography with regions of varying reflectivity, porosity, and chemical composition. The Hapi region, often described as the comet's "neck," displays particularly interesting properties, with the maximum escape speed detected in this area 6 .
Furthermore, the comet's slow rotation means that the gravitational potential is the dominant force on its surface, creating a unique environment where particles behave differently than they would on larger bodies 6 . Understanding how solar radiation pressure affects particles across 67P's surface is crucial—research shows that particles larger than approximately 10⁻³ cm at apocenter (farthest point from Sun) and 10⁻¹ cm at pericenter (closest point to Sun) are not significantly affected by solar radiation pressure 6 .
| Discovery | Significance |
|---|---|
| Four external equilibrium points identified around the comet, with two (E₂ and E₅) being linearly stable 6 | Helps understand orbital dynamics and potential locations where particles might accumulate |
| Particle motion varies significantly across different surface regions 6 | Explains observed surface features and material distribution |
| Solar radiation pressure has minimal effect on larger particles 6 | Provides constraints on dust behavior and lifetime in the coma |
| Pronounced morphological changes observed in southern active zones 6 | Demonstrates the comet's ongoing evolution and activity cycles |
One crucial experiment that exemplifies the ISSI team's approach combines telescopic observations, radiative transfer modeling, and laboratory measurements to map 67P's chemical distribution during different orbital phases.
The process begins with detailed observations using powerful ground-based telescopes like the 6-m Big Telescope Alt-azimuth (BTA) in Russia. During the 2021/2022 apparition, researchers used specialized instruments to obtain both spectra and images of the comet 1 . To increase the signal-to-noise ratio, they aligned and summed all the spectral images using a robust averaging algorithm 1 .
The resulting spectra revealed the comet's composition through its molecular fingerprints—strong emissions from CN molecules at about λ3883 Å, and weaker emissions from C₂ and C₃ molecules 1 . The continuum formed by sunlight scattering on dust grains provides additional information about the dust properties.
Next, researchers apply radiative transfer models to interpret these spectral signatures. These models consider various factors including the observing geometry, the comet's distance from the Sun, and the properties of potential surface materials.
Simultaneously, laboratory teams conduct controlled experiments measuring how likely ices and mineral analogs expected to be on cometary surfaces interact with light. These measurements provide essential reference data that make the models more accurate.
The analysis revealed intriguing patterns in 67P's composition. When comparing results from the 2015/16 and 2021/22 apparitions, researchers found that in both cases, the spectra showed strong CN emissions and relatively weak C₂, C₃, and CO⁺ emissions 1 . However, the emissions of NH₂ molecules were only observed in the 2015/16 spectra 1 , suggesting possible changes in the comet's activity or surface composition between apparitions.
The team also studied the spatial distribution of these molecules in the coma, using numerical simulations to reproduce the observed structures. By applying a simple model of an active region on the nucleus surface, they could explain the main features of the gas and dust distribution 1 .
| Molecule | Spectral Signature | Relative Strength | Potential Origin |
|---|---|---|---|
| CN | λ3883 Å (B²Σ⁺ - X²Σ⁺, Δν = 0) | Strong | Nitrogen-containing compounds in nucleus |
| C₂ | Multiple bands in blue-green | Moderate to weak | Carbon-chain molecules |
| C₃ | λ4050 Å | Weak | Carbon-rich material |
| NH₂ | Numerous weak emissions | Variable (only in 2015/16) | Ammonia ice or compounds |
Studying a comet from millions of miles away requires sophisticated technology and methodologies. The ISSI team's research relies on a diverse array of tools that bridge observational astronomy, theoretical modeling, and laboratory science.
Provide photometric, spectroscopic, and polarimetric observations of the comet during its close approach to Earth.
BTA, Liverpool TelescopeComputer algorithms that simulate how light propagates through cometary material.
ARMS-gb, RTTOV-gbReference libraries containing fingerprints of molecular transitions and scattering properties.
Reference DataControlled environments that replicate space conditions for measuring ice-dust mixtures.
Experimental SetupDetailed 3D representations of 67P's irregular shape for accurate calculations.
3D ModelingSpecialized software for aligning, calibrating, and enhancing raw observational data.
Processing AlgorithmsThe international collaboration studying comet 67P/Churyumov-Gerasimenko represents a fascinating convergence of observational astronomy, theoretical modeling, and laboratory science. By using radiative transfer modeling as a bridge between what we see through telescopes and what we measure in laboratories, scientists are gradually deciphering the complex composition of this ancient celestial visitor.
What makes this research particularly compelling is its relevance to fundamental questions about our origins. Comets like 67P are considered primordial building blocks of our Solar System—time capsules preserving material from the cloud of gas and dust that formed our Sun and planets over 4.6 billion years ago. Understanding their composition helps explain how water and organic molecules, the essential ingredients for life, were delivered to early Earth.
As these international teams continue their work, each discovery adds another piece to the puzzle of our cosmic heritage. The study of 67P reminds us that while comets may appear as distant, frozen rocks hurtling through space, they are in fact dynamic worlds holding profound secrets about the history of our Solar System—and potentially, the origins of life itself.
Comets like 67P are primordial building blocks of our Solar System, preserving material from the cloud of gas and dust that formed our Sun and planets over 4.6 billion years ago.