The NASA/ESA Cassini-Huygens mission explored Saturn and its moons from 2004 to 2017, providing the most detailed images and data on the system ever taken. This included Saturn's largest moon, Titan, which the probe examined closely during its many flybys, and with the deployment of the Huygens lander to its surface. The mission provided new insight into Titan's atmosphere, its methane cycle, and its rich prebiotic environment, and the organic chemistry taking place on its surface. Its findings even led to speculation about the possibility of life on Titan, possibly as methanogenic organisms living in its vast methane lakes.
Closer to home, the deployment of next-generation observatories like the James Webb Space Telescope (JWST) is revolutionizing how we study exoplanets. Thanks to Webb's advanced spectrometers, coronographs, and optics, this mission is effecting a transition from discovery to characterization. According to a new study, Cassini's examinations of Titan's atmosphere could inform these attempts to characterize the atmospheres of exoplanets. The probe's findings, the authors argue, could therefore serve as an aspirational study for future observations, allowing astronomers to anticipate and overcome potential difficulties interpreting mission data.
The research was led by Prajwal Niraula, a Graduate student at the Massachusetts Institute of Technology (MIT) working with co-author Juliet de Wit, an Associate Professor at MIT and the leader of its Disruptive Planet Group. They were joined by Robert J. Hargreaves and Iouli E. Gordon from the Atomic and Molecular Physics Division at the Harvard & Smithsonian Center for Astrophysics and Associate Professor Clara Sousa-Silva from Bard College. The paper describing their findings recently appeared online and is being reviewed for publication in Astronomy & Astrophysics
For their study, the team consulted data from Cassini's Visual and Infrared Mapping Spectrometer (VIMS). This instrument conducted high-fidelity observations of Titan using solar occultations, where sunlight passing through an atmosphere is analyzed with a spectrometer to detect chemical signatures. These observations confirmed that Titan's atmosphere is composed of nitrogen (95%) and methane (about 5%), with trace amounts of other hydrocarbons and organic compounds.
The data also revealed that Titan experiences a methane cycle similar to Earth's water, where liquid methane precipitates to form clouds and rained down onto the surface. As Niraula and de Wit explained to Universe Today via email, the success of this mission could inform future efforts to characterize exoplanet atmospheres. In particular, the Cassini mission demonstrated how identifying molecules in atmospheres can be challenging because different chemicals may have similar adsorption features. This can lead to mischaracterization, which would have drastic implications for scientists attempting to determine a planet's habitability. As they explained:
In this study, our primary focus is to leverage Titan's precise transmission spectrum and our existing knowledge of its atmosphere to investigate the strengths/limitations of exoplanet atmospheric retrievals. We focus on the underlying assumptions regarding what molecules should be retrieved.
This focus is timely owing to existing concerns associated with the possible misinterpretation of molecular features. It aims to assess if the impact is limited to inferences associated solely with the spectroscopic feature(s) in question or can lead to a bias on other atmospheric properties.
The characterization of exoplanet atmospheres has advanced considerably in recent years. Previously, astronomers relied on transmission spectra, where sunlight passing through an exoplanet's atmosphere is analyzed to determine chemical signatures. This is sometimes possible during planetary transits (Transit Spectroscopy), when planets pass in front of their star relative to the observer. Thanks to Webb and other next-generation observatories, astronomers are now at the point where exoplanets can be observed directly (aka. Direct Imaging) based on the light reflected by their surfaces and atmospheres.
For astronomers, the challenge remains the same: properly identifying what chemical spectra are present to determine the existence of biosignatures. The next step in their study consisted of running the publicly available Tierra model, a 1D spectroscopy code used to characterize absorption features. In a previous study, Niraula and de Wit relied on the model to account for seven chemical signatures: methane, carbon monoxide, carbon dioxide, water, hydrogen, nitrogen, and ozone. For this latest study, they expanded the model to include a wider range of molecules that may exist out there and the similarity of their signatures, based on existing astronomical data. Said Niraula and de Wit:
It reveals that spectral signatures could not only be easily misidentified, but their misidentification can also lead to biases on other atmospheric parameters, hence the title linking 'Detection' and 'Retrieval' because these two aspects were not connected in people's minds. In reality, we show that they are. In other words, what researchers set up as 'detectable' (i,e., choosing which molecules they may be retrieving for) affects much more than anticipated (incl. even the atmospheric temperature they derive).
Another insight gained in this study relates to our capability to identify the dominant background gas even if it doesn't have strong absorption features (e.g., nitrogen gas). This is key to providing the context for the type of atmospheric chemistry going on there, among other things.
As more exoplanets are added to the census roll, the search for potentially habitable planets is moving into its next phase. The Webb instrument has demonstrated its ability to characterize exoplanet atmospheres and has made direct detections (including the recent detection of TWA 7) since it commenced operations. In the near future, Webb will be joined by the successor of the venerable Hubble, the Nancy Grace Roman Space Telescope (RST).
Several ground-based telescopes will also begin operations soon, including the Extremely Large Telescope (ELT), the Giant Magellan Telescope (GMT), and the Thirty Meter Telescope (TMT). Together, these observatories will enable more direct imaging studies of exoplanets and characterizations. The ability to properly identify potential biosignatures based on their absorption features is indispensable if we are ever to find an Earth 2.0 or other habitable exoplanets.
In the meantime, Niraula and de Wit believe their work will help the astronomical and astrobiological community transition into a new era of information-rich data. As they summarized, this will require scientists to ask:
What can we reliably say from this data? That question comes in two forms: 'What can we reliably say from these data given our current opacity/stellar/atmospheric models?' and 'What could we say reliably from these data if we had perfect models?' The first helps us adequately account for the fact that most of our insights are limited by models developed to interpret data of lower quality in the past, and their limitations are now the bottlenecks (not the data quality).
Not accounting for the 'model-induced noise' would lead to overconfidence in our inferences and likely biased conclusions. The second helps us identify the dominant limitations of existing models and showcase the depth of science we could achieve by performing guided/targeted upgrades.
Further Reading: arXiv