Look at dwarf planets Pluto and Titan, Saturn’s largest moon, have perplexed astronomers after the James Webb Space Telescope (JWST) detected a chemical signature on their surfaces that did not match any recorded in spectroscopic databases. Scientists believe this is not an instrument error, but rather the signature of a compound whose identity remains a mystery – a mixture of materials that have never been tested in the laboratory, or even a compound whose chemical composition has not yet been characterized.
The discovery appears in test awaits publication in the journal Astronomy & Astrophysics. Scientists have identified an absorption band with a diameter of 5.113 micrometers on both Titan and Pluto – two worlds separated by billions of kilometers and characterized by very different physical conditions. The signal appeared in observations made with two different instruments on JWST, leading the team to rule out the possibility that it was a calibration problem or some other type of technical error.
The key to the discovery is a technique called spectroscopy. Each element or molecule interacts with lithe in a unique way, absorbing specific wavelengths and leaving a distinctive pattern, much like a fingerprint. For decades, scientists have built extensive catalogs of these spectral signatures to identify compounds such as water, methane, carbon dioxide and ammonia on planets and moons, as well as in other bodies outside the solar system.
In this case, the comparison did not produce any convincing results. Moreover, at this stage, the discovery of a chemical signature that cannot be associated with a known compound is highly unusual. Therefore, determining what is happening on Titan and Pluto may become a fundamental fresh question for planetary science.
Scientists have already explored several possibilities. They examined laboratory spectra of ices and organic compounds that may exist on these worlds, including acetylene, benzene, ketene and a family of molecules known as alenes. None of them exactly match the observed signature. The most likely explanation is that it comes from a known compound that exists in a physical state or mixture never previously tested in the laboratory, although the authors do not rule out that the signal comes from a material whose chemical composition has not yet been characterized.
The fact that the same signal appears in two such different places makes the mystery even more intriguing. Titan has an atmosphere prosperous in nitrogen and methane with a surface pressure of about 1.5 bar – higher than on Earth – as well as rivers and lakes containing liquid methane and a temperature of about -180 degrees Celsius (-292 Fahrenheit). Pluto, on the other hand, only maintains a slim atmosphere of about 10 microbars (about 150,000 times less dense); has a surface covered with ice composed of nitrogen, methane and carbon monoxide; and reaches temperatures close to –235°C (–391 Fahrenheit).