Thirteen years ago, the Cassini-Huygens spacecraft orbited Saturn, not yet through its first mission, when a set of telescopes on board detected an unknown ultraviolet signal. However, the intriguing data were only recently inspected, and an international research team suspects it may indicate the presence of hydrazine on Saturn’s second largest moon, Rhea.
The effort, which includes scientists from the United Kingdom, Taiwan, India and the United States, used the spectral data provided by UVIS, a telescopic beak that looks a bit like a refrigerator rotating on its side. (UVIS was much more technologically complex than a refrigerator and has been destroyed along with the rest of Cassini in 2017, when the vessel descended into Saturn’s atmosphere.) Taken during Rhea’s flights in 2007 and 2011, the data Cassini collects indicate an unidentified spectroscopic signature emanating from the icy moon. In other words, something on Rhea absorbs ultraviolet radiation, and the team tried to figure out which molecule is responsible. Their findings are published today in the journal Science Advances.
“The possible detection of hydrazine monohydrate in the Saturnian system (Rhea) is important because it may indicate the presence of ammonia in the ice layers of Saturn’s icy moons,” said Mark Elowitz, an astrophysicist at the Open University in the United Kingdom and lead author of the paper, said in an email. “Ammonia is important because it can suppress the freezing point of mixtures of water ice and thus increase the likelihood that ocean may exist in some of Saturn’s icy satellites.”
The recent research effort was derived from Elowitz’s dissertation, which also examined the reflection spectra of the moon Dione, another of Saturn’s 82 moons, but the analysis was not included in the recent article. It is noteworthy that Cassini used hydrazine fuel to propel it through space, meaning that it is possible that the spacecraft detected its own exhaust gas. However, the team does not think this happened, as the Rhea flying planes were not propelled by the hydrazine propellers, which did not fire at the time.
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Although hydrazine is the most likely culprit for the absorption band, a alternative explanation is a solitaire of chlorine-containing compounds. The hydrazine makes a little more sense, as it can chemically occur more easily than the chlorine chemicals, ‘which would require the presence of an internal ocean in Rhea’, Elowitz said.
In both scenarios, this is evidence that serious organic chemistry is taking place in the outer solar system. Some astrobiologists believe that two of Saturn’s moons, Enceladus and Titan, may even contain alien life.
“The presence of hydrazine is an indication that the surfaces of icy satellites serve as chemical factories to make the complex molecules, especially the precursors of biomolecules needed for the origin of life,” Bhalamurugan Sivaraman, an astrochemist at the Indian Physical Research Laboratory in Ahmedabad and co-author of the article, said in an email.
Although the absorption band was detected on Rhea, the team is not sure if the moon is native. Just around the corner lies Titan, Saturn’s largest moon and the only moon in our solar system with a significant atmosphere. The team argues that if hydrazine was not produced by chemical reactions between ammonia and water ice on Rhea, it could spray out of the nitrogen – rich atmosphere of Titan and end up on the smaller moon.
“The idea that hydrazine could be formed in the atmosphere of Titan before it was transferred to Rhea is a good reminder that the individual objects in planetary systems – and the young stellar objects that precede them – do not exist in isolation,” said Olivia Harper Wilkins, an astrochemist at the California Institute of Technology who was not involved in the new research, said in an email. ‘I would be curious to see if NASA’s planned Dragonfly mission would give us a better idea of whether hydrazine could form on Titan, and if so, whether the hydrazine (or other molecules) could be transported to Saturn’s other moons. ‘
Indeed, upcoming missions are likely to deepen our understanding of the outer solar system. Unfortunately, we will have to wait until the 2030s for Dragonfly’s voyage to Titan, which will hopefully answer many of these questions and raise quite a few new questions.