Thirteen years ago, the Cassini-Huygens spacecraft orbited Saturn, not yet on its first mission, when a set of telescopes on board detected an unknown ultraviolet signal. However, the intriguing data has only recently been inspected, and an international research team now suspects it may indicate the presence of hydrazine on Saturn’s second largest moon, Rhea.
The effort, which included scientists from the United Kingdom, Taiwan, India, and the United States, used spectral data from UVIS, a telescopic behemoth that looked a bit like a refrigerator turned on its side. (UVIS was much more technologically complex than a refrigerator and was destroyed along with the rest of Cassini in 2017, when the craft plummeted into Saturn’s atmosphere.) Taken during flybys from Rhea in 2007 and 2011, data collected by Cassini pointed to an unidentified spectroscopic signature coming from the icy moon . In other words, something on Rhea was absorbing ultraviolet rays and the team was trying to figure out which molecule was responsible. Their findings are published today in the journal Science Advances.
“The potential detection of hydrazine monohydrate in the Saturn system (Rhea) is significant as it may indicate the presence of ammonia in the ice sheets of Saturn’s icy moons,” Mark Elowitz, an astrophysicist at the Open University in the UK and author of the paper, said in an email. “Ammonia is important because it can lower the freezing point of water-ice mixtures, increasing the likelihood that oceans exist beneath the surface in some of Saturn’s icy satellites.”
The recent research effort grew out of Elowitz’s thesis, which also examined the reflection spectra of the moon Dione, one of Saturn’s 82 moons, although that analysis is not included in the recent paper. It’s worth noting that Cassini used hydrazine fuel to propel it through space, meaning the spacecraft may have detected its own exhaust fumes. However, the team doesn’t think this happened, as the Rhea flybys weren’t powered by the hydrazine thrusters, which were not firing at the time.
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While hydrazine appears to be the most likely culprit for the absorption band, one alternative explanation is a cabal of chlorine-containing compounds. The hydrazine makes a bit more sense, because it would be chemically easier to form than the chlorine chemicals, “which would require the presence of an internal ocean on Rhea,” Elowitz said.
In both scenarios, it 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 act as chemical factories in making the complex molecules, especially the precursors of biomolecules needed for life to form,” Bhalamurugan Sivaraman, an astrochemist at India’s Physical Research Laboratory in Ahmedabad and a co-author of the paper, said in an email.
Although the absorption band was detected on Rhea, the team isn’t sure its cause is native to the moon. Just around the corner is Titan, by far Saturn’s largest moon and the only moon in our solar system with a substantial atmosphere. The team states that if hydrazine weren’t produced by chemical reactions between ammonia and water ice on Rhea, it could have sputtered out of Titan’s nitrogen-rich atmosphere and landed on the smaller moon.
“The idea that hydrazine might have formed in Titan’s atmosphere before it was transferred to Rhea reminds us that the individual objects in planetary systems – and the young stellar objects that precede them – do not exist in isolation,” says Olivia Harper Wilkins. , an astrochemist at the California Institute of Technology who was not involved in the new research, said in an email. “I’ll be curious to see if NASA’s planned Dragonfly mission will give us a better idea of whether hydrazine could come from Titan, and if so, whether that hydrazine (or other molecules) could be transported to Saturn’s other moons.”
Indeed, upcoming missions will no doubt deepen our understanding of the outer solar system. Unfortunately, we’ll have to wait until the 2030s for Dragonfly’s journey to Titan, which will hopefully answer many of these questions and certainly bring many new ones.