Thirteen years ago, the Cassini-Huygens spacecraft was orbiting Saturn, not yet through its first mission, when a set of telescopes on board observed an unknown ultraviolet signal. The intriguing data was only recently inspected, though, and an international research team now 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 behemoth that looked a bit like a refrigerator turned on its side. (UVIS was much more technologically complex than a fridge and was destroyed along with the rest of Cassini in 2017, when the craft plummeted into Saturn’s atmosphere.) Taken during flybys of Rhea in 2007 and 2011, the data Cassini collected indicated an unidentified spectroscopic signature emanating from the icy moon. In other words, something on Rhea was absorbing ultraviolet radiation, and the team was trying to figure out what molecule was responsible. Their findings are published today in the journal Science Advances.
“The possible detection of hydrazine monohydrate in the Saturnian system (Rhea) is significant in that it may point to the presence of ammonia within the ice layers of Saturn’s icy moons,” Mark Elowitz, an astrophysicist at the Open University in the UK and lead author of the paper, said in an email. “Ammonia is important because it could depress the freezing point of water-ice mixtures, thereby increasing the likelihood that subsurface oceans could exist inside some of Saturn’s icy satellites.”
The recent research effort was borne out of Elowitz’s dissertation, which also explored reflectance spectra of the moon Dione, another of Saturn’s 82-odd moons, though that analysis is not included in the recent paper. It’s worth noting that Cassini used hydrazine fuel to propel it through space, which means it’s possible the spacecraft was detecting its own exhaust. The team doesn’t think this happened, though, as the Rhea flybys weren’t powered by the hydrazine thrusters, which were not firing at the time.
Though hydrazine seems the likeliest culprit for the absorption band, an alternative explanation is a cabal of chlorine-containing compounds. The hydrazine makes a bit more sense, as it would come about more easily, chemically speaking, than the chlorine chemicals, “which would require the presence of an internal ocean on Rhea,” Elowitz said.
In either scenario, it’s evidence that some serious organic chemistry is happening in the outer solar system. Some astrobiologists believe that two of Saturn’s moons, Enceladus and Titan, could potentially even contain alien life.
“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 that are needed for the origin of life,” Bhalamurugan Sivaraman, an astrochemist at India’s Physical Research Laboratory in Ahmedabad and a co-author of the paper, said in an email.
Though the absorption band was detected on Rhea, the team isn’t sure that whatever caused it is native to the moon. Just around the bend lies Titan, Saturn’s largest moon by far and the only moon in our solar system with a substantial atmosphere. The team argues that if hydrazine wasn’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 could have been formed in Titan’s atmosphere before being 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,” 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 whether NASA’s planned Dragonfly mission will give us a better sense of whether hydrazine could originate on Titan, and if so, whether that hydrazine (or other molecules) could be transported to Saturn’s other moons.”
Indeed, upcoming missions are bound to deepen our understanding of the outer solar system. Unfortunately, we’ll have to wait until the 2030s for Dragonfly’s trip to Titan, which will hopefully answer many of these questions and surely raise plenty of new ones.