Ammonia may lurk in the ice of Saturn’s moons, a clue to possible oceans

Thirteen years ago, the Cassini-Huygens spacecraft was in orbit around Saturn, not yet through its first mission, when a set of onboard telescopes observed an unknown ultraviolet signal. The intriguing data was only recently inspected, however, and an international research team now suspects it could point to the presence of hydrazine on Saturn’s second largest moon, Rhea.

The effort, which includes scientists from the UK, Taiwan, India and the US, used spectral data provided by UVIS, a telescoping monster that looked a bit like a refrigerator turned on its side. (UVIS was technologically much more complex than a refrigerator and was destroyed along with the rest of Cassini in 2017, when the craft fell into Saturn’s atmosphere.) Taken during overflights over Rhea in 2007 and 2011, data collected by Cassini indicated an unidentified spectroscopic signature emanating from the frozen moon. In other words, something on Rhea was absorbing ultraviolet radiation and the team was trying to figure out which molecule was responsible. Their conclusions are published today in the journal Science Advances.

“The possible detection of hydrazine monohydrate in the Saturnian system (Rhea) is significant in that it can indicate the presence of ammonia in the ice sheets of the icy moons of Saturn”, Mark Elowitz, astrophysicist at the Open University in the UK and responsible author of the article, said in an email. “Ammonia is important because it could lower the freezing point of water-ice mixtures, thus increasing the likelihood that underground oceans could exist inside some of Saturn’s icy satellites.

The recent research effort arose out of Elowitz’s thesis, which also explored the reflectance spectra of the moon Dione, another of Saturn’s 82 moons, although this analysis was not included in the recent article. It should be noted that Cassini used hydrazine to propel it into space, which means the spacecraft may be detecting its own exhaust. The team don’t believe this happened, as Rhea’s overflights were not powered by the hydrazine thrusters, which were not firing at the time.

Although hydrazine appears to be the most likely culprit in the absorption band, a alternative explanation is a cabal of chlorinated compounds. Hydrazine makes a little more sense, because it would occur more easily, chemically speaking, than chlorinated chemicals, “which would require an internal ocean to be present on Rhea,” Elowitz said.

In both scenarios, there is evidence that serious organic chemistry is occurring in the outer solar system. Some astrobiologists believe that two of Saturn’s moons, Enceladus and Titan, could even contain extraterrestrial life.

“The presence of hydrazine is an indication that the surfaces of icy satellites act as chemical factories to make the complex molecules, particularly the precursors of biomolecules necessary for the origin of life,” Bhalamurugan Sivaraman, astrochemist at the Indian Physical Research Laboratory in Ahmedabad and co-author of the article, said in an email.

Although the absorption band has been detected on Rhea, the team is not sure what is causing it originated in the moon. Just around the bend is Titan, by far Saturn’s largest moon and the only moon in our solar system with a substantial atmosphere. The team argues that if the hydrazine had not been produced by chemical reactions between ammonia and water ice on Rhea, it could have sprayed out of Titan’s nitrogen-rich atmosphere and landed on the smaller moon.

“The idea that hydrazine could have formed in Titan’s atmosphere before being transferred to Rhea is a good reminder that individual objects in planetary systems – and the young stellar objects that precede them – do not exist. not 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 if the Dragonfly mission planned by NASA will give us a better idea of ​​whether hydrazine could come from Titan, and if so, if this hydrazine (or other molecules) could be transported to the other moons.” of Saturn.

Indeed, upcoming missions are sure to deepen our understanding of the outer solar system. Sadly, we’ll have to wait until the 2030s for Dragonfly’s journey to Titan, which will hopefully answer many of these questions and surely raise a lot of new ones.