Thirteen years ago, the Cassini-Huygens spacecraft orbited Saturn, not yet through its first mission, when a set of telescopes on board observed an unknown ultraviolet signal. Interesting data has only recently been inspected, but an international team of researchers suspects it could 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 spectral data provided by UVIS, a telescopic behemoth that looked a bit like a refrigerator turned to the side. (UVIS was much more technologically complex than a refrigerator and it was destroyed along with the rest of Cassini in 2017, when the boat fell into Saturn’s atmosphere.) Taken during flights from Rhea in 2007 and 2011, data collected by Cassini indicated an unidentified spectroscopic signature emanating from the frozen moon. In other words, something in Rhea was absorbing ultraviolet radiation, and the team was trying to figure out which 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 indicate the presence of ammonia in the ice sheets of Saturn’s icy months,” said Mark Elowitz, an astrophysicist at the Open University in the UK and the lead author of the paper. in an email. “Ammonia is important because it could depress the freezing point of water-ice mixtures, increasing the likelihood that groundwater will exist inside some of Saturn’s ice satellites.”
The recent research effort was supported by Elowitz’s dissertation, which also explored the reflectance spectra of Dione, another 82-fold moon of Saturn, although this analysis is not included in the recent paper. It is noteworthy that Cassini used hydrazine fuel to propel it through space, which means that the spacecraft may detect its own evacuation. The team does not believe this happened, however, because the Rhea flies were not powered by hydrazine propellers, which were not firing at the time.
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Although hydrazine seems most likely to be the culprit for the absorption band, a alternative explanation is a cabal of chlorine-containing compounds. Hydrazine makes a little more sense because it would appear easier, chemically, than chlorine chemicals, “which would require the presence of an internal ocean on the Rhea,” Elowitz said.
In both cases, there is evidence that there is serious organic chemistry in the outer solar system. Some astrobiologists believe that two of Saturn’s moons, Enceladus and Titanium, could even contain extraterrestrial life.
“The presence of hydrazine is an indication that the surfaces of ice satellites act as chemical plants in the manufacture of complex molecules, especially the precursors of biomolecules that are necessary for the origin of life.” Bhalamurugan Sivaraman, an astrochemist at the Indian Physical Research Laboratory in Ahmedabad and co-author of the paper, said in an email.
Although the absorption band was detected on Rhea, the team is not sure that whatever caused it is from the moon. Right around the elbow is Titan, Saturn’s largest moon by far and the only moon in our solar system with a substantial atmosphere. The team claims that if the hydrazine had not been produced by chemical reactions between ammonia and ice-water on the Rhea, it could have escaped from Titan’s nitrogen-rich atmosphere and landed on the lower moon.
“The idea that hydrazine may have formed in Titan’s atmosphere before it was transferred to Rhea is a good reminder that the individual objects in the planetary systems – and the young stellar objects that precede them – do not exist in isolation,” said Olivia Harper Wilkins. The astrochemist at the California Institute of Technology was not involved in the new research, he said in an email. “I will be curious to see if NASA’s planned Dragonfly mission will give us a better sense of whether hydrazine could come from Titan and, if so, whether hydrazine (or other molecules) could be transported to the other months of Saturn. ”
Indeed, future missions must deepen our understanding of the outer solar system. Unfortunately, we will have to wait until the 2030s for the Dragonfly trip to Titan, which we hope will answer many of these questions and will certainly raise a lot of us.