Saturn’s moon Rhea has mysterious material on its surface

A new study reveals a curious relationship between Saturn’s moon, Rhea, and its neighbor, Titan.

The second largest A planet in the solar system, Saturn is adorned with icy rings and orbits a veritable swarm of some 80 moons.

These satellites have their own unique properties, some so unusual and intriguing that they stand out from other lunar bodies found throughout the solar system, including our own natural companion, the Moon.

One of these strange celestial bodies is Saturn’s second largest moon, Rhea. The moon can be a cold, airless object, but it also has three narrow, dense rings – a mirror of its host planet. These rings are the first ever discovered around a moon. However, Rhea’s unique characteristics don’t end there – previous spectroscopy studies had hinted at an unidentifiable substance on the moon’s surface.

And now, old Cassini data can solve this long-standing lunar mystery.

The new findings are detailed in a study published Friday in the journal Scientific advances.

Here’s the background – Rhea is the second largest moon to orbit Saturn – with a radius of 475 miles. The moon is locked around Saturn, which means that one side of Rhea is constantly facing the big planet. It completes an entire orbit of the gas giant in 4.5 Earth days.

Rhea is a cold and harsh world. Its temperatures range from -281 degrees Fahrenheit on the sunny areas, to -364 degrees Fahrenheit on its dark side. In keeping with the cool temperatures, the moon is largely made up of water ice.

The mystery of Rhea’s surface began decades ago, when NASA’s Cassini spacecraft flew over Rhea as part of its mission to explore Saturn and its moons. Cassini performed a spectrographic analysis, collecting ultraviolet imaging data of the moon and its chemical makeup, confirming that its surface was made of ice. But the data also showed evidence of an unidentifiable material.

How they did it – Bhalamurugan Sivaraman is Associate Professor at the Physical Research Laboratory in India and co-author of the new study. He says Reverse his team sought to study the chemical nature of this mysterious signal. To do this, the team reverted to the original Cassini data.

“We used data from the Cassini archives to understand exactly what is going on,” Sivaraman says.

The team reanalyzed data collected by one of Cassini’s Rhea flyovers and conducted experiments here on Earth to see if they could identify the chemical makeup of the molecules that produced the unknown absorption band.

The team behind the new study analyzed data collected by Cassini’s Rhea flyby and conducted lab experiments to test different molecules and see which one produced the unknown signal.

What they found – Browsing through potential candidates, the researchers finally found the likely culprit: hydrazine. This is the first time the compound has been detected on a moon, according to the study.

“When we did the experiment for hydrazine, it was a match,” Sivaraman says.

Hydrazine is an inorganic, colorless liquid with the same pungent odor as ammonia. Here on Earth, it is used in pharmaceuticals, agrochemicals, and as a propellant for spacecraft.

To figure out where hydrazine might come from, Sivaraman and his colleagues basically tried to recreate Rhea’s surface conditions in the lab.

Oddly enough, these simulations suggest that Rhea’s lunar neighbor, Titan, Saturn’s largest moon, could be linked to the presence of hydrazine. Titan could emit nitrogen molecules towards Rhea, which would then interact with the moon’s radiation to convert the nitrogen into hydrazine, the researchers suggest.

This type of interaction between two moons is a rarity in our solar system. But given Titan’s size, it’s likely the moon would exert some influence over its surrounding objects, Sivaraman says.

And after – Considering this is the first hydrazine detection on a moon, the team behind the study wants to observe more other moons to see if hydrazine forms elsewhere in the solar system. They also suspect that there may be more as yet unknown chemistry to be discovered.

“This particular work is helping us identify another molecule, which we didn’t know existed before,” says Sivaraman.

“We would like to search for molecules that are absorbed in other wavelengths as well.”

Abstract: We present the first analysis of far ultraviolet reflectance spectra of regions on the front and rear hemispheres of Rhea collected by the Cassini Ultraviolet Imaging spectrograph during targeted overflights. In particular, we aim to explain the unidentified broad absorption characteristic centered near 184 nm. We used laboratory UV spectroscopy measurements of a set of candidate molecules and found a good fit to Rhea spectra with both hydrazine monohydrate and several chlorine containing molecules. Considering the radiation dominated chemistry on the surface of icy satellites embedded in the magnetospheres of their planets, hydrazine monohydrate is considered the most plausible candidate to explain the absorption characteristic at 184 nm. Hydrazine was also used as a propellant in Cassini’s thrusters, but the thrusters were not used during icy satellite overflights and therefore the signal is not believed to come from the fuel of the spacecraft. We discuss how hydrazine monohydrate can be chemically produced on icy surfaces.