Signals from Saturn
An unusual signal from Saturn’s second-largest moon Rhea now has a possible explanation
When NASA’s Cassini spacecraft flew past Rhea, Saturn’s second-largest moon, it detected an unexpected and puzzling change in the ultraviolet radiation reflected from its surface. The data from Cassini’s flybys has led to a range of speculation and possibilities. Dr Amanda Hendrix, an expert in ultraviolet spectroscopy of planetary surfaces at the Planetary Science Institute in California, said that they noticed a dip in the spectrum and wondered if it was caused by some type of water ice. It was certainly an intriguing puzzle.
The signal was detected by the Cassini craft that was launched from Cape Canaveral on 15 October 1997. After seven years of travel it reached Saturn on 1 July 2004, and in total it orbited the planet for over 13 years. When it became very low on fuel it was decided to end the mission, and to avoid biological contamination of the planet or its moons it was deliberately sent into Saturn’s atmosphere, where it burnt up on 15 September 2017.
Cassini is one of the largest ever interplanetary probes to be built, weighing 2,150 kilograms. It carried the European Space Agency’s (ESA) Huygens lander probe, which it sent towards Titan, Saturn’s largest moon, on 25 December 2004. After 21 days of travel Huygens finally entered Titan’s atmosphere on 14 January 2005, and once on its frozen surface it transmitted data for 72 minutes until Cassini went out of range.
Since then scientists have researched this information to investigate the atmosphere of Titan and its geology. They made several important discoveries, including the fact that the levels of methane increased as the craft descended, whereas the amount of nitrogen remained constant. The presence of methane is exciting because it could be produced by micro-organic life, but ESA scientists think it is more likely large amounts of liquid methane are trapped under the surface ice and released into the atmosphere by cryovolcanism.
Besides the ill-fated Huygens, Cassini carried a large array of instruments to study Saturn and its moons. Some of these measured its magnetosphere and the presence of dust particles, and infrared, visible and ultraviolet images were captured using cameras and spectrographs. It was the Ultraviolet Imaging Spectrograph (UVIS) science package that detected the puzzling findings sent back from Rhea. The UVIS included a two-channel system for studying far and extreme ultraviolet light in wavelengths of 55.8 to 190 nanometres (nm).
The light reflected from planetary objects passed through the four UVIS telescopes into a spectrograph, where it was split into its component wavelengths. These wavelengths, invisible to the human eye, were able to show information and images of the night side atmospheres of Saturn and Titan. Hendrix, who analysed this data, said that this ability meant it could ‘see’ gases that were not seen by Cassini’s visible-light cameras. This ultraviolet light also showed patterns that revealed the chemical elements and compounds in the Saturn system. As an example, it identified a plume of material erupting from the south pole of Enceladus as being composed of water.
UVIS could also use an occultation technique to obtain ten times more detail of Saturn’s rings than Cassini’s visible-light cameras. This
involved UVIS locking onto a bright star and recording how the ultraviolet light changed when the rings of Saturn or a planetary body passed between them.
The perplexing dip in the far-ultraviolet from Rhea, centred near 184nm, is outlined in Dr R. Mark Elowitz’ PhD thesis Far-Ultraviolet Spectroscopy of Saturn’s Moons Rhea and Dione. In it he notes that data from Rhea and Dione showed a weak absorption feature near 184nm, and that as early as 2008 it was found that Phoebe presented similar readings. At the time, various ice mixtures of water, tholins, carbon, kerogen and poly-HCN could not explain this feature. Observations of Mimas, Enceladus and Tethys have also revealed absorption spectra in the same region of the spectrum.
To explain the Rhea signal, scientists decided the best route to an answer was to compare the spectra collected by Cassini to the spectra of thin-ice measurements in the laboratory. The far-ultraviolet data was extracted from targeted flybys of Rhea in 2007, 2010 and 2011 using datasets that completely filled the moon’s surface and provided the highest signal-to-noise spectra. Elowitz, who was one of the team members, says: “Over 20 modelled spectra of different chemical species of interest to studies of icy moons in the outer Solar System were compared with the Cassini observational data, with only two chemical species representing a good fit to the observed reflectance spectra. Those two chemical species included simple chloromethane molecules and hydrazine monohydrate. To determine the most likely of these two chemical species to exist in the upper surface ice layer on Rhea, the different sources and sinks of each chemical compound were explored, including the various chemical pathways that could lead to their production.”
Considering the two possible chemical compounds, it was determined that simple chloromethane compounds are least likely to be the answer. As Elowitz notes: “It would require the presence of a deep subsurface ocean under the ice shell of Rhea. It is unlikely the chloromethane compounds or salt derivatives of these compounds could migrate upwards through tiny cracks or fissures over hundreds of kilometres to the surface.”
The only other possible source of chlorine is via exogenic delivery by chondritic asteroids or micrometeoroids throughout the history of Rhea. If these simple chloromethane compounds were scattered over Rhea in this manner, they would produce by-products on the surface ice of Rhea. These chemical by-products were not found, so this possibility had to be ruled out.
That left the researchers having to explain why hydrazine monohydrate was detected. One immediate possibility was that the Cassini craft itself produced the hydrazine, as it was equipped with a 132-kilogram tank of hydrazine that fuelled its 16 attitude and small trajectory thruster motors.
Professor Nigel Mason, head of the School of Physical Sciences at the University of Kent and a co-author, along with Elowitz and Hendrix, of the science paper Possible Detection of Hydrazine on Saturn’s Moon Rhea, says: “We looked at spectra of other moons, like Dione and Tethys, since if the
signal was present on all moons it might suggest we should look for a common source, for example spacecraft fuel contamination of the spectrometer. The data from Tethys showed no signal, and we looked at other spectra to check for contamination and if the spacecraft motors were firing during or before observation to leave a ‘plume’ of hydrazine.”
The spacecraft’s own thrusters seemed a very likely culprit, but they had to be ruled out because they were never fired while Cassini made its flybys of Rhea. And, as Mason points out, hydrazine fuel would have contaminated data gained from other moons and not just showed up when it looked at Rhea.
Elowitz explains that if there is ammonia on the icy surface of Rhea, it could produce hydrazine “by irradiation from high-energy particles originating from Saturn’s magnetosphere”. However, he continues: “An alternative explanation is that hydrazine is produced on Titan from irradiation of ammonia present on its surface and/or atmosphere. The hydrazine then escapes from Titan’s atmosphere and is deposited over geologically long timescales on Rhea’s upper ice layer.”
Hydrazine therefore remains the prime candidate to explain the absorption signature seen in the Cassini far-ultraviolet spectral data, though Hendrix says we still need to figure out why this feature has been observed on some of the other moons of Saturn. In her view it indicates that this process is happening throughout the Saturn system, and possibly elsewhere.
Regarding future studies, Mason says: “In future work we are looking at spectra of other moons, not only to search for hydrazine, but to look for other compounds. The UV spectral database is used to look at other planets and moons, so we are exploring the chemistry of Jovian moons in preparation for the JUICE mission, and hopefully data from the Juno mission now it has been extended. Due to their volcanic nature, sulphur compounds are expected to be formed on those moons, so we are looking for these. Hendrix is also on the New Horizons team with the UV spectrometer, so data from Pluto and Kuiper Belt objects is being analysed as well.”
The New Horizons space probe was launched on 19 January 2006 to explore Pluto and the Kuiper
Belt. It swung past Jupiter in February 2007 and made its closest approach to Pluto on 14 July 2015. NASA’s Juno space probe has been orbiting Jupiter since 5 July 2016, where it has been studying the composition of the planet. The ESA’s JUICE (JUpiter ICy moons Explorer) is scheduled for launch in 2022 to search for possible habitable environments for organic molecules in the icy crusts and ocean layers of Jupiter’s moons Ganymede, Callisto and Europa.
The data from these missions might well help us further with the study of Saturn’s system and the mysterious presence of hydrazine.
To investigate the matter further, Elowitz, in his PhD thesis, proposes that future space probes should be equipped with infrared and ultraviolet spectrometers to examine the surface of Rhea and the similar icy moons of Saturn. Cassini’s UVIS instrument could be improved by employing a hyperspectral imaging system. Each pixel in the multi-layered sensor would obtain spatial and spectral information. The far-ultraviolet spectrum data received by each pixel would identify chemical compounds by reference to their reflective properties, and the whole instrument would be able to create detailed geochemical maps of selected areas of interest. “The detailed spectral maps would be used to characterise the spatial variability of the abundance of hydrazine monohydrate or chloromethane molecules, which could not be performed using the previous Cassini UVIS data due to limitations resulting from low signal-tonoise,” notes Elowitz.
The use of advanced spectrometers with higher sensitivities could be used to examine the upper layers of Rhea, Dione and Tethys for the presence of hydrazine monohydrate or chlorine molecules, and it would be great to send landing craft to these moons. The Curiosity rover carried mass spectrometers and gas chromatographs that detected dichloromethane on Mars, so any future surface landers on the icy moons should be equipped with similar equipment to help verify the existence of hydrazine or chloromethane. Certainly there is plenty more to learn and understand about this fascinating signal.