SIGNALS FROM SATURN

An unusual signal from the ringed planet’s moon Rhea now has a possible explanation

-  Nigel Watson

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.