The Cassini spacecraft, which began its tour of the Saturn system just over a month ago, has detected lightning and a new radiation belt at Saturn, and a glow around the planet’s largest moon, Titan.
The spacecraft’s radio and plasma wave science instrument detected radio waves generated by lightning. “We are detecting the same crackle and pop one hears when listening to an AM radio broadcast during a thunderstorm,” said Dr. Bill Kurth, deputy principal investigator on the radio and plasma wave instrument, University of Iowa, Iowa City. “These storms are dramatically different than those observed 20 years ago.”
Cassini finds radio bursts from this lightning are highly episodic. There are large variations in the occurrence of lightning from day to day, sometimes with little or no lightning, suggesting a number of different, possibly short-lived storms at middle to high latitudes. Voyager observed lightning from an extended storm system at low latitudes, which lasted for months and appeared highly regular from one day to the next.
The difference in storm characteristics may be related to very different shadowing conditions in the 1980s than are found now. During the Voyager time period when lightning was first observed, the rings cast a very deep shadow near Saturn’s equator. As a result, the atmosphere in a narrow band was permanently in shadow — making it cold — and located right next to the hottest area in Saturn’s atmosphere. Turbulence between the hot and cold regions could have led to long-lived storms. However, during Cassini’s approach and entry into Saturn’s orbit, it is summer in the southern hemisphere and the ring shadow is distributed widely over a large portion of the northern hemisphere, so the hottest and coldest regions are far apart.
A major finding of the magnetospheric imaging instrument is the discovery of a new radiation belt just above Saturn’s cloud tops, up to the inner edge of the D-ring. This is the first time that a new Saturnian radiation belt has been discovered with remote sensing.