Standing on the beach and watching the tide roll in is a reminder of Earth's special relationship with our moon. But Saturn and its moon Enceladus have something more between them. They communicate back and forth, and scientists have overheard the conversation.
Last year, the Cassini spacecraft captured an exchange of energy between the two heavenly bodies via sensors that could detect plasma waves. This discovery, made on September 2, 2017, was among the final findings of the Cassini probe that had spend two decades visiting Saturn and its moons. Two weeks later it took a kamikaze dive into Saturn’s atmosphere.
The recording has been altered to make the conversation easier for human comprehension. The same way a radio antenna converts electromagnetic waves into David Bowie songs, the JPL researchers converted the plasma energy into something we could hear. They compressed 16 minutes of plasma exchange between Saturn and Enceladus into 29 seconds of audio for human ears:
Enceladus is not like the Earth’s moon. Where our satellite is dead, Enceladus is geologically active. Enceladus has a subsurface ocean and shoots jets of water vapor that becomes ionized (stripped of some atoms) and wind up circling Saturn. What is new and intriguing is that Saturn returns the favor. The pair swap energy back and forth.
“Enceladus is this little generator going around Saturn, and we know it is a continuous source of energy," said Ali Sulaiman, planetary scientist at the University of Iowa and a member of the RPWS team. “Now we find that Saturn responds by launching signals in the form of plasma waves.”
Cassini carried 12 science instruments to study Saturn, often observing the planet and its moons in ways invisible to the human eye. Radio and plasma waves can tell researchers a lot about the relationship the planet has with its rings and moons. For example, they found Saturn's auroras emit radio waves at nearly the same frequency range as AM radio stations. Go figure!
The problem is that humans can’t study this kind of thing from Earth because our homeworld's ionosphere blocks the view of Saturn’s radiation. That meant sending a probe, Cassini, that could take a close look with the .
Why do we care about plasma waves? By mapping plasma around Saturn, researchers can determine the planet’s magnetosphere, or the magnetic bubble that surrounds some planets. (The bubbles are shaped like teardrops due to the influence of solar wind.) Earth’s protects life from deadly radiation. Saturn has a magnetosphere that cocoons Enceladus, unlike our own moon. Mapping changes in Saturn’s magnetosphere can tell us a lot about the solar wind, a critical component of space weather that effects probes, satellites and astronauts alike.
Finding a new relationship between Saturn and its water-bearing moon is a fringe benefit of the mission. Scientists now know there is a "circuit of magnetic field lines connecting it to Enceladus hundreds of thousands of miles away."
The universe just keeps getting weirder and more wondrous.