This is an excerpt of one of my absolute favorite answers to the question “what is your favorite deep, elegant, or beautiful explanation” posed by John Brockman in the book “This Explains Everything”.
The person who answers the question, Kevin P. Hand, is an astrobiologist and a deputy chief planetary scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California.
Back in the late 1990s, NASA’s Galileo spacecraft made several flybys of Europa, and the magnetic field sensors on the spacecraft found that Europa does not have a strong internal field of it own. Instead, it has an induced magnetic field, created as a result of Jupiter’s strong background magnetic field. In other words, the arm [detectors that can sense when an induced field is present] went off.
But in order for the alarm to go off, there needed to be a conductor. And for Europa, the data indicated that the conducting layer must be near its surface. Other lines of evidence had already shown that the outer 150 kilometers or so of Europa was water, but those datasets could not help distinguish between solid ice and liquid water. For the magnetic-field data, however, ice doesn’t work — it’s not a good conductor. Liquid water with salts dissolved in it, like our own ocean, does work. The best fits to the data indicate that Europa has an outer ice shell about 10 kilometers deep. Beneath that is a rocky seafloor, which may be teeming with hydrothermal vents and bizarre otherworldly organisms.
So the next time you’re in airport security and frustrated by that disorganized person in front of you who can’t seem to get it through his head that his belt, wallet, and watch will all set off the alarm, just take a deep breath and think of the possibly habitable distant oceans we now know of, thanks to the same beautiful physics that’s driving you nuts as you contemplate missing your plane.