On Oceans and Airport Security
It may sound odd, but for as much as I loathe airport security lines, I must admit that while I'm standing there, stripped down and denuded of metal, waiting to go through the doorway, part of my mind wanders to oceans that likely exist on distant worlds in our solar system.
These oceans exist today and are sheltered beneath the icy shells that cover worlds like Europa, Ganymede, and Callisto (moons of Jupiter), and Enceladus and Titan (moons of Saturn). The oceans within these worlds are liquid water (H2O), just as we know and love it here on Earth, and they have likely been in existence for much of the history of the solar system (about 4.6 billion years). The total volume of liquid water contained within these oceans is at least 20 times that found here on Earth.
From the standpoint of our search for life beyond Earth, these oceans are prime real estate for a second origin of life and the evolution of extraterrestrial ecosystems.
But how do we know these oceans exist? The moons are covered in ice and thus we can't just look down with a spacecraft and see the liquid water.
That's where the airport security comes into play. You see, when you walk through an airport security door you're walking through a rapidly changing magnetic field. The laws of physics dictate that if you put a conducting material in a changing magnetic field electric currents will arise and those electric currents will then create a secondary magnetic field. This secondary field is often referred to as the induced magnetic field because it is induced by the primary field of the doorway. Also contained within the doorway are detectors that can sense when an induced field is present. When these sensors detect an induced field, the alarm goes off, and you get whisked over to the 'special' search line.
The same basic principle, the same fundamental physics, is largely responsible for our knowledge of oceans on some of these distant worlds. Jupiter's moon Europa provides a good example. Back in the late 1990's the NASA's Galileo spacecraft made several flybys of Europa and the magnetic field sensors on the spacecraft detected that Europa does not have a strong internal field of its own, instead it has an induced magnetic field that is created as a result of Jupiter's strong background magnetic field. In other words, the alarm 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 the surface. Other lines of evidence had already shown that the outer ~150 km of Europa was water, but those datasets could not help distinguish between solid ice water and liquid water. With the magnetic field data, however, ice doesn't work—it's not a good conductor. Liquid water with salts dissolved in it, similar to our ocean, does work. A salty ocean is needed to explain the data. The best fits to the data indicate that Europa has an outer ice shell of about 10 km in thickness, beneath which lies a global ocean of about ~100 km in depth. Beneath that is a rocky seafloor that may be teeming with hydrothermal vents and bizarre other-wordly organisms.
So, the next time your in airport security and get frustrated by that disorganized person in front of you who can't seem to get it through their head that their belt, wallet, and watch will all set off the alarm, just take a deep breathe 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 try to reach your departing plane.