How Do They Know That: Europa’s Subsurface Ocean

Hello, friends!

This month is Europa month here on Planet Pailly!  For those of you who haven’t met Europa before, she’s one of the moons of Jupiter, and she’s generally counted among the top four places in the Solar System where we might find alien life.  This is in large part because Europa has a vast, global ocean of liquid water hidden beneath her surface.  By most estimates, Europa has twice as much liquid water as Earth!

But one might reasonably ask how we know, for certain, that Europa’s ocean of liquid water exists.  I mean, no space probe has ever cracked through Europa’s surface to check.  Not yet, anyway.  Which brings us to another episode of “How Do That Know That?”


There are three main lines of evidence pointing to the existence of Europa’s ocean: spectroscopic evidence, gravitational evidence, and magnetic evidence.

  • Spectroscopy: Every chemical substance in the universe interacts with light in its own unique way.  Very specific wavelengths of light will be absorbed and/or emitted, depending on what chemical substance you’re looking at.  So by measuring the wavelengths of light reflecting off Europa, scientists could determine what Europa’s surface is made of.  I won’t leave you in suspense.  The answer is water.  Frozen water.
  • Gravity: In the 1990’s, NASA’s Galileo spacecraft conducted several close flybys of Europa.  Each time, Europa’s gravity nudged Galileo ever so slightly off course.  By measuring exactly how much gravitational nudging Galileo experienced, scientists could calculate what Europa’s internal structure must be like.  Turned out there was a thick layer of low density material near the surface.  Water, in either a frozen or liquid phase, has a pretty low density.
  • Magnetism: Jupiter has an absurdly powerful magnetic field.  As Europa orbits Jupiter, a mysterious something inside Europa responds to Jupiter’s magnetism, creating what’s called an “induced magnetic field” around Europa.  Once again using data from the Galileo spacecraft, scientists could measure the shifting and changing intensity and orientation of Europa’s magnetic field as she orbited Jupiter.  As it so happens, a large volume of saltwater would react to Jupiter’s magnetic field in much the same way as the mysterious something inside Europa.

Taken individually, each line of evidence would have to be considered inconclusive.  Suggestive, perhaps, but ultimately inconclusive.  Sure, spectroscopy tells us there’s frozen water on Europa’s surface, but that layer of frozen water might only be skin deep.  Gravity data tells us there’s a very deep layer of low density material, but gravity data, by itself, cannot tells us what that low density material is.  And if you didn’t know anything else about Europa’s internal structure or chemical composition, then her induced magnetic field could be explained in many different ways.  Taken together, though, these three lines of evidence leave little room for doubt: there’s an ocean of liquid water (specifically saltwater) beneath the surface of Europa.

Science is, in my mind, a little like trying to solve a crossword puzzle.  Not all the answers are obvious at first, but with each word in the puzzle you find, the intersecting words become a little easier to figure out.  Maybe you thought the answer to 17 across (What’s beneath the surface of Europa?) could be three or four different things.  But then you found out the middle letter is a T, and the last letter is an R, and now you can narrow down the possibilities to one and only one solution.

By following multiple lines of evidence, scientists can now say, with a very high degree of certainty, that there’s an ocean of liquid water beneath the surface Europa.  Exactly how thick is the ice above that ocean?  And what minerals are present in the ocean?  How much hydrothermal activity occurs at the bottom of that ocean?  Those are some of the next questions that need answers.


There’s a lot of information out there about Europa.  A little too much, actually.  It’s hard to sort through it all.  So if you want to learn more about Europa, I highly recommend Alien Oceans: The Search for Life in the Depths of Space by Kevin Peter Hand.  It’s got all the best Europa facts you could ever want, all together in a single book.  And Hand devotes a full chapter to each of those lines of evidence that I listed above.

Somewhere Over the Rainbow: The Discovery of Infrared Light

Hello, friends!

The way I see it, there are two kinds of people who call themselves skeptics.  There are skeptics who question everything because they genuinely want to learn more, and then there are skeptics who question everything that does not conform to their own particular worldview.

I was once sitting in a bar with a young woman who turned out to be that second type of skeptic.  The conversation turned to outer space (as conversations inevitably do when I’m around), and this young woman kept asking: “How could they possibly know that?”  And when I said I honestly didn’t know, she concluded: “I think scientists just make all this stuff up!”

So today, I’d like to start what I’m hoping will become a series of posts on this blog answering the question: “How could they possibly know that?”  And we’ll begin with the discovery of infrared light.


You may be surprised to learn that infrared light was discovered in the year 1800.  Sir William Herschel (the same Sir William Herschel who’d previously discovered the planet Uranus) was tinkering with his telescope, trying to find a safer way to observe the Sun.  He thought that, perhaps, different colored filters might do the trick.

So Herschel set up an experiment to measure the temperatures of different colors of light.  It was an elegantly simple experiment.  A ray of sunlight passed through a prism, and the rainbow of light that came out of the prism hit some thermometers.

Herschel found that the blue/violet side of the spectrum was associated with lower temperatures; the red/orange side was associated with higher temperatures.  This was not, actually, what Herschel had expected.  He’d thought temperatures would peak somewhere in the middle: in the yellow/green part of the spectrum.

Curious, Herschel decided to place a thermometer outside the visible spectrum, somewhere beyond red.  The dark area beyond red turned out to be hotter than any of the visible colors.

Herschel called this new, invisible kind of light “calorific rays,” from a Latin word meaning “heat.”  The word calorie comes from the same Latin root.  The term infrared light would not be introduced until many decades after Herschel’s death.


Of course you do, because you’re the first kind of skeptic I mentioned, not the second!  Here are some links, organized from “easiest and most accessible” at the top to “most technical” at the bottom.  Enjoy!

P.S.: The word infrared literally means “under red.”  So this blog post really should have been titled “Somewhere Under the Rainbow.”