A very small person asks a very big question: why aren't the moons of gaseous planets also made of gas?
It took a very small person to ask a big question, one that planetary scientists pondered for a long time. There are four gas giants in our solar system – Jupiter, Saturn, Uranus and Neptune – but why are their moons not made of gas? They’re solid, unlike the planets they orbit.
As always, Bill Nye is on standby to explain the workings of the world and universe around us. The widely accepted theory on how Earth’s moon was created is known as the Giant Impact Hypothesisis. It states that a very young proto-Earth was struck by an asteroid, and in that collision both Earth and the debris that chipped off it became molten. They both cooled off, and now we have a moon; nicely spherical (although not perfectly) and solid, in orbit around us.
So why don’t gas giants have gas moons? The simple answer: gravity. When Jupiter or Saturn have undergone similar collisions or events that resulted in a fragment being spun off into their orbits, those new moons were too small to sustain themselves in a gaseous state. Gravity pulled the gas down until it formed a solid sphere. A gas giant, on the other hand, stays gaseous because of its size; there is so much heat generated from its spin that it cannot cool down into a solid state. Jupiter, for example, has 67 known moons and even the bigger ones like Io, Callisto, Europa and Ganymede, have cooled off – they're non-gaseous, unlike Jupiter itself.
Much can be learned after we know the state of moons; we now know that Europa has twice as much seawater as Earth, but that the water is under a crust of ice. One day, we may find life in those oceans – they’ve been around for 4.5 billion years, so it’s likely Europanians may be living there, says Nye.
Bill Nye's most recent book is Unstoppable: Harnessing Science to Change the World.