How Tides are Caused

Why Io Erupts: Gravitational Tidal Forces

Io, a moon of Jupiter, is the most volcanically active body in the solar system. It is so hot and fluid that it turns itself entirely inside out ever few hundred millennia. How a moon sized object could be so energetic is a fascinating study.

Our perception, stuck on the surface of the Earth, is that gravity is constant, an even force pulling down on us steadily. While the rule of Gravity is universal our perception of it as static is dependant on our relatively steady position on Earth.

Gravity actually decreases very quickly as you move away from the gravitational object, halves every time the distance squares to be exact, therefore and object moving toward or away from an object would report that the strength of gravity rises and falls quickly. All a gravitational tidal force is is the pressure created on a body by the changing gravitational pull by another body or bodies. Let’s look at a very mild example before we tackle Io.

First, a I say first because this won’t be obvious in this example, let us remind ourselves that any two molecules that pass each other generate a tiny amount of heat in friction. Rub your palms together if you want to test this.

Next consider the tide. The ocean is primarily affected by the gravity of the Earth, however, when the moon passes over, her gravitational pull is strong enough to raise the level of the ocean many feet. This means that tons of water beyond counting are displaced as the tide shifts around the Earth. This causes billions and billions of tiny, heat causing interactions between water molecules. On Earth, with only one moon, we this keeps the temperature in the oceans less than one degree hotter than it would be otherwise but its there.

The final thing to consider before we move to Io, is the way this same pressure effects the crust and interior of the Earth. We don’t see the crust move under the moon because it’s rigid but, when the moon is over a continent it is applying the same pressure it applied over the water. The Earth is very slightly hotter because of its interaction with the moon. This very slight heating reaches clear to the core as gravity is not stopped by intervening matter.

Now consider Io. Jupiter is more than a hundred times larger than the Earth and exerts many times more gravitation. Additionally, Jupiter has more than a dozen large moons and several dozen small moons all with their own gravitation as well. Europa and Castillo are much larger than our own moon even. So poor Io, close to Jupiter and pulled by the powerful tides of her sister moons begins to heat slowly under the pressure.

No single orbit through a complex tidal system like this would be enough to super heat a moon but Io has been locked in this dance for billions of years. A planet can only shed heat by radiating it into space. This means that while the crust of Io remains relatively cool, heat generated deep in the planet by the pull of multiple orbiting partners can only be released by and endless series of volcanos and boiling calderas. A million years or so of this and Io completely erupts its interior to its exterior like a cake constantly refrosting itself.

Io, cool study in gravity, bad place for a summer home.