Worlds in Eruption

The reason is simple. Both worlds are small, compared to the Earth, Venus and Mars. A golden rule in physics, which applies to volcanic activity on a planet, is that small bodies cool more quickly than larger bodies. The Moon and Mercury have cooled faster than the Earth and Venus, hence their quicker decline in volcanic activity.

Furthermore, because of their smaller size, the Moon and Mercury had less primordial heat leftover from their creation. They also contained a smaller fraction of radioactive elements, the decay of which is all-important in providing planets with a continued source of internal heat. The Moon and Mercury have cooled to form thick solid crusts. While their interiors may be molten, the magma is trapped beneath a deep rigid crust and is sealed in forever.

NASA Io - the most volcanically active place in the solar system.

Erupting moons

To the surprise of many scientists, the spacecraft armada has revealed that volcanism is not confined to the rocky planets of the inner Solar System. One of the highlights of the Voyager missions to Jupiter in 1979 and 1980 was the discovery of active volcanism on its moon Io. Navigation engineer Linda Morabito was running a image processing technique to brighten an image returned from the Voyager 1 spacecraft, when she was greeted by a huge semicircular plume extending outward some 280 km above the moon’s multicoloured surface. Previously invisible in the raw image, the faint plume represented a huge volcanic eruption, spewing a fountain of molten rock faster than the speed of sound. Further careful processing of other images revealed no less than eight active volcanoes on this world.

Io is about the same size as our Moon, which is now volcanically dead. Why, then, should Io be the most active volcanic world in the Solar System?

The answer comes the third option for internal heat, tidal forces. Io is under constant stress. It orbits Jupiter at approximately the same distance the Moon orbits Earth - but Jupiter is 318 times more massive. As a result, Io is constantly stretched by Jupiter’s immense gravitational pull, and - to make matters worse - its sister moons Europa and Ganymede tug on Io in the opposite direction. This repetitive tug-of-war raises and lowers Io’s surface by up to 100 metres as it revolves around Jupiter. Such tidal forces produce enormous amounts of energy, in the form of heat in the moon’s interior. Although Io contains some radioactive elements, it’s the heat liberated in the continuous flexing that has melted the moon’s interior, resulting in a surface peppered with over 300 volcanoes, some of which are active today.