Worlds in Eruption

The Magellan radar images also reveal that Venus’s crust is fundamentally different from the Earth’s, in that it shows no sign of plate tectonics. On our planet, the process of plate tectonics drives large slabs of the crust – plates – in perpetual motion around the Earth, jostling and diving below each other as they go. The action of plate tectonics is fundamental in cooling the Earth’s molten interior. First, a vast amount of heat energy is consumed in melting old cold crust that descends into the hot mantle below. Second, heat escapes via volcanic eruptions which are concentrated along the margins of the plates.

Venus appears to be a one-plate planet, so how does it cool off? Presently there are two main theories. Venus may lose its internal heat mainly through hot-spot volcanism, at the large shield volcanoes and coronae, in a controlled gradual process. Or, periodically, Venus may literally boil over like an oven pot with a lid, spilling vast quantities of lava onto its surface and releasing heat in the process. Perhaps this is how the extensive planitia were formed. The jury is still out on this controversy.

NASA Olympus Mons on Mars is the largest volcano in the solar system. It is three times the height of Mauna Loa and its area is as large as the entire Hawaiian archipelago.

Gigantic volcanoes of Mars

Mars has also been the target for numerous robotic Earth emissaries. They have revealed that Mars today is almost volcanically extinct - but that hasn’t always been the case. The Mariner 9 spacecraft in 1971 and the two Viking spacecraft in 1976 first returned images that tell a story of more dynamic times. Like Venus, Mars boasts an impressive assortment of volcanic features.

Indeed, the largest volcano in the Solar System is found on Mars - Olympus Mons. Situated in the Tharsis province, a huge bulge in the planet’s crust, Olympus Mons towers some 27 km high, three times higher than Mauna Loa, Earth’s largest volcano. At least twelve other volcanoes are found within the Tharsis province including Arsia Mons, Pavonis Mons and Ascraeus Mons - three giants in their own right.

The Martian volcanoes are also strikingly similar to the shield volcanoes found on Earth. They are surrounded by numerous lava flows and channels, and have collapsed pits or calderas at their summits. The main difference is size - in every detail the Martian volcanoes are bigger, even though the planet is only half the Earth’s diameter. Why?