Venus has often been called Earth’s twin. It is her second planet out from the sun. We live in the third. It is about the same size as Earth, with a diameter of 12,104 km, while the world’s diameter is 12,756 km.
Our world is hot inside due to residual heat and decay of radioactive elements from its formation some 4.5 billion years ago. Because Venus is about the same size, formed at the same time, and is made of more or less the same material, we would expect Venus to also be hot and at least partially molten inside. The similarities end there. Venus has a corrosive atmosphere, is permanently shrouded in a thick layer of clouds, and has surface temperatures hot enough to melt lead and tin.
The first actual observation of the surface of Venus was made by the Magellan spacecraft, which used radar to map the planet’s surface. Radio waves are not affected by clouds. Magellan revealed a world with hundreds of volcanoes on the surface, some much larger than any other on Earth. As expected, there was no sign of the ocean or any living creatures. What was strange was that none of these volcanoes appeared to be erupting. The interior of Venus is expected to be more or less as hot as the interior of our world. The volcanoes on Venus are not steep cones like Vesuvius or Mount St. Helens, but large dome-like structures like the volcanoes that make up the Hawaiian islands.
There are two main types of volcanoes on Earth. One is a volcano associated with a subduction zone, and the other is a volcano located on a hot spot in the Earth’s mantle. There is a subduction zone just off the west coast of British Columbia, where the ocean floor of the Pacific Ocean is pushed beneath the continent. This mixture of rock, sediment, and seawater melts and bubbles to form volcanoes like Mount St. Helens. Upwelling magma forms sticky lava that tends to clog the volcano’s pipes. When pressurized with superheated steam, the volcano erupts explosively.
These volcanoes form the familiar sharp cones and periodically self-destruct and reform. The Hawaiian Islands are formed from a different type of volcano. Magma rises in hot spots in the mantle and penetrates to the surface. Lava is mostly molten basalt and freely flowing. These volcanoes rarely explode and the lava flows very easily, forming flat domed volcanoes that resemble thick pancakes. Lava flows for many kilometers and covers vast areas.
Although Magellan’s space mission ended long ago, scientists continue to analyze the data. Evidence of a volcanic eruption was recently discovered. The Ma’at Mons volcano has side craters that recently filled with lava, which spilled down the sides of the volcano and onto the surrounding land. What’s surprising is that Venus’ surface is the result of continuous volcanic activity, yet only one example of this has been found so far.
The moon is covered in impact craters. That’s because it has been geologically dead for billions of years, and its surface has remained unchanged for a long time, aside from the cumulative effects of countless meteorite impacts. If you use a telescope or binoculars, you can see numerous craters. On the other hand, Earth, which is at least as old, has fewer than 200 impact craters. This is because the surface of our world is continually recycled through erosion and plate tectonics. Venus has few craters, so its surface is geologically young.
There are no signs of plate tectonics, but frequent lava flows from numerous volcanoes would fill the craters and renew the surface. Our inability to observe this volcanic activity may be primarily due to the difficulty of observing our twins.
Venus lies low in the dawn glow. After sunset, Jupiter shines high in the southern sky. Saturn is sinking into the sunset and becoming difficult to see. There will be a new moon on the 9th.
Ken Tapping is an astronomer at the National Research Council’s Dominion Radio Astrophysical Observatory in Penticton.