Venus, the Earth’s unlucky twin
Venus is 12,100km across, 95 percent the diameter of the Earth. It has 82 percent of its mass, 90 percent of its surface area, 90 percent of its surface gravity… You get the idea. It even seems to have a core, mantle and crust much like the Earth's, although its feeble magnetism rules out a molten iron core.
But this is where the similarities end. For one thing, Venus takes 243 days to rotate on its axis, compared to one day for the Earth, and does it backwards compared to us. The Earth is rotating at over 1600kph at the Equator, but on Venus the figure is 6kph.
And then there is that atmosphere. Venus is the brightest object in the sky apart from the Sun and the Moon. Part of the reason is that it is near – it comes nearer than any other planet to the Earth. But the other reason is that its albedo is massively high at 65 percent. That is because it is covered with continuous cloud, and we are seeing the silver lining.
Despite being further from the Sun than Mercury, Venus is far hotter, with surface temperatures exceeding 500°C.The reason is that Venus has the severest case of the greenhouse effect in the known universe.The dense clouds trap heat in abundance – indeed they also transmit it so effectively around the planet that the night-time side is pretty much as hot as the sunlit hemisphere.
The Earth's greenhouse effect is mainly caused by carbon dioxide, but this makes up less than 0.1 percent of the Earth's atmosphere. It accounts for 97 percent of the Venusian atmosphere, with nitrogen making up most of the rest. Some global warming activists regard Venus as an awful warning about our own future if we burn too much fossil fuel. But in fact there is no way things on Earth can work out remotely as they have on Venus.
Venus's dense atmosphere means that – with the honourable exception of fewer than a hundred pictures taken by the Soviet Union's Venera landers – nobody has ever seen its surface. But it has nevertheless been mapped in some detail. Most of the hard work was done by NASA's Magellan spacecraft, which lugged a 3.4 tonne radar to Venus to map the surface from orbit. As a result, all the big features have been identified and named – after women, an inspired decision by the International Astronomical Union.
Magellan showed a surface with few of the small meteor craters that litter the Moon or Mars. The atmosphere is too thick for any but the biggest meteorites to get through. It is also so dense that it erodes the surface almost as water does on the Earth. The big difference between Venus and the Earth, however, is that Venus is without active surface-building like the plate tectonics that constantly redraws the map of the Earth. The reason is probably that the crust is too stiff and thick to be mobile. Although there is a liquid mantle beneath, it cannot shift the crusty carapace on top. Instead, it seems to have catastrophic volcanic eruptions every so often, most recently about 800 million years ago, that completely remake the surface with fresh basalt lava.