On 5-6 June many of us will have the chance to witness a cosmic event that won’t happen again for another 105 years. On those two days our planetary neighbour, Venus, will pass in front of the Sun. If you’re lucky enough to live in the right part of the globe, and the skies are clear, then you too can be part of space history.
The transit is the sort of thing us astronomical twitchers love to try to see. It’s a similar idea to a solar eclipse — Venus will move between us and the Sun, blocking out some of its light. But there are none of the long shadows and spooky ethereal darkness you get with solar eclipses. Instead, observers (please don’t look directly at the Sun) will see just a tiny black dot move across the face of our nearest star.
But this is no ordinary black dot. Venus is one of the most extraordinary places in the Solar System. The surface temperature on the planet, at 470C (880F), is enough to melt lead and the pressure, of some 90 atmospheres, is roughly equivalent to diving a kilometre beneath the ocean on Earth. Venus is surrounded by a toxic soup of gases with sulphuric acid clouds and winds of up to 350km/h (220 mph). This dense atmosphere prevents heat escaping into space – a process often described as a ‘runaway’ greenhouse effect.
What’s weird is that Venus is only slightly smaller than the Earth and has a very similar chemical composition and density. The fact that it’s turned out so differently has led those who study the planet to refer to it as ‘Earth’s evil twin’. The one the family don’t like to talk about. But that hasn’t stopped scientists trying to figure it out. So far, there have been more than twenty missions to Venus, including two Russian landers, which survived for a couple of hours before getting frazzled by the extreme conditions. The most recent mission, Europe’s Venus Express, is in orbit right now analysing the planet’s atmosphere. But despite all this international effort, the second planet from the Sun still holds many mysteries.
“When we look at Venus we think this is a planet that once, in the very early solar system, might have been very Earth-like,” says Geoffrey Landis from Nasa’s Glenn Research Centre in Cleveland, Ohio. “But obviously it changed, it overheated, it turned into the very hot thick atmosphere planet. Nevertheless, when we study Venus we can get a view of the very different ways that a rocky planet like the Earth can evolve.”
And, he says, there are basic things that we don’t understand. “It seems to be a planet that doesn’t have plate tectonics – but the Earth does – so we’d like to know how come the Earth is so different. We don’t know if it has a very thin crust… or maybe it has a very thick crust; we see a lot of evidence of volcanoes but we don’t even know if any of them are still active.” In short, Venus turns much of what we understand about Earth-sized rocky planets on its head.
There might even be life. “It’s not out of the question,” says Landis. “Although the surface is very hot – and not very hospitable to life – as you go up into the middle atmosphere it becomes very Earth-like... researchers have proposed that maybe the middle atmosphere does have life.”