The Planets. Andrew Cohen

      © NASA/JPL-Caltech, Illustrations by Jessie Kawata

      David Grinspoon, astrobiologist, on Venusian life:

      ‘So when we say, as we often do, that Venus is completely uninhabitable, we should put a little asterisk next to that statement, because, we’re talking about the surface environment. But actually if you go up from the surface about 50 kilometres you reach a zone that may be habitable on Venus; in the clouds the pressure and temperature is roughly what it is here on the surface of Earth. There are energy sources in terms of radiation and chemical energy, there are nutrients, there is even liquid water medium – although it’s concentrated sulphuric acid in the clouds – but we now know of organisms on Earth that love concentrated sulphuric acid. So there’s nothing to rule out life in the clouds of Venus and there are even some, I would say circumstantial, facts that suggest the possibility of a biosphere there. I wouldn’t bet on life in the clouds of Venus, but I wouldn’t rule it out until we’ve explored a little more carefully.’

      As our Sun gets older, it’s gradually burning hotter and hotter. This is because as it ages the process of nuclear fusion – the fusion of hydrogen into (mainly) helium – gradually leads to an increase in the amount of helium in its core. This rise in helium causes the Sun’s core to contract, which in turn allows the whole star to shrink in on itself, creating an increased pressure that results in a rise in the rate of fusion, and so the energy output of the Sun goes up. If tomorrow the Sun is burning hotter than today, it of course makes sense that in the early days of the Solar System our Sun burned far less brightly. It’s a life cycle that is common to all main-sequence stars, the category of star that includes our Sun, and as the most common type of star in the Universe we have been able to study this life cycle in intimate detail, allowing us to make immensely detailed predictions about the characteristics of our Sun in the past and in the future.

      Winding back the clock, the current consensus amongst astronomers is that 4 billion years ago the faint young Sun was at least 30 per cent dimmer than it is today. This cooler Sun would have undoubtedly had a big impact on all of the terrestrial planets. Earth would have been much colder, and as it was receiving far less solar energy it remains something of a mystery as to why our planet wasn’t frozen solid. Instead, at this time on Earth first life was just beginning, in the liquid water that we are pretty certain covered its surface.

      At the same time, 3.5 to 4 billion years ago, the young Sun would have bathed Venus in a warmer glow. This ocean world found itself in its very own sweet spot, a world held in a delicate balance. With the Sun weakened and restrained, the Earth-like atmosphere of Venus could act as a gentle blanket, keeping the surface temperate and covered in an abundance of liquid water. But even with this additional solar energy we think Venus would have been cooler than the Earth is today; in fact we believe temperatures at that time would have been like a pleasant spring day here on Earth.

      It wasn’t to last. Slowly the young Sun grew brighter, its increased energy output causing temperatures to gradually rise, which in turn began to lift more and more water vapour into the air, thickening the atmosphere and sealing the planet’s fate. Although the oceans of Venus may have persisted for billions of years, as the surface warmed and the atmosphere thickened, the destiny of this planet was already set, driven by an unstoppable process we have recently become very familiar with here on Earth.

      The greenhouse effect is a process that has the power both to protect and to destroy a planet, but despite this power it actually boils down to some pretty simple physics. It’s all about how sunlight – solar radiation – interacts with the constituent parts of an atmosphere. In the case of the Earth, as solar radiation hits our atmosphere some of it is reflected straight back out into space, some is absorbed by the atmosphere and clouds, but most of the sunlight (about 48 per cent) passes straight through the atmosphere and is absorbed by the Earth’s surface, where it is heated up. The reason so much solar radiation makes it to the surface is because the gases in our atmosphere, like water vapour and carbon dioxide, are transparent to light in the visible spectrum. When you think about it, that’s pretty obvious because there’s a source of visible light in the sky, the Sun, and we can all see it! But it’s a different story when that sunlight heats the surface of the Earth and re-radiates back out not as visible light but as the longer-wave infrared light – thermal radiation.