How unique is the Earth in its structure and composition? Our knowledge of the other planets in our solar system has been increasing apace in recent times. As the astronomer Carl Sagan (1934-96) put it, we are the generation in whose lifetime the planets of the solar system have turned from being ill-seen lights in the sky to being familiar worlds. We now have the surface of Mars essentially under continuous observation. We know that it has had rivers – perhaps recently – and live volcanoes. And we know that Mercury has a magnetic field not too unlike the Earths, suggesting it probably has a big iron core like the Earths, or did comparatively recently. So far Mercury has only been visited by one spacecraft, Mariner 10, although at the time of writing the US Messenger spacecraft was on the way, while Europe's Bepi-Colombo Mercury mission was at the planning stage.
It might seem unlikely that the outer solar system contains planets with much in common with the Earth. But although the gas giants seem radically different from the Earth, some of their satellites do resemble our own planet in certain ways. One of Jupiter's satellites, Io, has what may be the only active volcanoes in the solar system beyond the Earth. These volcanoes have lava flows of molten rock like those found on the Earth. The plumes of material they emit into space are mainly sulphur dioxide – itself a common component of volcanic emissions on the Earth. However, while Etna and Mount St Helens are powered by heat generated deep inside the Earth when radioactive atoms disintegrate, Io's volcanoes get their energy from the gravitational energy of Jupiter itself.
Two of these satellites in particular merit a closer look.
Titan is a satellite of Saturn discovered by the Dutch astronomer Christiaan Huygens in 1655. It is smaller than the Earth, about 5150km across to the Earth's 12,800km. Given everything we know about the formation of the solar system, it is almost certainly about the same age (4.54 billion years) as the Earth and the other major planets, having been formed by aggregation at the same time. It is certainly a rocky world like the Earth because its density is very similar to the Earth's.
Titan is one of the few places we know (another is Venus, see box) to share a key attribute of the Earth – a thick, dense atmosphere. In fact, like Venus, Titans atmosphere is so dense that passing spacecraft, or telescopes on Earth, can never see the surface in visible light.
Titan even has a weather system as the Earth does. In 2005 a European spacecraft called Huygens was launched into Titans atmosphere to take a look. It found a world which seemed eerie in its familiarity to human onlookers. The atmosphere had rain, wind and clouds. On the surface could be seen seas, rivers, lakes, bays and valleys and, at the smallest scale, rocks.
However, all this is cosmic trickery on a grand scale. Like the Earths atmosphere, Titans is mostly nitrogen, a gas known for its reluctance to get involved in any serious chemistry, at least by comparison with oxygen. But instead of oxygen, which makes up most of the rest of the Earths atmosphere, the remainder of Titans atmosphere is made up of methane, the smallest possible molecule of carbon and hydrogen, and its close chemical relatives, collectively called the hydrocarbons. The rocks lying about are in fact the only thing there made of any familiar material, and even they seem to be mainly lumps of ice rather than stone. The seas and rivers are mostly of methane and so is the rain. To us, methane is the “natural gas” that we use for cooking, although it is also shipped around the world in liquid form in refrigerated containers. On Earth, methane mainly comes from rotting vegetable matter trapped in the ground, as do coal and oil, but on Titan it was probably formed by chemical processes in the early solar system.
It is certainly possible to observe something very like the erosion that goes on at the Earths surface in action on Titan, with the difference that on Earth, water does most of the hard work. On Titan, deposits of dark hydrocarbons are washed away by methane rain.
But although erosion and the other processes at work there will have changed the detailed map of Titan over time, Huygens would have seen essentially the same phenomena at any time in the last few billion years. By contrast the Earth is a dynamic planet with active systems that change its surface the whole time. With the arrival of human beings, it has become even more exciting, with big changes happening fast. Io is probably the only quicker-changing object in the solar system. Its active volcanoes rework the surface so fast that there is no point in taking a map if you are sent there as an astronaut. It will be out of date before any possible spacecraft can get you there.
There is one further difference between the Earth and Titan: while the Earth is about 150 million km from the Sun, Titan – like Saturn and the rest of its extensive satellite entourage – is over 1.4 billion km away, nearly ten times as far. This means that unlike the Earth, which basks in a non-stop flow of solar energy, the surface temperature on Titan is minus 180°C.
This is responsible for the most significant difference of all between Earth and Titan: while the Earth has a rich and varied biology, Titan is devoid of life. The Earths proximity to the Sun has allowed photosynthesis (the process by which plants use light to grow and release oxygen) to take place, transforming the planets surface.
Europa: the real Waterworld
Another of Jupiter's satellites, Europa, was discovered by Galileo in 1610 along with three others. In the pre-space-probe era, Europa fascinated scientists by being one of the brightest objects in the solar system, reflecting back 64 percent of the light that tell on it. (This figure is called the albedo; in comparison, the Earth's is about 30 percent.)
Once we got a closer view, Europa, like Titan, turned out to look oddly familiar. But unlike Titan, Europa has almost no atmosphere. No thick clouds defend the surface from the intense cold of outer space. What slight atmosphere Europa does have consists almost entirely of oxygen. The reason is that the planets surface is covered with ice, and cosmic rays striking it split the water into hydrogen (which escapes) and oxygen, which is held back because its atoms are heavier.
The surface looks like nothing so much as an aerial photograph of the Arctic Ocean, complete with ice floes crunching together and throwing up raised icebergs. Indeed, if you believe current wisdom about global warming on Earth, Europa may have some educational value in a few years' time: it may become the last place in the solar system to feature pack ice. There are even remains of bizarre geysers or volcanoes, caused by the movement of the ice. When it shifts and a gap opens up to the water beneath, huge amounts of water vapour spray out into the vacuum of space until enough of it has built up to close the rift.
Before the space age, our telescope-based knowledge of the solar system told us mainly about its inner planets, with the result that water was thought to be rare except on Earth. We now know that water is common in the outer solar system. For example, another of Saturn's moons, Enceladus, has an even higher albedo than Europa (about 75 percent) and once again its surface is pretty much solid ice. But Europa seems to be a unique ally of the Earth, at least in the known universe, in having a supply not only of ice but of liquid water. At more than five times the Earth's distance from the Sun, how does the water avoid freezing? The answer is probably that, as with Io, Jupiter's gravity sets up forces within Europa that turn into heat. The result is a deep ocean with a thin crust of ice – perhaps about 5km thick – formed by the cooling effect of deep space.
Europa may have lessons for our thinking about the origin of life here on Earth. Somewhere that has abundant liquid water and heat, after all, has in place several of the assets that make it possible for life to exist on Earth. There are even proposals to send a space probe to Europa which could melt a hole in the ice and swim about beneath to look for any passing haddock. While they are there, they would be wise to head on down to the ocean floor. As well as the water volcanoes on the surface, there may be more normal volcanism going on at the point where the water meets Europa's rocky core. On Earth such locations are a prime site for life, and maybe the same is true of Europa. This would, however, be some trip for the robot concerned as the ocean is probably about 20km deep, and it will be at least a decade before this question is answered.