Why is the Earth so different from Venus, and for that matter from Mars and Mercury? Venus, as massive as the Earth, has a thick atmosphere of carbon dioxide, which keeps the surface of the planet well above the boiling point of water. Mars, smaller than the Earth and further from the Sun, has a tenuous atmosphere of carbon dioxide. Mercury, the innermost planet, is simply bare rock. How did the inner planets of the solar system acquire these features?
The truth is that nobody knows for sure. At some level, the explanation must involve accidents going back to the start of the solar system-between the formation of the Sun 5,000m years ago and that of the Earth 4,500m years ago.
Now the Earth's history has been put in a clearer light by two people from Harvard-Charles Harper and Stein Jacobsen. Writing in Science, they use geological evidence to show that the formation of the Earth took no more than 100,000 years after the Sun itself had formed.
How can geological evidence throw light on questions such as the composition of the Earth's early atmosphere? It's not the conundrum it may seem, for the Earth should contain traces of the gases from the Earth's early atmosphere. So much has been recognised for years. The new development is that Harper and Jacobsen have been able to make sense of previously conflicting data.
The starting point must be the formation of the Sun, which emerged by the condensation of the solar nebula-a cloud of gas and dust. The solar nebula must have consisted mostly of the primordial material of the universe, mostly hydrogen and helium.
So where does the Earth come from? The general assumption is that particles of dust began sticking together to form lumps of matter that were eventually big enough to serve as centres of gravitational attraction which became the core of the Earth. The dust would have carried with it samples of the gas from which the Sun was formed, so that there should still be a stock of that gas somewhere deep in the earth.
By putting together data about the traces of the isotope of helium and of other rare gases such as neon and xenon, the authors come to a firm, if surprising, conclusion: about 100,000 years after the formation of the Earth, the young planet had a thick atmosphere of gas left over from the formation of the Sun. The atmosphere, they say, would have been so thick that the surface rocks would have remained molten. And in these conditions, atmospheric gases would have been dissolved in the surface rocks. Only that can explain the large proportions of primordial helium that have been extracted from rocks carried up to the surface from the interior of the Earth at places where the ocean floor is fractured by upwelling rock.
But the thick atmosphere did not last for long. The Sun would have been a powerful source of ultraviolet radiation, which would quickly have blown the atmosphere of the young Earth away. Then the surface rocks would have solidified into the Earth's crust and the rocks beneath, still molten, would have had to get rid of their heat by convection-a linked pattern of upwelling and downwelling such as still continue within the Earth, driving continental drift among other things.
The idea that large amounts of primordial material, and hydrogen in particular, were dissolved in the early molten rocks, goes a long way to explain why there is a large mass of molten iron at the centre of the Earth at present. Hydrogen would have turned iron oxide into metallic iron which, with the convection currents there would have found its way to the centre of the Earth.
What this idea does not explain is the subsequent history of the Earth's atmosphere. Once the primordial atmosphere had been swept away, some of it would have begun seeping out again at the same time as the bombardment of the Earth's surface by meteorites from elsewhere in the solar system. Harper and Jacobsen reckon that their isotope data show that between 10 per cent and 20 per cent of the Earth's mass was accumulated in that way.
All that makes sense. Rocks recovered from the surface of the Moon by the Apollo project in the early 1970s show that the Moon-and presumably the Earth-was heavily cratered by meteoritic impact 4,000m years ago, 500m years after the formation of the Earth proper. But it also follows that the composition of the Earth's atmosphere 4,000m years ago would have been determined chiefly by the incoming meteorites rather than the primordial gases buried in the Earth's interior. That is a vital question: the origin of life on the surface of the Earth hangs on the composition of the atmosphere at about that time. Whether it is possible to work out from the new data what that atmosphere was like is another question.
The properties of the other inner planets are not directly addressed by the new research. But Venus-or the difference between Venus and Earth-remains the big puzzle. On the face of things, the first 100,000 years in the history of both of them should have been essentially the same. The present high temperature on the surface of Venus is an extreme case of global warming, which among other things has dehydrated the surface of the planet. The historical accident, for which we should be grateful, is that this did not happen on Earth.
