John Maddox looks at new research which sheds light on the differences between Earth and the other planetsby John Maddox / November 20, 1996 / Leave a comment
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…