NEW VIEW of the young earth covered in oceans of liquid water as early as 4 4 billion years ago contrasts sharply with the hot hostile world typically depicted in textbooks 58 SCIENTIFIC A MERIC A N COPYRIGHT 2005 SCIENTIFIC AMERICAN INC A Cool Early Earth The textbook view that the earth spent its first half a billion years drenched in magma could be wrong The surface may have cooled quickly with oceans nascent continents and the opportunity for life to form much earlier By John W Valley COPYRIGHT 2005 SCIENTIFIC AMERICAN INC SCIENTIFIC A MERIC A N 59 the earth glowed like a faint star Incandescent yellow orange oceans of magma roiled the surface following repeated collisions with immense boulders some the size of small planets orbiting the newly formed sun Averaging 75 times the speed of sound each impactor scorched the surface shattering melting and even vaporizing on contact Early on dense iron sank out of the magma oceans to form the metallic core liberating enough gravitational energy to melt the entire planet Massive meteorite strikes continued for hundreds of millions of years some blasting craters more than 1 000 kilometers in diameter At the same time deep underground the deOLD VIEW of a hot young cay of radioactive eleearth Life ments produced heat at magazine rates more than six December 8 times greater than they 1952 do today These fiery conditions had to subside before molten rock could harden into a crust before continents could form before the dense steamy atmosphere could pool as liquid water and before the earth s fi rst primitive life could evolve and survive But just how quickly did the surface of the earth cool after its luminous birth Most scientists have assumed that the hellish environment last ed for as long as 500 million years an era thus named the Hadean Major support for this view comes from the apparent absence of any intact rocks older than four billion years and from the fi rst fossilized signs of life which are much younger still In the past five years however geologists including my group at the University of Wisconsin Madison have discovered dozens of ancient crystals of the mineral zircon with chemical compositions that are changing our thinking about the earth s beginnings The unusual properties of these durable minerals each the size of the period in this sentence enable the crystals to preserve surprisingly robust clues about what the environment was like when they formed These tiny time capsules bear evidence that oceans habitable to primitive life and perhaps continents could have appeared 400 million years earlier than generally thought Cooling Down si n c e t h e 19 t h c e n t u ry scientists have attempted to calculate how quickly the earth cooled but few expected to find solid evidence Although magma oceans initially glowed at temperatures exceeding 1 000 degrees Celsius a tantalizing suggestion of a more temperate early earth came from thermodynamic calculations showing that crust could have solidified on the surface within 10 million years As the planet Overview Zircon Time Capsules 60 Geologists have long thought that the fiery conditions of our planet s birth 4 5 billion years ago gave way to a more hospitable climate by about 3 8 billion years ago Now tiny crystals of the mineral zircon which retain clear evidence of when and how they formed suggest that the earth cooled far sooner perhaps as early as 4 4 billion years ago Some ancient zircons even bear chemical compositions inherited from the cooler wet surroundings necessary for life to evolve SCIENTIFIC A MERIC A N hardened over the thickening layer of consolidated rock would have insulated the exterior from the high temperatures deep within the interior If there were suitably quiescent periods between major meteorite impacts if the crust was stable and if the early hothouse atmosphere did not trap too much heat surface temperatures could have quickly fallen below the boiling point of water Furthermore the primitive sun was fainter and contributed less energy Still for most geologists an undisputed fiery birth and scant clues in the geologic record seemed to point instead to a prolonged ultrahot climate The oldest known intact rock is the four billionyear old Acasta gneiss in Canada s Northwest Territories This rock formed deep underground and bears no information about conditions on the surface Most investigators assumed hellish conditions at the planet s surface must have obliterated any rocks that formed earlier The oldest rocks known to have originated underwater and thus in relatively cool environs did not form until 3 8 billion years ago Those sediments which are exposed at Isua in southwestern Greenland also contain the earliest evidence of life see Questioning the Oldest Signs of Life by Sarah Simpson Scientific American April 2003 Single crystals of zircon began to add new information about the early earth in the 1980s when a few rare grains from the Jack Hills and Mount Narryer regions of Western Australia became the most ancient terrestrial material known at that time the oldest dating back almost 4 3 billion years But the information these zircons carried seemed ambiguous in part because geologists were unsure of the identity of their parent rock Once formed zircon crystals are so durable that they can persist even if their parent rock is exposed at the surface and destroyed by weathering and erosion Wind or water can then trans COPYRIGHT 2005 SCIENTIFIC AMERICAN INC OC TOBER 2005 D O N D I X O N p r e c e d i n g p a g e s T I M E L I F E P I C T U R E S G E T T Y I M A G E S t h i s p a g e In its infancy beginning about 4 5 billion years ago OLDEST PIECES OF THE PLANET Isua sediments oldest evidence for life 3 8 billion years old Acasta Gneiss oldest intact rock 4 billion years old Rocks older than 2 5 billion years Jack Hills zircons oldest earth material 4 4 billion years old Inferred Exposed Fossilized gravel bed in the Jack Hills above contained the world s oldest zircons yet discovered Geologists crushed and sorted hundreds of kilograms of this rock below to find the 20 crystals that bear signs of cool conditions more than four billion years ago port the surviving grains great distances before they become incorporated into deposits of sand and gravel that may later solidify into sedimentary rock Indeed the Jack Hills zircons separated by perhaps thousands of kilometers from their source were found embedded in a fossilized gravel bar called the Jack Hills