Early Water – Early Life?
Dr Hugh Murdoch
An article by American Geologist John Valley in Scientific American (October 1985) is introduced somewhat sensationally in large type with the prominent provocative statement:
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 have formed earlier.
Massive meteorite impacts struck the Earth (and the rest of the Solar System) during that first half billion year period, sometimes known as the Hadean (or Hades like) era, regarded as having ended with a substantial bombardment at about five billion years. Much of the evidence has come from the moon exploration and dating of the rocks there. A more hospitable climate conducive to life did not arise until about 3.8 billion years. The earliest geological evidence for life occurs at 3.8 billion years (in Greenland).
Valley argues the case for a modified scenario of intermittent quiet periods between bombardment interludes when water could have occurred and hence, even possibly, primitive life, only to be snuffed out by the next bombardment. The evidence is based on a surprising source – the existence of tiny crystals known as zircons, many of which are no larger than a full stop on a page. These crystals (ZrSiO4) have a unique structure and they can be dated because occasionally uranium substitutes as a trace impurity. As the uranium decays ultimately to lead on a long but well-known time scale, the uranium-lead clock can be used to date the crystal.
The story told here concerns zircons found in an area of Western Australia about 800 kilometres north of Perth occupied by large sheep stations. A few zircon grains were found embedded in a gravel bed known as the Jack Hills Conglomerate. This was presumably not their original source but zircon grains are explained as very durable. These were dated in 1986 in a collaboration by two Australian Groups. William Compton and colleagues from A.N.U., Canberra, used a special kind if ion microprobe called SHRIMP to blast off a few atoms from a targeted part of a crystal surface and these were dated by Simon Wilde and John Baxter from the Curtin University of Technology. The age was found to be 4.3 billion years. At the time the age seemed hardly credible and appears to have not been taken too seriously.
The current author, John Valley, after originally not taking the above result seriously, was in 1999 himself investigating the Jack Hills zircons, and he approached Wilde who, using an improved SHRIMP, measured 56 previously undated zircons, finding 5 older than 4 billion years, including one which dated back to 4.4 billion years!
Now comes the sting in the tale! Valley and his PhD student William Peck were looking for well-preserved samples of the oldest oxygen on earth. They were using the isotopic ratio of O18 to O16 to measure the temperatures of processes leading to the formation of magmas and rocks. The proportion of the higher isotope (dO18) incorporated in a crystal as it is formed depends on the ambient temperature at the time. Zircon from the Earth’s mantle typically has dO18 of 5.3 (on a scale where seawater is zero) and that was the value expected for the Jack Hills zircons, assuming their origin in the Earth’s mantle. They sent a sample of them, including the oldest, to the University of Edinburgh in Scotland for measurement expecting to find a value of 5.3, characteristic of the mantle. They were stunned to discover that the values of dO18 ranged up to 7.4. This can apparently only occur if the crystals chemically interact with water on formation, as is common among much younger rocks where water is present.
This leads to the striking statement: “Thus liquid water and low temperatures are required on the surface of the Earth to form zircons and magmas with high dO18; no other process is known to do so (emphasis mine). Finding high oxygen isotope ratios in the Jack Hills zircons implied that liquid water must have existed on the surface of the Earth at least 400 million years earlier that the oldest known sedimentary rocks. If correct, entire oceans probably existed, making the Earth’s early climate more like a sauna than a Hadean fireball”. Presumably the Hadean bombardment was not continuous but spasmodic with extended quiet interludes.
Valley and Peck were so stunned by their results that they delayed publishing for over a year while they checked their results. Then another group based at the Universities of Colorado and Los Angeles confirmed their results and they published back to back papers. Since then, they report, supporting results have come from investigators as far afield as Perth, Canberra, Los Angeles, Edinburgh, Stockholm and Nançay, France. “Hundreds of newly discovered zircons have been reported from several localities with ages from 4.4 billion to 4 billion years old. Colleagues from the Geological Survey of Western Australia have found similarly ancient zircons as far as 300 km south of the Jack Hills. Geochemists are scrutinizing other ancient regions of the Earth, hoping to find pre-4.1 billion year old zircons outside Australia”. As this was written 2 years ago, much could have happened since.
Meanwhile the Mars Rovers “Spirit” and “Opportunity” continue to explore their region of Mars and find more evidence for the probable existence of early water (and hence possibly life) on Mars. The rovers have long surpassed their expected time limit of six months. That expectation was not because of anticipated equipment failure, but because they were expected to become covered in dust and unable to transmit.
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