Summary: Astrobiologists have a strong interest in understanding the conditions on the early Earth, but the record for the first 700 million years of Earth history is gone. The stages that made our planet fit for life are not recorded in the rocks we have today.
Astrobiologists have a strong interest in understanding the conditions that prevailed on the early Earth, but the record for the first 650 million years of Earth history is gone. The earlier stages that made our planet fit for life are not recorded in the rocks we have today.
The Earth was not a habitable planet when it first formed: it was a seething cauldron of molten material, with impact material raining down as the young Earth swept up those remnants of the solar disk that fell within its gravitational influence. In order to recount the history of life on Earth, we will first need to understand the structure of the planet at the end of this accretion era.
Francis Albarède, of the Ecole Normale Supérieure in Lyon, France, has pointed out that we know nothing about the initial mineralogy and structure of the Earth. That's because plate tectonics as well as alteration and erosion have destroyed the most ancient geological records. The oldest terrestrial material so far discovered, is a mineral - a zircon - dated at 4.4 billion years. The most ancient rock you can hold in your hand is 4.1 billion years old. And if you want to take a stroll across early Earth, you must go to Isua, Greenland where there is a swath of continental crust dated at 3.8 billion years.
Recovering the record of the early Earth owes much to a decade of rapid progress in which geologists improved the accuracy of rock dating. For example, the decay of uranium into lead allows scientists to determine the age of a rock with a precision of less than one million years (if the rock still contains some uranium). As a result of such improvements in chronology, our picture of Earth's formation is becoming clearer.
The first planetary objects in our solar system probably formed 4.568 billion years ago. It took just a few million years of collisions to change the dust in the solar nebula into objects up to 100 km in size. The oldest meteorites, chondrites, survive from that era. Collisions continued, building ever larger objects by consolidation and fusion. By looking at the radioactive decay of hafnium to tungsten, geologists have concluded that the formation of Earth's molten metalic core was probably completed in 30 million years.
What was Earth like just 30 million years after the onset of planetary formation? The outer layer, known as the mantle, was molten as a result of the gravitational energy released during accretion. Our planet was entirely covered by an ocean of magma, (a magma is a molten rock similar to present day volcanic lava).
| Lava flow - Mount Merapi, one of Indonesia's most active volcanoes.
"Without a stable continental crust, you cannot have life as we know it," says Hervé Martin of the Observatoire de Physique du Globe in Clermont-Ferrand, France, who studies the oldest continental crust and the magma that generated them. Martin and his colleagues have derived the conditions of pressure and temperature under which the ancient rocks formed, and determined that it derived from basalt. "Later the melting of basalt gave rise to the granite that made the continents," says Martin. "The question is, where did these melting and formation processes take place?"
To answer that question, Martin and his colleagues have turned to analogues of the early Earth that exist today on the surface of our planet. They study subduction zones, regions where the ocean basaltic crust is being sucked back into the mantle.
Whereas most modern subduction zones display low geothermal gradients, in some rare places ( Chile, Aleutians, Ecuador, Philippines, etc..) high thermal regimes, similar to those expected on the hot early Earth are found. In these places, the generated magmas have exactly the same composition as the early Earth’s continental crust. This leads Martin and his colleagues to conclude that the formation of primitive crust, and continents, occurred in a subduction environment by melting of basalt at depth.
This research on early Earth has obvious important connections with astrobiology. Plate tectonics, which endlessly remould the surface of our planet, were more active in the past when the plates were smaller. Crucially, a planet with active tectonics is able to more efficiently distribute the mineral and chemical nutrients required for the development of life, for instance through more efficient hydrothermal activity.
It is well-known that terrestrial life requires water and a source of energy; in this connection the 4.4 billion year old zircons tell an interesting story. Micro-analysis of their crystals reveals that the early continental crust was stable enough and extensive enough to escape complete obliteration as cosmic debris continued to bombard the Earth (during the Late Heavy Bombardment). Analysis of the oxygen isotopes in zircon crystals suggests that, when they formed, the early Earth's crust was interacting with liquid water. The implication is that Earth probably had liquid water and perhaps even oceans on its surface as early as 4.4 billion years ago.
| Ancient zircon crystals suggest liquid water on Earth's surface 4.4 billion years ago.
Although astrobiologists now have a reasonable understanding of the origin of the continental plates and the importance of tectonics for the origin of life, the same claim cannot be made for the oceans. Water does not leave much of a geological signature. According to Daniele Pinti of the Université du Québec in Montréal, Canada, astrobiology has two central questions. First, how and when did water arrive on Earth? And second, when did internal heat flow decrease sufficiently to allow liquid water to be stable on the surface?
The scientific community generally agrees that water arrived at Earth during the final stage of accretion from the solar nebula. There are differences of opinion on the mode of transport. Some have proposed impacts of comets as the source of water, an idea that excites astrobiologists because comets are also a great resource of the organic molecules needed for the bricks of life. On the other hand, others in the community propose asteroidal impacts. An approach based on hydrogen isotopes concludes that the comet contribution cannot exceed 10-15 %.
The final aspect that needs to be considered when building a habitable Earth is the origin of its atmosphere. Here the situation is even more uncertain. The first atmosphere would have had a composition similar to that of the Sun, in which hydrogen dominates. But Earth's gravity was too weak to retain the hydrogen, so it leaked away into space. As is the case with water, to find the source of our atmosphere an extraterrestrial origin beckons. Cosmic dust may be responsible for the nitrogen, which comprises some 80% of the atmosphere today. There are strong suggestions too that the carbon required for carbon dioxide must have come from meteoritic impacts.
The consensus opinion by Martin and his colleagues is that the very early Earth (before 4.4 billion years) was completely unsuited to fostering life as we know it. After 4.4 billion years, the Earth presented environments potentially suitable for prebiotic chemistry and live apparition (stable continental crust, plate tectonics, ocean of liquid water, low meteoritic flux, etc.) If the planet was potentially habitable, there is no evidence that life started during this period. Between 4.0 and 3.85 billion years the Earth was submitted to an intense catastrophic meteoritic bombardmentt, that could have replenished Earth’s surface, vaporise oceans, and possibly destroy life if it existed before.
by Simon Mitton
This feature draws on the following
Building a habitable planet, H.Martin et al,
"From suns to life: a chronological approach to the history of life on Earth",
p97-151, Gargaud et al, Springer, 2006