Tumultuous Beginnings

Araging ocean of magma, bombarded by a constant barrage of asteroids, beneath a thick, dense atmosphere completely devoid of oxygen. For scientists, there’s little doubt: this is what our planet was like several tens of millions of years after the birth of the solar system.
As for the rest, the Earth’s early history remains something of a mystery. From what and how did the Earth form? How did it become a planet able to sustain complex and varied forms of life? What is the origin of the water that makes this life possible at all and which fills our oceans? Laboratories the world over are buzzing with activity as scientists try to find answers to these questions. Using increasingly advanced techniques, researchers have gotten better at extracting valuable information about those turbulent times from samples of ancient matter, found everywhere from scorching volcanoes to the icy depths of Greenland, and even in the tails of comets. But though new discoveries are coming thick and fast, many essential questions remain unanswered.

Shaping Matter
The first major question puzzling scientists is that of the Earth’s birth, approximately 4.5 billion years ago. The current theory on how Earth formed is as follows: In the gas that surrounded the early Sun, dust clumped together to form meteorites, called “chondrites,” which in turn clustered together in ever bigger chunks, eventually creating our planet. This process, known as accretion, is not yet fully understood. By studying chondrites, of which several tons still fall to Earth every year, scientists at the CRPG¹ in Nancy may have discovered the existence of a new stage at the beginning of this process. Its precise nature is not yet entirely clear, but it would appear to involve “planetesimals,” small planet-like objects a few kilometers in diameter that already have a structure, including a small core. “When we analyzed the composition of chondrules–the tiny spheres of silicate² found inside chondrites–we realized that they couldn’t have been produced solely from the dust formed around the Sun, but that they also contained fragments of planetesimals,” explains Guy Libourel, professor at ENSG³ and CNRS researcher.
The time it took for the accretion process to build up the bulk of our planet is still open to debate, and only recently the previous estimation of 100 million years was revised down to 30 million years. “Indeed, the observation of Mars and of exoplanets, as well as the improvement of models, have convinced us that planets form much faster than previously believed,” says Marc Chaussidon, a researcher at CRPG.

Earth’s Many Lives
Researchers have also much to learn about the Earth’s early history, in other words the 600 million years following its formation–a period known as the Hadean. Towards the end of this era, the Earth was struck by an extremely intense meteorite bombardment, which also left its mark on the Moon. This “late heavy bombardment,” as it is known, wiped out most traces of previous events–making it very difficult to reconstruct them. “Until recently, we thought that the Earth had remained in a molten state from its formation until the late heavy bombardment,” Chaussidon explains. But a new theory suggests that, “during this period, our planet had certainly already taken its present-day structure of a core surrounded by a mantle, possibly with continents and oceans on its surface–perhaps even life. Until the late heavy bombardment remelted the Earth, and everything had to start all over again.” Researchers don’t exclude the possibility that this sequence of events occurred several times during the Hadean; each time, the development of the Earth would have been wiped out by the bombardments, reducing it back to its original molten form.
Though the actual number of times the Earth had to start over is unknown, the scenario is fairly well understood. “Each time, the Earth would have had to be subjected to an unbelievably powerful impact, either through an intense meteorite bombardment, or through a major collision, like the one that occurred with an object the size of Mars, giving rise to our Moon,” explains Jean-Louis Birck, senior researcher at IPGP.⁴ Such a cataclysm could cause massive melting, covering the planet with an ocean of magma several hundred kilometers deep with an average temperature in the thousands of degrees.
The chemical elements then migrated, depending on their nature. For example, metallic iron, a heavy component incompatible with the silicates that make up most of the magma, traveled down towards the center of the Earth, forming its core. Then, when the surface cooled, for instance when the meteorite bombardment subsided and the planet was able to emit a large part of its heat into space, a solid crust  formed at the surface. In theory, oceans–and possibly even life–could then have appeared, only to disappear again when fresh impacts triggered further melting. Yet one problem remains: each new melting phase would have destroyed all traces of the previous stage. But according to Birck, that is not entirely true: “In 2006, by testing for the presence of various isotopes⁵ of an element called neodymium found in the rocks at Isua, a site in Greenland, we were able to prove that the Earth had a mantle very early on in the Hadean.”

The Birth of the Atmosphere
The Hadean period is also of interest to specialists of atmospheric evolution. And on one point, everyone agrees: The atmosphere’s current composition bears no resemblance to the one it originally had. Rather, it resembled what can be found on present-day Venus. Carbon dioxide (CO₂), nitrogen, hydrogen, water vapor, and possibly methane–this is the composition that scientists have reconstructed based on the nature of chondrites, the original material from which Earth was made.
Furthermore, atmospheric pressure was tremendously high, between 20 and 480 times that of today. “This hostile atmosphere was certainly the result of a massive mantle outgassing during the first 150 million years after the Earth’s formation,” explains IPGP’s Manuel Moreira. “But there was also a cosmic contribution to this process, in other words the release of gases trapped inside the meteorites and comets that collided with Earth on a daily basis.”
At this early stage, temperature and gravity conditions meant that the planet was unable to retain all the gases in its atmosphere, letting the lighter components escape to space. And this is a major hurdle when trying to reconstruct the history of our atmosphere. This is why IPGP researchers study changes in concentrations of the noble gases (xenon, helium, argon, neon, etc.), which have the advantage of having been present in the atmosphere right from the start. This is done by hunting down the gases’ various isotopes. “For example, argon, which is too heavy to escape out to space and has been building up in the atmosphere for 4.5 billion years, would enable us to reconstruct precisely the evolution of mantle outgassing,” Moreira points out. This would give us a better understanding of the atmosphere’s composition during the first 2 billion years, the period on which most of the questions are focused. After that, things become clearer: 2.3 billion years ago, oxygen started to appear in sizeable quantities due to early photosynthesis, and this triggered the exponential rise of life. Taken up by plants, and also trapped by silicates, CO₂–at that time the atmosphere’s undisputed top dog–saw its share rapidly fall from 98% of the atmosphere to under 1% (today, it stands at 0.03%). In other words, the atmosphere’s composition gradually became similar to that of today.

The Origins of Water
Researchers are also trying to elucidate another mystery: Where does Earth’s water come from? “We now believe water to have two different origins,” says Marc Javoy, a specialist on the subject at IPGP. “On the one hand, there is water that has been present since the formation of the Earth, and which was outgassed by the primitive mantle. On the other, there is water of cosmic origin, which was brought in during the early Earth’s Late Veneer, a bombardment by meteorites and above all comets, the latter being essentially made of water ice. To identify the respective importance of these two origins, researchers rely on the measurement of deuterium, the heavy hydrogen isotope, more abundant in comets’ water than in mantle-derived water. They determine and compare the amounts detected in the mantle and in the oceans. “Although the jury is still out, recent data lead us to believe that water of cometary origin may account for as much as half of the Earth’s water,” Javoy concludes.
That leaves one question: Where do the oceans, in their present form, come from? Surprising though it may seem, they were probably formed by rainfall. When the Earth was in a molten state, water turned into water vapor, trapped in the atmosphere until conditions improved. When temperatures began to fall, the water vapor condensed to a liquid, and the Earth was subject to a continuous torrential downpour. Some scientists estimate that as much as 4 to 7 meters of rain fell per year. The first oceans, whose surface temperature may have been as high as 80°C, have been dated at 3.9 billion years old, according to sediments found in the Isua area. But they may have been preceded by primitive oceans during the Hadean. These early oceans would have been completely vaporized into the atmosphere by the impact of huge meteorites, restarting the entire process. In fact, the recent discovery of particularly ancient rocks raises new questions. “Zircons⁶ found in Australia have been dated at 4.3, or even 4.4 billion years old,” explains Pierre Agrinier, a researcher at IPGP. “And yet the formation of some of these minerals requires sediments, which means liquid water.” Could there already have been an ocean at the time? Just one of the many unanswered questions about our planet’s early history.

Matthieu Ravaud

Tracking Down the Primeval Materials To discover the raw materials from which the Earth was formed, researchers at CRPG have developed the nebulotron. This machine can be used to reproduce the emergence of the first minerals that condensed out of the cloud of gas surrounding the early Sun, the solar nebula. “We put a piece of glass that has a similar composition to what we think solar gas had inside the machine, and we heat it up with a laser,” explains Guy Libourel. “This results in a plasma1 that we are able to ‘stabilize’ using an oven. Then all we have to do is analyze the composition of the minerals formed from the gas.” Using this experiment, the scientists have obtained the minerals found in chondrites, rocks that were forming near the sun at the time. But to refine their research, they need to further improve their understanding of solar gas. This is why CRPG is taking part in the analysis of samples from two international space missions: the Genesis mission,2 which gathered particles emitted by the Sun at a distance of 1.5 kilometers from the Earth, and the Stardust mission,3 which collected dust from the tail of the comet Wild 2, itself a direct witness to the earliest years of the solar system. M.R. 1. Ionized gas, a fourth state of matter that is not solid, liquid, or gas. 2. http://genesismission.jpl.nasa.gov 3. http://stardust.jpl.nasa.gov