ScienceWeek

PLANETARY SCIENCE: HISTORY OF EARTH'S ATMOSPHERE

The following points are made by H.J. Melosh (Nature 2003 424:22):

1) The Earth was born in violence. Modern scenarios of its origin suggest that, as the Earth grew, matter arrived in progressively larger chunks. The final crescendo included the impact of a Mars-size protoplanet that added mass and energy to the nascent Earth and, incidentally, created the Moon. So great was the energy delivered in this impact that the proto-Earth probably melted completely, and silicate vapour formed a fiery (although short-lived) envelope around the planet. Amidst such hostility, it seems hardly possible that the fragile envelope of atmospheric gases could survive. Planetary scientists have taken it virtually for granted that the primordial atmosphere of the proto-Earth would have been stripped by such a stupendous impact, and have looked for mechanisms that might have regenerated the gaseous envelope after the tumult subsided. But some researchers have recently proposed that Earth's atmosphere is not quite as fragile as it once seemed.

2) The idea that impacts of kilometer-size comets or asteroids might strip off a small fraction of a terrestrial planet's atmosphere has been discussed for many years. In these relatively small events, the atmosphere in the vicinity of the impact is driven off by the high-speed vapour and debris that are ejected from the crater. For planet-size impactors, however, the loss mechanism is different. Of course, the atmosphere close to the impact site will be ejected with the other high-speed gases from the vaporized projectile. However, because the depth of the atmosphere is only a small fraction of the Earth's radius, this kind of direct stripping cannot remove the atmosphere that lies far from the impact site. Instead, a planetary-scale impact creates a strong shock wave in the Earth's mantle that propagates through its dense interior and emerges as a sudden jump in surface velocity at locations far from the impact site. If the velocity jump is big enough, a further shock wave forms that may accelerate the atmosphere to a high enough velocity for it to escape the Earth.

3) Chen and Ahrens (1997) were the first to analyse this global atmospheric ejection process. They assumed that the velocity of the motion generated at the surface reaches 8 km/s and found that at this velocity most of the atmosphere is ejected. Genda and Abe (2003) used a one-dimensional atmosphere model similar to that of Chen and Ahrens, but they benefited from more recent giant-impact modelling that shows that the actual surface velocities following a Mars-size impact are smaller than those assumed by Chen and Ahrens. According to such computations, the maximum surface velocities only reach about 6 km/s at the antipode of the impact and are smaller elsewhere. The difference in velocity between 6 and 8 km/s may sound trivial, but it corresponds to a steep step in the curve of ejection fraction versus velocity and so makes a big difference: Chen and Ahrens found that "almost all atmosphere" is lost at 8 km/s, but Genda and Abe find that less than 30% of the atmosphere is lost at 6 km/s.

Nature http://www.nature.com/nature

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ON THE EARLY DEVELOPMENT OF AN OXYGEN-RICH EARTH ATMOSPHERE

It is currently believed that the oxygen concentration in Earth's atmosphere may have remained at 1 percent of its present level until approximately 2 billion years ago, after which the concentration gradually increased to its present value with the increasing success of photosynthetic life forms. Fossil oxygen-generating cyanobacteria have been dated as far back as 3.5 billion years ago, but the rate at which oxygen accumulated in the atmosphere because of photosynthesis is not known.

Although the time-frame of the increase in oxygen concentration of the atmosphere is uncertain, the consensus among researchers is that the initiation of an oxygen atmosphere increased the number and kinds of organisms capable of using aerobic metabolic pathways. By the start of the Cambrian period 570 million years ago, or somewhat earlier, oxygen levels had apparently increased enough to permit rapid evolution of large oxygen-utilizing multicellular organisms.

The following points are made by Norman H. Sleep (Nature 2001 410:317):

1) The author points out that although oxygen now constitutes approximately 20 percent of the gas in the atmosphere, before approximately 2.5 billion years ago it was apparently only a trace constituent. The Earth is unique among the planets in the Solar System in having an oxygenated atmosphere. The atmospheres of the other planets are anoxic because oxygen levels are kept relatively low by an equilibrium system involving chemical processes in crusts, mantles, and volcanic gases. On the Earth, oxygen levels increased over geological time apparently mainly as a result of photosynthesis, which can be expressed as the general reaction carbon dioxide --> carbon + oxygen.

2) Over the eons, a vast amount of organic carbon has become locked up in sedimentary rocks, a small part of it in coal and oil, thus preventing the reverse reaction to equilibrium that would create carbon dioxide and lower atmospheric levels of oxygen. However the advent of oxygen-producing photosynthesis cannot be the entire story of the evolution of Earth's atmosphere, since photosynthesis apparently existed long before the well-documented rise in oxygen levels 2.5 billion years ago (the Archaean-Proterozoic transition).

3) L.R. Kump et al (Geochem. Geophys. Geosyst. 2001) are now proposing that changes in the deep interior of the Earth affected the composition of volcanic gases, and that this led to the rise in atmospheric oxygen levels 2.5 billion years ago. The idea is that although photosynthesis is presently the main source of oxygen in Earth's atmosphere, it may have been geological activity that first allowed an oxygen-rich atmosphere to develop.

4) The author (Sleep) concludes: "It was over two centuries ago that Antoine Lavoisier figured out that we breathe oxygen. But we still don't know how an oxygen-rich atmosphere arose. Clearly, processes at both the Earth's surface and in its bowels were involved. Exactly how and by how much each contributed remain open questions."

Nature http://www.nature.com/nature

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