by Carl Strang
Last year’s winter series was a review of the prehistoric life and geological history of northeast Illinois (if you are interested, access it through “Prehistoric Life series” under “Categories” in the left sidebar). Paleontology as a field of research has the advantage that it is covering billions of years of time, and so there is always much to learn each year from newly published findings. The remainder of this season’s reviews of last year’s literature will focus on this subject. It makes sense to go chronologically, so I’ll begin with the Hadean Eon, the time when Earth formed but before it cooled enough for the first rocks to crystallize that have survived to the present day.
Bottke, William F., et al. Stochastic Late Accretion to Earth, the Moon, and Mars. Science, 10 December 2010: 1527-1530 DOI: 10.1126/science.1196874 They studied the composition of mantle-derived rocks on Earth, Moon and Mars to test the degree to which these bodies were struck by large impactor bodies after their basic structures were established. Such bodies weren’t so large that they completely restructured the planets (as did Theia on Earth), and so their contents became added to the more surface portions rather than the cores. In particular the presence of “siderophile” metals (gold, platinum, palladium, osmium and iridium for example) is indicative of such impacts, and would not be present in the usable amounts we see on Earth without such additions. The time frame is a few tens of millions of years after the Theia impact. The largest of these hitting Earth is estimated to have been the size of Pluto, 1500-2000 miles in diameter. It may be responsible for as much as a 10° adjustment of the Earth’s tilt from its previous orientation. The largest Moon impactors are estimated in the 150-200 mile diameter range, and are the likely source of water there.
Kerr, Richard A. 2011. Planetary two-step reshaped solar system, saved Earth? Science 332:1255. This news article described a new model, published in Nature, of planetary dynamics in the developing Solar System. The model, which focuses on gravitational interactions of the sun, developing planets, planetesimals and gases explains today’s endpoint better than earlier ones, and answers a lot of puzzles. Earlier work suggested that Jupiter should have swept in close enough to the Sun to wipe out the inner planets, including the Earth. This model shows that Jupiter came to within 1.5 Earth orbits of the sun, but then was drawn back out by a combination of factors, particularly the draw of Saturn. That close approach kept Mars small, as Jupiter limited the planetesimals available to the fourth planet. The model also accounts well for the asteroid belt and aspects of its structure (mainly dry, rocky asteroids inward and others rich in ice and organic compounds dominating the outer belt). All of this happened over a relatively brief period of 5 million years, 4.6 billion years ago.
Dustin Trail, E. Bruce Watson, Nicholas D. Tailby. The oxidation state of Hadean magmas and implications for early Earth’s atmosphere. Nature, 2011; 480 (7375): 79 DOI: 10.1038/nature10655 As described in a ScienceDaily article. They looked at the oxidation state of elements in zircon minerals from the earliest rocks, and found that these indicate an early atmosphere high in oxygen-containing gases such as carbon dioxide, water and sulfur dioxide (though not, of course, free oxygen). This contradicts the current understanding that the early atmosphere (in which life arose) was a reducing atmosphere with ammonia, methane, carbon monoxide and hydrogen sulfide as dominant gases.