by Carl Strang
Today’s post begins a series of weekly updates from last year’s literature on prehistoric life and the associated geology. In this one I include selected studies of our planet’s first two eons, covering the first 2 billion years (out of 4.6 total) of the Earth’s existence. The Hadean Eon is defined by the lack of surviving crust. It is known mainly from moon rocks, which along with certain deep-Earth data have told of a collision between the early Earth and a Mars-sized object named Theia. The moon was a product of that collision. The following Archean Eon brought the first-formed planetary crust, oceans, and the origin of life.
Arpita, Roy, et al. 2014. Earthshine on a young moon: explaining the lunar farside highlands. Astrophysical Journal Letters, DOI: 10.1088/2041-8205/788/2/L42 The far side of the moon has hardly any maria, unlike the familiar near side which has large areas covered by those ancient lava flows. This paper provides an explanation as to why the far side crust is so much thicker, so that meteor strikes did not so readily punch through. It is built on the collision that formed the moon. Both Earth and moon were much closer together at first, and the moon became tidally locked, so that the one side always faced the Earth. The heat of the Earth kept the near side hotter and molten longer, so that aluminum and calcium compounds cooled sooner and fell out more thickly on the far side, ultimately combining with silicates to form a thicker, feldspar-rich crust there.
Herwartz, D., A. Pack, B. Friedrichs, and A. Bischoff. 2014. Identification of the giant impactor Theia in lunar rocks. Science 344 (6188): 1146-1150. Data casting doubt on the Theia collision hypothesis were based on lunar rocks that had been contaminated through contact with Earth. New measurements taken from samples returned by NASA missions from the moon confirm that the proportion of Earth material in the moon is low enough to fit collision models.
Valley, John W., et al. 2014. Hadean age for a post-magma-ocean zircon confirmed by atom-probe tomography. Nature Geoscience, DOI: 10.1038/ngeo2075 They found zircon crystals in certain Australian rocks that formed 4.4 billion years ago, pushing back the earliest crust formation time and potentially permitting the formation of life earlier than had been thought. They suggest a hydrosphere may have existed as soon as 100 million years after the Theia collision.
Russell, Michael J., et al. 2014. The drive to life on wet and icy worlds. Astrobiology 14 (4): 308. DOI: 10.1089/ast.2013.1110 From a ScienceDaily article. They are examining one way life could make a start, around alkaline thermal vents at the bottom of an otherwise acidic (carbon dioxide rich) ocean. “Life takes advantage of unbalanced states on the planet, which may have been the case billions of years ago at the alkaline hydrothermal vents,” said Russell. “Life is the process that resolves these disequilibria.” The article describes two possible geological analogs to processes that go on in mitochondria. One imbalance is in protons, or hydrogen ions, which would have been more concentrated on the outsides of vent chimneys. The other would be the gradient from the methane and hydrogen in the vent to carbon dioxide in the surrounding ocean, which could have produced an electron transfer. The mineral analogs to enzymes are thought to have been “green rust” (not further identified in the article; its participation could have stored energy from the proton imbalance in a phosphate-containing molecule) and molybdenum (known to transfer two electrons at a time in physiological processes). They point out that these are processes that could be common on other watery planets.
Martin, William F., Filipa L. Sousa, and Nick Lane. 2014. Energy at life’s origin. Science 344:1092-1093. They compare the energy-releasing chemical reactions common to living things and find them to be similar to those going on at alkaline hydrothermal vents, suggesting that such places were where life began.