The influx of meteorites. Changes due to radioactivity
Heavy cratering on the surfaces of the Moon, Mars, Mercury, the Jovian moon Callisto and the Saturnian moon Dione attests to the intensity of meteorite bombardment of the planets and their satellites. Examination of the density of craters and their ages on the Moon reveals a tremendous rate of influx about 4400 Ma ago, rapidly diminishing by 3800 Ma ago and gradually tailing off subsequently.
4.20: A meteorite embedded in the shell of a straight shelled nautiloid (Mollusca) in a 463-million-year-old Ordovician limestone. It seems likely that the meteorite was the cause of death
The cumulative masses of meteorites swept up by the gravitational fields of the Moon and Earth from 4400 Ma ago to the present are estimated to be about 1019 and 20 x 1019 kg respectively, representing about 0.003 Earth mass. Taking into account the volatile content of meteorites (about 10 per cent for carbonaceous chondrites), the capture process would account for less than 0.1 per cent of the present mass of water in the world's oceans and only a small fraction of the Earth's total inventory of carbon or other volatile elements.
The fiery trail of meteorites heated to incandescence by atmospheric friction is vivid evidence for the infall of rocky material from space. Such visible falls are comparatively rare, however, and by far the largest mass of debris captured from space by the Earth's gravity consists of a steady rain of microscopic meteorites or dust particles which are too small to be observed directly. Measurement of their total influx requires elaborate chemical techniques based on exotic nuclides or anomalous quantities of the rare metal iridium.
Meteorites and dust particles in space are constantly bombarded by cosmic rays emitted in a powerful stream from the Sun. Cosmic rays produce exotic nuclides such as 26Al, 53Mn and 59Ni in collisions with dust particles, but the Earth's atmosphere effectively shields surface rocks from exposure. Thus the presence of 59Ni in ocean sediments can be attributed entirely to microscopic meteorites and is found to be about 2 per cent of the total nickel present. Meteorites contain a 'normal' cosmic abundance of iridium, but the Earth's surface rocks are strongly depleted in this metal because of its siderophile character (preference for the metallic iron core rather than the rocky crust).
Hence the quantity of iridium in surface rocks provides a measure of the influx of meteorites. Amajor iridium anomaly has been detected in rocks at the Cretaceous-Palaeocene boundary (65 Ma ago), which coincides with the extinction of the dinosaurs. It has been suggested from this evidence that the impact of a large meteorite, or a nearby supernova, severely modified surface terrestrial conditions.
Changes due to radioactivity. Finally radioactive decay involves conversion of a parent nuclide into a daughter nuclide, accompanied by the release of energy. Of the various long-lived radioactive elements present in rocks, only two daughter nuclides are produced in sufficient quantity to increase appreciably the Earth's inventory. First, potassium-40 undergoes decay to argon-40. producing a progressively larger content of this noble gas in the Earth's atmosphere. Second, uranium and thorium both undergo decay to lead plus helium : new daughter lead represents only a few per cent of the Earth's intrinsic complement of lead, but the extra helium production is significant.
5.1: Rock composed of crystals of calcite (white), pyrite (gold), sphalerite (black), and galena (silver).
Date added: 2023-01-09; views: 166;