Cosmic rain

Natural History, March, 2007 by Stephan Reebs

Cosmic rays--charged particles emitted by supernovas and other highly energetic sources in space--continually strike the Earth's atmosphere. Most scientists, however, had assumed they could have little effect on terrestrial life. Then last year, Henrik Svensmark, a physicist at the Danish National Space Center in Copenhagen, published experimental evidence that cosmic rays could increase the formation of cloud droplets, with obvious implications for climate and thus for life. Now another study by Svensmark reveals a remarkable link between cosmic rays and the stability of biological productivity on Earth.

Two main factors, Svensmark assumes, have accounted for most of the changes in Earth's cosmic-ray exposure through geological time: the amount of shielding from cosmic rays afforded by the Sun's magnetic field, and the rate of supernova formation throughout our Milky Way. Svensmark estimated the Sun's shielding by studying other sun-like stars for clues to our star's history, and the supernova rate from straightforward astrophysical records. He also estimated changes in Earth's biological productivity through time by Hot gas cloud from a supernova, a cosmic-ray source measuring the ratio of the isotopes carbon-13 to carbon-12 in ancient sediments. Life processes, such as photosynthesis, preferentially use carbon-12, so the higher the relative amount of carbon-13 left behind in sediment, the greater Earth's biomass must have been when the sediment was deposited.

Svensmark discovered that when cosmic rays were most intense (between 2 billion and 2.5 billion years ago, for instance), life was particularly unstable: periods of high productivity alternated with leaner times. Thus a surprising connection exists between distant supernovas and life on Earth: intense cosmic rays appear to cause climate fluctuations that bring on alternating periods of feast and famine. (Astronomische Nachrichten)