It takes the right amount of carbon

"On Earth, carbon is a relatively rare element. It is enriched close to Earth´s surface, but as a fraction of the total matter on Earth it is a mere one-half of 1/1000th. In primitive comets, however, the proportion of carbon can be ten percent or more," states Prof. Trieloff. According to the geochemist, comets originate in the cool outer regions of the solar system where volatile water and carbon compounds condensed into ice. Researchers assume that asteroid and comet impacts contributed these volatile elements to the newly formed Earth. But it is a puzzle why the amount of carbon on Earth is so low. "A substantial portion of the carbon in asteroids and comets is in long-chain and branched molecules that evaporate only at very high temperatures. Based on the standard models that simulate carbon reactions in the solar nebula where the sun and planets originated, Earth and the other terrestrial planets should have up to 100 times more carbon," states Prof. Gail.

The Heidelberg researchers assume that the short-duration flash-heating events were responsible for the "loss" of carbon. They suspect that all the matter in the inner regions of our solar system was heated, in some cases repeatedly, to temperatures between 1.300 and 1.800 degrees Celsius before small planetesimals and ultimately the terrestrial planets and Earth formed. The researchers believe the evidence lies in chondrules, the round grains that formed as molten droplets during these heating events before their accretion to meteorites. "Only the spikes in temperature derived from the chondrule formation models can explain today's low amount of carbon on the inner planets. Previous models did not take this process into account, but we apparently have it to thank for the correct amount of carbon that allowed the evolution of Earth's biosphere as we know it," says Hans-Peter Gail.

The researchers speculate that a carbon "overdose" would have probably been detrimental to the evolution of life. In its oxidised state, carbon forms the greenhouse gas CO2, which is removed from Earth's atmosphere especially by the silicate-carbonate cycle, which acts like a thermostat. "Whether 100 times more carbon would permit effective removal of the greenhouse gas is questionable at the very least. The carbon could no longer be stored in carbonates, where most of Earth's CO2 is stored today. This much CO2 in the atmosphere would cause such a severe and irreversible greenhouse effect that the oceans would evaporate and disappear," states Mario Trieloff.