Comets contain the key elements needed for the emergence of life on Earth and may have indeed provided our planet with them, according to Tel Aviv University researchers. While investigating the chemical makeup of comets, Prof. Akiva Bar-Nun - a former director of the Israel Space Agency who works at TAU's geophysics and planetary sciences department - found they were the source of missing ingredients needed for life in Earth's ancient primordial soup. "When comets slammed into the Earth through the atmosphere about four billion years ago, they delivered a payload of organic materials to the young Earth, adding materials that combined with Earth's own large reservoir of organics and led to the emergence of life," says Bar-Nun. It was the chemical composition of comets, he believes, that allowed them to kickstart life. He has published his theory widely in scientific journals, including Icarus. Using a unique machine he devised at TAU, he and colleagues simulated comet ice and found that comets contain ingredients necessary for providing the basic nutrients of life. Specifically, Bar-Nun looked at the "noble" gases argon, krypton and xenon because they don't interact with any other elements and are not destroyed by Earth's oxygen. These elements have maintained stable proportions in the Earth's atmosphere throughout the lifetime of the planet, he explains. "If we look at these elements in the atmosphere of the Earth and in meteorites, we see that neither is identical to the ratio in the sun's composition. Moreover, the ratios in the atmosphere are vastly different than the ratios in meteorites, which make up the bulk of the Earth. "So we need another source of noble gases which, when added to these meteorites or asteroid influx, could change the ratio. And this came from comets." Formed in the early days of the solar system far away from our sun, comets originated as water vapor condensed directly into ice, thus creating tiny grains. These grains came together to form the comets, says Bar-Nun. During the comets' formation, the porous ice trapped gases and organic chemicals from outer space, creating a certain ratio of argon to krypton to xenon. This ratio, together with that of gases coming from rocky bodies, gives us the ratio that we observe in the atmosphere of the Earth, he adds. Thus, the arrival on Earth of comets and asteroids led to the necessary ratio of materials for organic life "that eventually were dissolved in the ocean and started the long process leading to the emergence of life on Earth," says Bar-Nun. This process began between 4.6 and 3.8 billion years ago when both the moon and the Earth were bombarded by asteroids and comets. "On the Earth, most of the craters were obliterated by continental movement and weathering winds and water erosion. On the moon, they remained as they were," says Prof. Bar-Nun, who adds that no life could thrive during this period of bombardment. But the Earth recovered, and 300 million to 400 million years later, fragile forms of life emerged after the comet-delivered elements precipitated into the ocean. "There was another chemical development of these molecules in water, which became more and more complex," says Bar-Nun, leading to the origin of life on Earth.