An alternative to Earthly abiogenesis is the hypothesis that prime time life may have originally formed extra terrestrially. Organic compounds are relatively common in space, especially in the outer solar system where volatiles are not evaporated by solar heating. Comets are encrusted by outer layers of dark material, thought to be tar- like substance composed of complex organic material formed from simple carbon compounds after reactions initiated mostly by irradiation by ultraviolet light. It is supposed that a rain of cometary's material on early Earth could have brought significant quantities of complex organic molecules and that it is possible that primitive life itself may have formed in space was brought to the surface along with it. A related hypothesis holds that life may have formed first on early Mars, and been transported to Earth when crust material was blasted off of Mars by asteroid and comet impacts to later fall to Earth's surface. Both of there hypothesis are even more difficult to find evidence for, and neither of them actually answers the question of how life first originated, merely shifting it to another planet/ comet.