It is increasingly obvious that to study the origins of life on Earth we cannot limit our investigations to our own planet, but we have to look at the various planetary objects of the Solar System. All traces of the history of the first half-billion years on Earth have been erased. By looking in detail at Venus and Mars now, in particular with the help of spacecrafts' atmospheric probes and landers, we can perform a systematic comparison of the present status of the three terrestrial planets. From such a comparison we can deduce the main physical–chemical processes that governed these planets' evolution from their formation up to now. We can thus model for each planet, including the Earth, not only its evolution but also the initial conditions, which are currently unreachable by direct observation.

By looking in detail at comets and meteorites, we can study the organic matter currently carried by these extraterrestrial objects. We can thus approach the chemical nature of the organics that may have been imported onto the primitive Earth from these carriers.

By looking in detail at the outer Solar System, we can study the organic physical and chemical processes that are currently involved in all the atmospheres of the outer planets and, first of all, as this chapter will try to demonstrate, in the atmosphere of Titan, the largest satellite of Saturn. We can thus constrain our models of the organic physical and chemical evolution on Earth, before the emergence of life.