From the abundance of craters on the Moon and Mercury, we know that numerous asteroids and meteoroids struck the inner planets early in the history of our solar system. It is reasonable to suppose that numerous comets also pelted the planets 3.5 to 4.5 billion years ago. There must have been many large icy planetesimals in the Kuiper belt and Oort cloud that collided with each other, producing many large fragments, some that remain out there today. Those that lost a lot of energy during such collisions fell in towards the Sun and some peppered the inner planets with impacts. Much of the water that now exists on Earth, the water that was known to exist on Mars, and evidently the water existed for a brief time on Venus and Mercury were most likely delivered to these planets by these icy fragments.
(a) These impacts probably completely obliterated any atmosphere the planets might have had at that time.
(b) Comets are known to contain lots of organic hydrocarbons, so they probably generated thick atmospheres laden with organic polymers, much like the atmosphere of Titan.
(c) Much of the methane, ammonia and water vapor that marked the early planetary atmospheres also arrived on these fragments. Indeed, methane is broken down by ultraviolet light and hydrogen and nitrogen are released. The hydrogen usually escapes from terrestrial planets but the nitrogen remains in the atmosphere.
(d) They probably punched holes in the crusts of the planets causing natural gas trapped in the crust to be released.
(e) None of the above are likely.
