Water, having a relatively high freezing point, will solidify first, followed by heaver compounds such as Carbon Dioxide and Methane. These compounds form separate snow lines, differing in colour and chemical make-up. The area where Water freezes relates to an orbital distance between that of Mars and Jupiter in our Solar System, whereas the Carbon Monoxide snow line would approximate the orbit of Neptune.
In a new theory, scientists have postulated that the reason why the Earth is so unexpectedly dry (less that 1% of the Earth's mass is locked up in water) is that the Earth was created in the zone before the Water snow line, where temperatures were hotter, instead of inside it, as was previously thought. In fact, the reason we have the water we do, rather than being much dryer like Mercury or Venus, may be down to the debris left by passing and impacting comets.
This discovery has given scientists "the first real picture of a snow line around a young star, which is extremely exciting because of what it tells us about the very early period in the history of the Solar System," said Chunhua "Charlie" Qi (Harvard-Smithsonian Center for Astrophysics, Cambridge, USA). It will also give scientists important new information about the formation and evolution of planets and comets.