Formation and evolution of the Solar System depletion period that brought the asteroid belt down close to its present mass is thought to have followed when Jupiter and Saturn entered a temporary 2:1 orbital resonance (see below). The inner Solar System's period of giant impacts probably played a role in the Earth acquiring its current water content (~6 kg) from the early asteroid belt. Water is too volatile to have been present at Earth's formation and must have been subsequently delivered from outer, colder parts of the Solar System. The water was probably delivered by planetary embryos and small planetesimals thrown out of the asteroid belt by
Extraterrestrial liquid water Should planetary mass objects be present, a single, gas giant planet, with or without planetary mass moons, orbiting close to the circumstellar habitable zone, could prevent the necessary conditions from occurring in the system. However, it would mean that planetary mass objects, such as the icy bodies of the solar system, could have abundant quantities of liquid within them. Lunar maria are vast basaltic plains on the Moon that were thought to be bodies of water by early astronomers, who referred to them as "seas". Galileo expressed some doubt about the lunar 'seas' in his "Dialogue Concerning the Two Chief