ocr: - < e I - - - MARS AND EARTH: Earth (cont.) A COMPARATIVE HISTORY Volcanic activity on HE SEARCH FOR Volcanos thus became active, and changed the planets drastically. Almost the entire surface of the Earth, for example, was at one time covered with lava. The heaviest substances solidified, remodeling the Earth's surface. The lightest remained in the gas: state, and modified the planet's primitive atmosphere. In fact, carbon dioxide and water vapor made up 90% of the material ejected by volcanos. On Earth, the. atmospheric pressure was high enough to condense the water vapor, eventually forming ...
