Labels: text | screenshot OCR: MARS AND EARTH: A COMPARATIVE HISTORY Volcanic activity on Earth (cont.) THE SEARCH FOR LIFE 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 oceans that covered a large part of the globe. But subjected to the Sun's rays, water evaporated from the vast surface of the oceans at the rate of 100 billion tons per day. Cooling in the upper atmosphere, the water vapor condensed, forming clouds and causing rain. A small amount of water molecules, however, reached the highest altitudes where they broke down into hydrogen and oxygen. The very light hydrogen escaped into space, while the heavier oxygen molecules fell back in the atmosphere, further modifying its composition. MARS BETAT
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