(ËvÉr bÉl) adj. 1. of or consisting of words: verbal ability. 2. spoken rather than written; oral: verbal communication. 3. concerned with words only, rather than with the ideas, facts, or realities expressed: a purely verbal distinction. 4. corresponding word for word; verbatim: a verbal translation.
Because their hypothesis requires that Earth arose from water-containing materials already present in the inner solar system, it's called the wet-accretion hypothesis. Most of Earth's water has an indigenous origin, says Drake.
This continuous process of precipitation and evaporation is called the water cycle, or hydrologic cycle. It's been going on ever since oceans were formed on this planet 3.8 billion years ago. Earth was formed 4.6 billion years ago, but water was not present at the start. At some point, possibly because of the heating of hydrogen and oxygen as Earth developed, water vapor began to form in the atmosphere. Oceans formed and the cycle began. Seventy percent of Earth is covered by water.