How did water form on Earth?

1. How did water form on Earth? A mini lesson in a series of scientific controversies

2. Some basics • Approximately 2/3 of Earth is covered in water • It is the only planet known that has liquid surface water • Water is one of the requirements for life • Until recently, scientists had an answer for how water appeared on Earth • As often happens; not one but two popular hypotheses have now emerged!

3. Refresh your memory… • Remember that when we follow the scientific method, a theory is a well tested and accepted concept • A new, untested idea that has been arrived at by inductive or deductive reasoning is a hypothesis • Inductive reasoning is based on our own experiences • Deductive reasoning is based on the use of logical assumptions

4. Did water arrive from outer space? • Planets like Mercury, Venus, Earth and Mars are terrestrial, made of metals and rocks • These are held together by the force of gravity • All around us in the solar system are billions of other rocks that did not form planets, but were formed at the same time that Earth was, approximately 4 ½ billion years ago • When these collide with Earth, we call them meteors • Occasionally; meteors contain surprises inside.

5. Idea #1: Icy chunks of rock hit Earth • Recently, scientists have discovered that inside some meteorites with hard iron shells, there are small cavities containing salts. • Inside the salt molecules are drops of water. • The water drops from space have the same proportion of hydrogen isotopes as water on earth! • This has lead some scientists to hypothesize that during the early life of our planet (several 100 million years after it formed), when Earth was struck by countless meteors, enough water was released from them to form our oceans.

6. Idea #2 – Water is homemade • For many years, geologists believed that water always existed on Earth, but did not exist in a liquid state. • When the planet was forming, volcanoes erupted with lava and steam, which is water in a vapor or gas phase. • When Earth cooled after millions of years, the vapor condensed, forming liquid water that formed the oceans.

7. Reminder: matter has phases or states • All matter, including water, has different phases or states depending on its temperature: • Coolest – Solid, molecules are held in place in a crystalline form. Water forms ice. • Warm – Bonds weaken as molecules start to move and the solid turns to a less stable liquid. Ice melts to form water. • Hot – Bonds between molecules break and molecules move freely in space. Water boils to form steam (vapor). • Extreme – Molecules absorb so much heat energy that they take on new properties. Stars are made of atoms that are so hot that they emit energy (light). This state is called a plasma.

8. Why does this matter? • Understanding how water formed on Earth provides clues on where else in the solar system to look for it. • One source of water that has been confirmed is on the moon, which has enough water to support human colonies. • There are many other searches underway across the solar system based on new knowledge that may find locations with water that will support life.