| Answer | Crossword Clue |
|---|---|
| EON | About 1-5th of Earth's existence |
| ARGON | About 1 percent of the Earth's atmosphere |
| ionosphere | The layer of the earth's atmosphere that contains a high concentration of ions and free electrons and is able to reflect radio waves. It lies above the mesosphere and extends from about 50 to 600 miles (80 to 1,000 km) above the earth's surface |
| ionospheres | The layer of the earth's atmosphere that contains a high concentration of ions and free electrons and is able to reflect radio waves. It lies above the mesosphere and extends from about 50 to 600 miles (80 to 1,000 km) above the earth's surface |
