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Across 2. Regions on Mercury that are relatively smooth. 3. Long-lived storm system in Jupiter's atmosphere. 5. Martian polar ice cap that grows and shrinks. 6. Theory suggesting that the Moon formed from the Earth. 11. Rotating storm system near the Great Red Spot. 13. Regions on the lunar surface that are elevated above maria. 14. Long-lived hole in clouds that permits astronomers to see into Jupiter's lower atmosphere. 15. Dark, smooth, basaltic features on the lunar surface. 19. Moons that share the same orbit. 20. Theory suggesting that the Moon formed from an impact of a Mars sized object. 21. Type of greenhouse effect where a planet's atmosphere increases its ability to retain heat. 22. Eastward and westward atmospheric flow; symmetrical about the equator. 23. Bowl-shaped depression. 24. Occurs when a planet's or moon's orbit is equal to its average orbital period. 25. Surface dust.

Down 1. When rotation rates are unequal at the equator and the poles of a gaseous planet. 4. An apparent backward loop traced by a planet. 7. Martian polar ice cap that remains permanently frozen. 8. Thin, flat rings surrounding gaseous planets. 9. Gravitational pull of a moon that helps keep the planet's ring(s) in shape. 10. Long-lived storm system in Neptune's atmosphere. 12. Theory suggesting the Moon formed as a separate oject close to Earth. 16. Theory suggesting that the Moon was captured by Earth's gravitational pull and brought closer. 17. The four large moons of Jupiter. 18. Forces that cause a satellite to rotate at the same rate it revolves around its parent planet.