To Mars and beyond!

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Across
  1. 3. A substance that flows
  2. 6. Natural environment
  3. 7. A group of two or more atoms
  4. 9. What's left when a meteorite hits Mars
  5. 12. To turn from a solid directly into a gas
  6. 13. The study of life on planets
  7. 14. The distance between two peaks of a wave
  8. 17. Space craft that lands
  9. 18. The building blocks of some human-made materials
  10. 19. Removing harmful gases
  11. 20. This layer of the atmosphere is protective
Down
  1. 1. Tiny planet-like body
  2. 2. The path Mars takes around the Sun
  3. 4. The force that keeps our feet on the ground
  4. 5. What a rocket carries
  5. 8. The Milky Way is one
  6. 10. The smallest part of an element
  7. 11. A small celestial body
  8. 15. Something that lives in an extreme environment
  9. 16. Indigestible part of a plant
  10. 19. Something that dissolves substances