The Earth's Moon

1. The Earth's Moon • The Earth's Moon, in many ways, is prototypical of a substantial fraction of the objects in the Solar System. • Like many other moons and planets it exhibits a heavily cratered surface that preserves a history of internal geological activity. • All that being said, as we'll see, the Moon is also a truly bizarre object, unrepresentative of most other Solar System objects!

2. The Earth/Moon as a Double Planet • With the exception of Earth, all planets in the Solar System have much smaller Moons.

3. The Earth/Moon as a Double Planet • The Earth and Moon are more similar in size.

4. The Earth/Moon as a Double Planet • The Earth and Moon are more similar in size. Seen from Mars orbit by Mars Reconnaissance Orbiter Seen from a distance (Galileo spacecraft on its way to Jupiter) True orbital separation

5. Tidal Coupling • Tides have worked to slow the rotation rate of the Earth and Moon. • The Moon's rotation has been slowed to the point where it always keeps the same hemisphere turned to Earth. • We only ever see one side of the Moon.

6. Tidal Coupling • Tides have worked to slow the rotation rate of the Earth and Moon. • The Moon's rotation has been slowed to the point where it always keeps the same hemisphere turned to Earth. • We only ever see one side of the Moon.

7. Tidal Coupling • Tides have worked to slow the rotation rate of the Earth and Moon. • The Moon's rotation has been slowed to the point where it always keeps the same hemisphere turned to Earth. • We only ever see one side of the Moon.

8. General Features of the Moon • It's round... • Gravity plays a significant role in shaping large objects in the Solar System • Rocks (e.g small asteroids) are cohesive and strong and do not change shape under their internal gravitational stresses. • If an object is large enough, gravity provides enough force to deform rock and make it flow. The object behaves like a liquid/plastic droplet and becomes spherical. Note that this asteroid is 500 times smaller relative to the Moon than depicted here.

9. General Features of the Moon • Contrast between distinct “dark” and “bright” terrains. • Dark regions (the Maria) are relatively crater free and at a lower elevation than the bright terrain. • Bright regions (the Highlands) are heavily cratered.

11. The Lunar Highlands • The heavily battered lunar highlands represent the oldest surviving crust of the Moon. • These regions are saturated with craters since they have been exposed to infalling debris since nearly the beginning of the Solar System. • Rocks brought back by the Apollo astronauts indicate an radioactive dating age of about 4.2 billion years for highland rocks – indicating the formation age of the original lunar crust.

12. The Lunar Highlands • The heavily battered lunar highlands represent the oldest surviving crust of the Moon. • These regions are saturated with craters since they have been exposed to infall debris since nearly the beginning of the Solar System. • Rocks brought back by the Apollo astronauts indicate an radioactive dating age of about 4.2 billion years for highland rocks – indicating the formation age of the original lunar crust.

13. The Lunar Maria • The Lunar Maria are huge impact basins where the previous history of cratering has been erased by lava flows. • Radioactive dating of rocks brought back from the maria indicate ages of about 3.5 billion years. • The highlands and maria were shaped in the first billion years of lunar history. The Moon has been largely geologically dead since then.

15. The Lunar Maria • The maria are dark because the lavas that filled them are dark, comparable to basaltic lava flows on Earth. • The highlands are brighter because • the original crust of the Moon consisted of a lighter colored material (anorthosite) • the roughness of these regions also makes them appear brighter.

16. The Lunar Maria • Huge impacts fractured the crust enabling the flow of magma from deep inside the relatively young Moon. • The Moon has since cooled to the point where it has nearly completely solidified. • Little or no geologic activity since 3 billion years ago!

17. The Lunar Maria • The huge impacts fractured the crust enabling the flow of magma from deep inside the relatively young Moon. • The Moon has since cooled to the point where it has nearly completely solidified. • Little or no geologic activity since 3 billion years ago!

19. Lunar History • Early crust formation • Heavy bombardment • Huge impacts • Filling of Basis with lava flows • Geologic death and light subsequent cratering

20. Lunar History • Early crust formation • Heavy bombardment • Huge impacts • Filling of Basis with lava flows • Geologic death and light subsequent cratering

22. Bombardment History • Few craters have accumulated since the maria were resurfaced. • What does this say about the history of bombardment in the Solar System??

23. Cratering History • Cratering has tapered off to a drizzle from the time of the “heavy bombardment”

24. Crater Counting and Surface “Age” • Turn this around, and the degree of cratering of a surface can reveal the “age” of the surface (if you know the cratering rate). • Where “age” means the time since last resurfacing.

25. Crater Counting and Surface “Age” • Turn this around, and the degree of cratering of a surface can reveal the “age” of the surface (if you know the cratering rate). • Where “age” means the time since last resurfacing. Saturn's moon Enceladus

26. The Lunar Farside • Farside not Darkside!

27. The Lunar Farside • The far side of the Moon is completely dominated by highland terrain. There are virtually no maria???

28. The Lunar Farside • The crust is thicker on the far side.

29. An Explanation for the Lunar Farside?? Two moons originally formed from the debris of the moon-forming collision. Not long thereafter these two Moons collided at relatively low velocity, plastering one face of the Moon with an extra later of material (click on the left image for the article in Nature).

30. Shaping the Lunar Surface • Overall, impacts both large and small have been responsible for sculpting the Lunar surface. • Even the Moon's mountains are piles of impact debris.

31. Shaping the Lunar Surface

32. Regoliths and Breccia • Impacts have fractured and pulverized the surface and fused previously fractured rocks together. • An impact fractured planetary surface is a “regolith” (blanket rock literally) • An impact fused rock made of different rock types is a “breccia”.

35. Lunar Exploration • A dozen humans walked on the Moon in six separate missions flown between 1969 and 1972. Apollo 11 30th anniversary site Apollo mission summaries and images Landing sites Top 10 Apollo results First person history

43. Going Back? • A return to the Moon was under discussion, but an expensive proposition. • The most recent set of plans have been scrapped by the administration as too unrealistic. • At the same time lunar exploration continues – especially the search for water.

44. Going Back? • A return to the Moon is under discussion, but an expensive proposition. • The most recent set of plans have been scrapped by the administration as too unrealistic. • At the same time lunar exploration continues – especially the search for water (rocket fuel!).

45. Going Back? • A return to the Moon is under discussion, but an expensive proposition. • The most recent set of plans have been scrapped by the administration as too unrealistic. • At the same time lunar exploration continues – especially the search for water (i.e. rocket fuel!)

46. The Latest...from NASA This is the beginning of a new era in space exploration where we will build the capabilities to send humans deeper into space than ever before. We will use the International Space Station as a test bed and stepping stone for the challenging journey ahead. We are changing the way we do business, fostering a commercial industry that will safely service low Earth orbit so we can focus our energy and resources on sending astronauts to an asteroid and eventually to Mars. The road ahead is challenging but this approach and space exploration architecture puts us in a position to go where no human has gone before..