The Moon and Mercury: Airless Worlds

1. The Moon and Mercury: Airless Worlds Chapter 21

2. Guidepost The two preceding chapters have been preparation for the exploration of the planets. In this chapter, we begin that detailed study with two goals in mind. First, we search for evidence to test the solar nebula hypothesis for the formation of the solar system. Second, we search for an understanding of how planets evolve once they have formed. The moon is a good place to begin because people have been there. This is an oddity in astronomy in that astronomers are accustomed to studying objects at a distance. In fact, many of the experts on the moon are not astronomers but geologists, and much of what we will study about the moon is an application of earthly geology.

3. Guidepost (continued) While no one has visited Mercury, we will recognize it as familiar territory. It is much like the moon, so our experience with lunar science will help us understand Mercury as well as the other worlds we will visit in the chapters that follow.

4. Outline I. The Moon A. The View From Earth B. Highlands and Lowlands C. The Apollo Missions D. Moon Rocks E. The History of the Moon F. The Origin of Earth's Moon II. Mercury A. Rotation and Revolution B. The Surface of Mercury C. The Plains of Mercury D. The Interior of Mercury E. A History of Mercury

5. The Moon: The View from Earth From Earth, we always see the same side of the moon. Moon rotates around its axis in the same time that it takes to orbit around Earth: Tidal coupling: Earth’s gravitation has produced tidal bulges on the moon Tidal forces have slowed rotation down to same period as orbital period

6. Lunar Surface Features Two dramatically different kinds of terrain: • Highlands: mountainous terrain, scarred by craters • Lowlands: • about 3 km lower than highlands; smooth surfaces • Mare (mah-ray): • Basins flooded by lava flows; once thought of as oceans and still called “seas” • (Maria: plural of mare)

7. Highlands and Lowlands Sinuous rilles are remains of ancient lava flows May have been lava tubes which later collapsed due to meteorite bombardment. Apollo 15 landing site

8. The Highlands Saturated with craters Older craters partially obliterated by more recent impacts … or flooded by lava flows

9. Impact Cratering Impact craters on the moon can be seen easily even with small telescopes. Ejecta from the impact can be seen as bright rays originating from young craters

10. The Moon’s Craters (SLIDESHOW MODE ONLY)

11. History of Impact Cratering Rate of impacts due to interplanetary bombardment decreased rapidly after the formation of the solar system. Most craters seen on the surface of the moon (and Mercury) were formed within the first 1/2 billion years.

12. Missions to the Moon Major challenges: Need to carry enough fuel for: • in-flight corrections • descent to surface • re-launch from the surface • return trip to Earth Need to carry enough food and other life support for one week for all astronauts on board. Solution: • land with a small, light lunar module; keep a command module orbiting the moon. • leave everything behind that is no longer needed. Lunar module (LM) of Apollo 12 on descent to the surface of the moon

13. The Apollo Missions

14. Apollo Landing Sites First Apollo missions landed on safe, smooth terrain. Later missions explored more varied terrains. Apollo 17: Taurus-Littrow; lunar highlands Apollo 11: Mare Tranquilitatis; lunar lowlands

15. Apollo Landing Sites (2) Selected to sample as wide a variety as possible of different lowland and highland terrains. Lowlands (maria) Highlands

16. Moon Rocks All moon rocks brought back to Earth are igneous (solidified lava) No sedimentary rocks (no sign of water ever present on the moon.) Different types of moon rocks: Vesicular(contain holes from gas bubbles in lava) basalts, typical of dark rocks found in mare Breccias (fragments of different types of rock fused together), also containing anorthosites (bright, low-density rocks typical of highlands) Older rocks become pitted with small micrometeorite craters

17. The History of the Moon • Moon is small, low mass so it rapidly cooled off. • Low escape velocity • No atmosphere, so unprotected against meteorite impacts. • Must have formed in a molten state -“sea of lava” • Heavy rocks sunk to bottom; lighter rocks at the surface • No magnetic field means small core with little metallic iron. • Surface solidified 4.6-4.1 billion years ago. • Heavy meteorite bombardment for the next 1/2 billion years. Alan Shepard (Apollo 14) analyzing a moon rock, probably ejected from a distant crater.

18. Formation of Maria Impacts of heavy meteorites broke the crust and produced large basins that were flooded with lava

19. Formation of Maria (2) Major impacts forming maria might have ejected material over large distances. Apollo 14 Large rock probably ejected during the formation of Mare Imbrium (beyond the horizon!)

20. Origin of Mare Imbrium Terrain opposite to Mare Imbrium is jumbled by seismic waves from the impact.

21. The Origin of Earth’s Moon Early (unsuccessful) hypotheses: Fission hypothesis: Fission (break-up) of Earth during early period of fast rotation Problems: No evidence for fast rotation; moon’s orbit not in equatorial plane capture hypothesis: Capture of moon that formed elsewhere in the solar system Condensation hypothesis: Problem: Requires succession of very unlikely events Condensation at time of formation of Earth Problem: Different chemical compositions of Earth and moon

22. Modern Theory of Formation of the Moon The Large-Impact Hypothesis • Impact heated material enough to melt it consistent with “sea of magma” • Collision not head-on Large angular momentum of Earth-moon system • Collision after differentiation of Earth’s interior Different chemical compositions of Earth and moon

23. Mercury Very similar to Earth’s moon in several ways: • small; no atmosphere • lowlands flooded by ancient lava flows • heavily cratered surfaces Most of our knowledge based on measurements by Mariner 10 spacecraft (1974 - 1975) View from Earth

24. Rotation and Revolution Like Earth’s moon (tidally locked to revolution around Earth), Mercury’s rotation has been altered by the sun’s tidal forces, but not completely tidally locked: Revolution period = 1.5 times rotation period • Revolution is about88 days • Rotation is about59 days This causes extreme day-night temperature differences: -280 oF (100 K) to 620 oF (600 K )

25. The Surface of Mercury Very similar to Earth’s moon: • Heavily battered with craters, including some large basins. • Largest basin is called Caloris Basin • Terrain on the opposite side jumbled by seismic waves from the impact.

26. Lobate Scarps Curved cliffs, probably formed when Mercury shrank while cooling down

27. The Plains of Mercury No large maria, but intercrater plains: Marked by smaller craters (< 15 km) and secondary impacts Smooth plains: Even younger than intercrater plains

28. The Interior of Mercury Large, metallic core. Over 60% denser than Earth’s moon Magnetic field only ~ 0.5 % of Earth’s magnetic field. Difficult to explain at present: Liquid metallic core should produce larger magnetic field. Solid core should produce weaker field.

29. History of Mercury Dominated by ancient lava flows and heavy meteorite bombardment. Radar image suggests icy polar cap.

30. New Terms tidal coupling terminator limb mare sinuous rille ejecta ray secondary crater micrometeorite multiringed basin relative age absolute age vesicular basalt anorthosite breccia regolith jumbled terrain fission hypothesis condensation hypothesis capture hypothesis large-impact hypothesis resonance lobate scarp intercrater plain smooth plain