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The Moon and Mercury: Comparing Airless Worlds Chapter 21

Want to fly to the moon? You will need to pack more than your lunch. There is no air and no water, and the sunlight is strong enough to kill you. Mercury is the same kind of world. Take shelter in the shade, and you will freeze to death in moments. Earth seems normal to you, and other worlds are, well, unearthly, but they are related to Earth in surprising ways. Exploring these two airless worlds will answer five essential questions: Why is the moon airless and cratered? How did the moon form and evolve? In what ways is Mercury similar to, and different from, the moon? How did Mercury form and evolve? How are the histories of the moon and Mercury connected to Earth? Guidepost

You are beginning your detailed study of planets by exploring airless worlds; in the next chapter you will move on to bigger planets with atmospheres. They are not necessarily more interesting places, but they are just a bit less unearthly. Guidepost (continued)

I. The Moon A. The View From Earth B. The Apollo Missions C. Moon Rocks D. The History of the Moon E. 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 Outline

From Earth, we always see the same side of the moon. The 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 the rotation down to the same period as the orbital period. The Moon: The View from Earth

Lunar Surface Features Two dramatically different kinds of terrain: Highlands: Mountainous terrain, scarred by craters Lowlands: ~ 3 km lower than highlands; smooth surfaces: Maria (pl. of mare): Basins flooded by lava flows

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

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

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.

Impact Craters Impact craters are best seen when they are near the day-night boundary, called the Terminator. Ejecta from a major impact sometimes leave traces of secondary craters.

History of Impact Cratering The age of the moon rocks provide evidence of a late heavy bombardment 4.1 to 3.8 billion years ago. Rate of impacts due to interplanetary bombardment decreased rapidly within the first ½ billion years after the formation of the solar system.

Missions to the Moon Major challenges: Lunar module (LM) of Apollo 12 on descent to the surface of the moon 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 ~ 1 week for all astronauts on board

Missions to the Moon Lunar module (LM) of Apollo 12 on the surface of the moon Solution: only land a small, light lunar module; leave everything behind that is no longer needed

The Apollo Missions

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

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 (= containing holes from gas bubbles in the lava) basalts, typical of dark rocks found in maria Breccias (= fragments of different types of rock cemented together), also containing anorthosites (= bright, low-density rocks typical of highlands) Older rocks become pitted with small micrometeorite craters.

The History of the Moon Alan Shepard (Apollo 14) analyzing a moon rock, probably ejected from a distant crater. Moon is small; low mass  rapidly cooling off; small escape velocity  no atmosphere  unprotected against meteorite impacts Moon must have formed in a molten state (“sea of lava”); Heavy rocks sink to bottom; lighter rocks at the surface No magnetic field  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

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

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

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

The Origin of Earth’s 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

Mercury Very similar to Earth’s moon in several ways: Small; no atmosphere lowlands flooded by ancient lava flows heavily cratered surfaces Until very recently, most of our knowledge was based on measurements by Mariner 10 spacecraft ( ). View from Earth Ground-based image The MESSENGER spacecraft will go into orbit around Mercury in 2011.

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.

Rotation and Revolution Revolution period = 3/2 times rotation period Revolution: ≈ 88 days Rotation: ≈ 59 days  Extreme day-night temperature contrast: 100 K (-173 o C) – 600 K (330 o C)

The Surface of Mercury Very similar to Earth’s moon: Heavily battered with craters, including some large basins Most craters on Mercury were formed after the era of heaviest bombardment.

The Surface of Mercury (2) Largest basin: Caloris Basin Terrain on the opposite side jumbled by seismic waves from the impact

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

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

A History of Mercury Innermost planet in the Solar system → Only heavy elements could condense out Later bombardment removed more of the ligher rocks from Mercury’s surface. No atmosphere → Heavy bombardment left many impact craters. Stronger gravity than the moon → Secondary impacts less spread out over the surface Cooling interior contracted → crust broke to form lobate scarps