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. Guidepost

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. Guidepost (continued)

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 Outline

The Moon and Mercury: Airless Worlds Chapter 21

Mare Highlands

1. Original crust forms 2. Impacts rework surface (late heavy bombardment) 3. Lunar mare form (volcanism) billion years: volcanism stops; cratering only since then

Formation of Maria: Lava flows 3.8 to 3.2 billion years ago Impacts of massive objects broke the crust and produced large basins that were flooded with lava

The Lunar Surface Older: heavily cratered Younger surface: flooded with lava

Impact Cratering Ejecta from the impact : bright rays around young craters

History of Impact Cratering Most craters seen on the moon’s (and Mercury’s) surface were formed within the first ~ 1/2 billion years. Rate of impacts decreased rapidly after the solar system formed.

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

The Moon landing was not faked! Moon rocks were brought back Black sky: no stars expected to show up Weird shadow angles caused by wide-angle shots

Moon Rocks: Igneous No sedimentary rocks => No sign of water ever present on the moon. Vesicular = bubbles from gases in lava, basalts (mare) Breccias (= “broken”) highlands Older rocks are pitted with small micrometeorite craters (highlands)

LCROSS impact plume

Mercury Very similar to Earth’s moon in several ways: Small; no atmosphere lowlands flooded by ancient lava flows heavily cratered surfaces View from Earth Visited by Mariner 10 ( ) Messenger (NOW!)

Mercury is more massive than the Moon, so an impact does not alter Mercury’s surface as much as the Moon’s. Result: Mercury holds a better record of solar system impact history Moon: lots of overlapping craters Mercury: easier to count craters because they don’t overlap as much as the moon’s

Mercury, Mariner 10 spacecraft similar to Moon but no mare day : 700 K night : 100 K

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

Discovery scarp: 500 km long, 2km high Mercury’s crust split and cracked as the planet cooled and shrank Lobate scarps

Scarps: unique to Mercury

(Extinct) Volcanoes on Mercury Mercury’s geology: heavily cratered. There are no large volcanoes like Mars’ Olympus Mons, but there are many smooth, flat plains with few craters Ancient plains are caused by volcanic activity The latest closeup images by NASA’s MESSENGER support the volcano theory MESSENGER false color image of Caloris impact basin (light orange is the basin interior). Extinct volcanoes were imaged in several of the bright orange regions just inside the southern crater rim.

Evidence for Volcanoes MESSENGER has found shield volcanoes and vents suggesting explosive volcanism inside the large Caloris basin The Mercury volcanoes smaller versions of the Hawaiian Islands or Olympus Mons on Mars Lava appears to have partly filled impact craters both inside and far from Caloris basin (not shown) MESSENGER image (left) of a shield-like volcanic dome, multiple vents and associated bright deposits, and partially buried nearby features. Shield volcanism formed the island of Hawaii (right). vents partly filled crater

The Big Picture Mercury’s widespread plains formed by volcanism Mercury’s volcanic style was more similar to the Moon than Mars or Earth MESSENGER will enter orbit around Mercury in 2011 Volcanic features in the inner solar system Hawaii Venus Mars Alaska Mercury

For more information… Press Releases space.com - 7/3/08 - “Volcanoes on Mercury Solve 30-year Mystery” Images Global view of Caloris basin and Mercury shield volcano courtesy of Science / AAAS Aerial view of Hawaii courtesy of NASA/JSC STS61A Aerial view of erupting Mauna Loa in Hawaii courtesy of HVO/USGS Image of Alaska’s Redoubt Volcano courtesy of AVO/USGS, taken by Heather Bleick Image of Olympus Mons on Mars and Maat Mon on Venus courtesy of NASA/JPL Source Article (on-campus login may be required to access journals) Head et al., ‘Volcanism on Mercury: Evidence from the First MESSENGER Flyby’, Science, 321(5885), p. 69, DOI: /science , Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider Released 24 April 2009

Fig Caloris basin 1300 diameter! OUCH!

Caloris Basin km across Discovery Scarp Geological features on Mercury “scarps” are long, steep cliffs, not found anywhere else

The Plains of Mercury intercrater plains, no large mare Marked by smaller craters (< 15 km) and impacts from ejecta Smooth plains: Even younger than intercrater plains

Intercrater plains: lots of small craters, no large ones

Smooth plains: similar to mare. Result of ancient volcanic flooding? We can learn about the timing of the volcanic activity relative to cessation of heavy bombardment

Smooth plains near Caloris Basin Smooth plains: younger than intercrater

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.

Why does Mercury have such a large core? Massive impacts may have blown away much of its mantle after the planet differentiated

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

Outstanding questions about Mercury: 1)Why is it so dense? 2)What is that stuff at its poles? 3) What can it tell us about planetary evolution?

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 New Terms

1. Old science-fiction paintings and drawings of colonies on the moon often showed very steep, jagged mountains. Why did the artists assume that the mountains would be more rugged than mountains on Earth? Why are lunar mountains actually less rugged than mountains on Earth? 2. From your knowledge of comparative planetology, propose a description of the view that astronauts would have if they landed on the surface of Mercury. Discussion Questions

Quiz Questions 1. Why does the same side of the Moon always face Earth? a. The Moon does not rotate. b. The Moon rotates in the same direction that it revolves. c. The Moon's period of rotation is equal to its orbital period. d. Sometimes the backside of the Moon is lit by the Sun. e. Both b and c above.

Quiz Questions 2. How did the Moon achieve its synchronous rotation? a. When the Moon formed it just happened to have this synchronous rotation. b. The Earth raises tidal bulges on the Moon. As the Moon rotated through these bulges, internal friction slowed the Moon's rotation until it achieved tidal coupling. c. Competing gravitational tugs on the Moon by the Earth and Sun set up this synchronous rotation. d. The Moon pulls up a tidal bulge on Earth, and Earth rotates so fast that it has locked the Moon into this synchronous rotation. e. As the Earth and Moon orbited their common center of mass, the centrifugal forces sent the Moon outward until this synchronous rotation was achieved.

Quiz Questions 3. How do we know that Copernicus is a young impact crater? a. It is on the side of the Moon that faces Earth. b. It has a central peak and raised rim. c. It has scalloped slopes along its inner crater walls. d. Blocks of material in its ejecta formed secondary craters. e. It has bright rays that extend onto the surrounding maria.

Quiz Questions 4. How do we find the relative ages of the Moon's maria and highlands? a. By counting the number of impact craters. b. By measuring the depth of the lunar regolith. c. By measuring the lunar latitude and longitude. d. By measuring the size of the smallest impact craters. e. By measuring variations in the Moon's gravitational field.

Quiz Questions 5. Why do almost all impact craters have a circular shape? a. High-speed projectiles vaporize explosively upon impact, sending out spherical compression waves. b. The impacting projectiles have a spherical shape and thus punch out circular penetration holes. c. Erosion has reduced the irregular craters to circular shapes. d. Most impacts occur from directly overhead. e. A circle is the most perfect form.

Quiz Questions 6. Why did the first Apollo missions land on the maria? a. The most interesting geology is at these locations. b. To maintain a continuous communication link with the command module. c. To search for fossils that are more likely to exist where water was once present. d. It was thought to be safer due to the smoother terrain and thinner regolith. e. The lunar air is thicker at low elevation.

Quiz Questions 7. Why do we suppose that the Moon formed with a molten surface? a. The Moon is covered with volcanic craters of all sizes. b. Samples from the maria regions are basalt, a common igneous rock. c. The oldest lunar rock samples are about 4.4 billion years old and composed of anorthosite, a mineral that crystallizes and rises to the top of a lava ocean. d. Both a and b above. e. All of the above.

Quiz Questions 8. What are the characteristics of a rock that is a breccia? a. Breccia is igneous rock, with large crystals that form by slow cooling of magma deep beneath the surface. b. Breccia is igneous rock, with small crystals that form by rapid cooling of lava flows on the surface. c. Breccia is rock consisting of broken rock fragments that are cemented together by heat and pressure. d. Breccia is a sedimentary rock composed of calcium and magnesium carbonates. e. Breccia is sedimentary rock formed by the evaporation of salty shallow seas.

Quiz Questions 9. Why are so many lunar rock samples breccias? a. The many violent volcanic eruptions have formed a lot of breccia. b. The numerous impact events produce a lot of brecciated rock. c. Slow evaporation of shallow seas in the maria regions left breccia deposits. d. Plate motion has pushed the deeply formed breccias to the lunar surface. e. Carbon dioxide dissolves in water, combines with calcium, and precipitates onto the sea floor. These deposits are later lithified by the heat and pressure that accompany deep burial. Impact events bring the breccias to the lunar surface.

Quiz Questions 10. On the large scale, which of the four states of development of a planetary body could be termed arrested development in the case of the Moon? a. Melting and differentiation. b. Impact cratering. c. Flooding of low-lying regions. d. Slow surface evolution. e. None of these stages took place on the Moon.

Quiz Questions 11. What single factor resulted in the Moon today being so very much different than the Earth is today? a. The long, continued period of occasional impacts. b. The flooding of lowland basins with basalt. c. The early torrential bombardment. d. The late heavy bombardment. e. The Moon's small size.

Quiz Questions 12. Why does the Moon have large maria on the Earth-facing side, yet no large maria on the opposite side? a. The maria regions are the same on both sides; we normally don't see those on the far side. b. The late heavy bombardment only occurred on the Earth- facing side. c. The maria on the far side are not as dark as those on the near side. d. The Moon's crust is thicker (or elevations higher) on the far side. e. No large impact basins exist on the Moon's far side.

Quiz Questions 13. Which of the following is due to the Moon's small size? a. The Moon has no atmosphere. b. The Moon does not have a dipole magnetic field. c. The Moon does not have plate tectonics. d. The Moon's surface geology is dominated by impact craters. e. All of the above.

Quiz Questions 14. For what reasons do we reject the condensation (double planet) hypothesis of the Moon's origin? a. The Moon has a much lower density than Earth. b. The Moon is very low in volatiles, compared to Earth. c. The Moon is much smaller and less massive than Earth. d. Both a and b above. e. All the above.

Quiz Questions 15. How does the large impact hypothesis explain the Moon's lack of iron? a. The impact occurred before either planetesimal had differentiated and formed an iron core. b. The ejected orbiting material that formed the Moon was initially at a high temperature. c. Both planetesimals were differentiated, and the two iron cores went to Earth. d. The impacting planetesimal was not differentiated and thus had no iron core. e. The Moon's lack of iron is the major problem of the large impact hypothesis.

Quiz Questions 16. How is the planet Mercury similar to Earth's moon? a. Their surfaces both appear heavily cratered by impacts. b. Their lowland regions were flooded by ancient lava flows. c. Their rotational periods are equal to their orbital periods. d. Both a and b above. e. All of the above.

Quiz Questions 17. How is the planet Mercury different than Earth's moon? a. The lowland maria on Mercury are not much darker than the cratered highlands. b. Mercury has a much higher density. c. Mercury has a dipole magnetic field. d. Both a and b above. e. All of the above.

Quiz Questions 18. How do we suppose that the lobate scarps on Mercury's surface formed? a. Lobate scarps are huge dormant lava tubes. b. As Mercury cooled and shrank, the crust wrinkled. c. Plate tectonics created a chain of folded mountains. d. One side along a strike-slip boundary was forced upward. e. As a chain of volcanic mountains along the edge of a subduction zone.

Quiz Questions 19. What is the difference between the intercrater plains and the smooth plains that are found on Mercury, in terms of time of formation? a. The intercrater plains are older than the smooth plains. b. The intercrater plains are younger than the smooth plains. c. These two types of plains formed at the same times at different locations. d. Their times of formation overlap due to the Sun's tidal influence. e. Their times of formation overlap due to the formation of the Caloris Basin.

Quiz Questions 20. What evidence do we have that Mercury has a partially molten, metallic core? a. The rate at which the orbit of Mercury's moon precesses indicates that Mercury has a high-density center. b. The recent volcanic activity seen on Mercury's surface indicates that it still has a molten interior. c. The S waves created by the impact that formed Caloris Basin did not appear on the opposite side of Mercury. And we know that S waves cannot travel through liquids. d. The peculiar tidal coupling of Mercury's spin to its orbit can only be due to a partially molten, metallic core. e. Mercury has a weak dipole magnetic field.