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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.
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.
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
The Moon and Mercury: Airless Worlds Chapter 21
Mare Highlands
2. Impacts rework surface (late heavy bombardment) 1. Original crust forms 4. 3.2 billion years: volcanism stops; cratering only since then 3. Lunar mare form (volcanism)
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 Rate of impacts decreased rapidly after the solar system formed. Most craters seen on the moon’s (and Mercury’s) surface were formed within the first ~ 1/2 billion years.
Apollo Landing Sites First Apollo missions landed on safe, smooth terrain. Later missions explored highlands. Apollo 17: Taurus-Littrow; lunar highlands Apollo 11: Mare Tranquilitatis; lunar lowlands
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 www.badastronomy.com
Moon Rocks: Igneous No sedimentary rocks => No sign of water ever present on the moon. Vesicular= bubbles from gases in lava, basalts (mare) Older rocks are pitted with small micrometeorite craters (highlands) Breccias (= “broken”) highlands
Mercury Very similar to Earth’s moon in several ways: • Small; no atmosphere • lowlands flooded by ancient lava flows • heavily cratered surfaces • Visited by • Mariner 10 (1974-75) • Messenger (NOW!) View from Earth
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 Mercury: easier to count craters because they don’t overlap as much as the moon’s Moon: lots of overlapping craters
day : 700 K night : 100 K Mercury, Mariner 10 spacecraft similar to Moon but no mare
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
(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 partly filled crater vents • 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).
The Big Picture Alaska Hawaii Venus Mars Mercury • 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
For more information… Prepared for the Division for Planetary Sciences of the American Astronomical Society by David Brain and Nick Schneider dpsdisc@aas.org - http://dps.aas.org/education/dpsdisc/ - Released 24 April 2009 Press Releases • space.com - 7/3/08 - “Volcanoes on Mercury Solve 30-year Mystery” http://www.space.com/scienceastronomy/080703-mercury-messenger.html Images • Global view of Caloris basin and Mercury shield volcano courtesy of Science / AAAS http://messenger.jhuapl.edu/gallery/sciencePhotos/pics/caloris_color_MB.jpg http://messenger.jhuapl.edu/gallery/sciencePhotos/pics/Head_Fig1.jpg • Aerial view of Hawaii courtesy of NASA/JSC STS61A http://tinyurl.com/maunaloashieldvolcano • Aerial view of erupting Mauna Loa in Hawaii courtesy of HVO/USGS http://hvo.wr.usgs.gov/ • Image of Alaska’s Redoubt Volcano courtesy of AVO/USGS, taken by Heather Bleick http://www.avo.alaska.edu/image.php?id=17872 • Image of Olympus Mons on Mars and Maat Mon on Venus courtesy of NASA/JPL http://pds.jpl.nasa.gov/planets/captions/mars/olympus.htm http://photojournal.jpl.nasa.gov/catalog/PIA00106 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: 10.1126/science.1159256, 2008. http://www.sciencemag.org/cgi/content/abstract/321/5885/69
Fig. 10.6 Caloris basin 1300 diameter!OUCH!
Caloris Basin - 1300 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
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: Why is it so dense? What is that stuff at its poles? What can it tell us about planetary evolution?
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
Discussion Questions 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.
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.
