Earth and the Geology of the Terrestrial Planets (Bennett et al. Ch. 9)

Transcription

1 Earth and the Geology of the Terrestrial Planets (Bennett et al. Ch. 9)

2 Major Ideas In This Chapter Terrestrial planets looked (largely) the same when they were formed. Differences due to geological processes. Geological activity is driven by internal heat Planetary size plays a large role in retaining heat Distance from the Sun, rotation affects erosion Crater density can indicate surface age Earth has a unique geology

3 Terrestrial Planets Compared to Jovian planets: Smaller size/mass Large core to atmosphere ratio Higher density Closer to Sun and closer together Warmer Few or no moons No rings (NASA)

4 Planetary Surfaces and Interiors Terrestrial planets + Moon were similar when young Subjected to heavy bombardment Differences due to processes that occurred after formation Understanding the surface features: planetary geology Processes in the interior drive activity at the surface Your book uses terrestrial worlds to refer to the terrestrial planets + the Moon.

5 (from Bennett et al.)

6 (from Bennett et al.)

7 How Do We Learn About Planetary Interiors? Average density determinations Local gravity variations as measured with artificial satellites Magnetic fields: molten core/convection Lava flow: internal composition Earthquakes: internal structure

8 Earthquakes: Seismic Waves Earthquakes generate vibrations Typical wavelength ~ several km Reconstruct interior Two types of waves: P-waves: compressional waves S-waves: shear waves S-waves cannot pass through liquid Earth's interior has liquid layer (from Bennett et al.) Monitoring also done on the Moon (from Morrison and Owen)

9 Interior Structure of Terrestrial Planets crust Density stratification Core Mantle iron, nickel Earth has liquid outer core core Rocky layer (minerals with silicon, oxygen,...) Crust mantle granite, basalt lithosphere

10 Interior Strength crust Most of earth's interior: solid rock Rock varies in strength Can deform and flow Lithosphere Below lithosphere: higher T rock flows easier core Lithosphere floats on the soft rock below mantle Thickness important lithosphere How does lithosphere thickness affect volcanic eruptions/mountain formation?

11 Why Layering? Differentiation Gravitational separation of materials with different densities Interiors were hot initially rock/metal molten Why are planets round? Phobos and Deimos (the moons of Mars) (NASA)

12 Planetary Interiors (from Bennett et al.) We expect smaller planets to have smaller cores Mercury? Moon? Small planets = thicker lithospheres

13 What Drives Geological Activity? Heat In general: bigger = more heat How do we heat? Accretion Differentiation Radioactivity Which of these processes is still taking place in terrestrial planets? What about sunlight? These processes result in the core/mantle/crust structure

14 What Drives Geological Activity? How do we cool? Convection Conduction Radiation Example: Earth: Convection in interior (flowing solid rock) Above lithosphere, too rigid to flow conduction takes over At surface: radiation

15 What Drives Geological Activity? (from Bennett et al.)

16 Planetary Size Larger planets remain hotter longer Mercury/Moon Cooled quickly (~ 1 billion years) Lithosphere thickens, mantle convection stops Geologically dead Venus Similar in size to Earth, so probably still active Mars Cooled more unclear if the deep interior is still convecting

17 Cooling Terrestrial Planets Interiors Total store of heat is proportion to the planet's volume, Energy is only lost through the surface rate of energy loss is proportion to the surface area of the planet, Cooling time is related to the total amount of heat/energy stored / rate of energy loss (volume to surface ratio)

18 Planetary Cores and Magnetic Fields Magnetic fields are generated in some planets What is needed to generate a magnetic field? B-field Mercury Venus Earth Moon Mars yes no yes no no why? large metal core (despite slow rotation) rotation too slow molten rock cooled off no metallic core or cooled

19 Planetary Cores and Magnetic Fields (from Bennett et al.)

20 Shaping Surfaces Impact Cratering More small than large craters All terrestrial planets had impacts Impact at 40,000 to 250,000 km/h Craters are circular D ~ 10x impactor size Depth ~ 10-20% diameter Sometimes: central peak Tycho crater on the Moon ( NASA) (from Bennett et al.)

21 Impact Craters

22 Shaping Surfaces Volcanism The eruption of molten lava onto surface Magma rises: lower density / trapped gases / squeezed Result depends on how easily lava flows (from Bennett et al.)

23 Shaping Surfaces Volcanism (cont.) Volcanic plains and shield volcanoes made of basalt (high density, but runny) all terrestrial planets and some Jovian moons show volcanic plains or shield volcanoes basalt common Stratovolcanoes made of lower-density rock rare outside of Earth. Volcanoes outgas atmospheres Atmospheres of Venus, Earth, and Mars, and Earth's oceans came from outgassing

24 Solar System Volcanos The Culann Patera volcano on Jupiter's moon Io (Galileo Project, JPL, NASA) Olympus Mons on Mars the largest volcano in the solar system (Mars Global Surveyor Project, MSSS, JPL, NASA)

25 Shaping Surfaces Tectonics Surface changes due to forces acting on lithosphere Most tectonic features arise from mantle convection Compression features Cracks and valleys Fractured lithosphere plate tectonics (from Bennett et al.)

26 Shaping Surfaces Erosion Breakdown/transport of rock Glaciers Rivers Wind... Erosion can build (sand dunes, river deltas,...) Erosion makes sedimentary rock

27 Effect of Planetary Properties Volcanism/Tectonics Requires internal heat planetary size matters Which planets had volcanism/tectonics initially? Moon/Mercury already cooled Earth large still active Venus similar to Earth still active? Mars should be cooler inside, much less activity than past

28 Effect of Planetary Properties Erosion Requires weather (wind, rain,...) How does planetary size affect an atmosphere? Distance from Sun (how does this affect things?) Rotation (why?) Moon/Mercury: no atmosphere no erosion Mars: thin atmosphere little erosion Venus/Earth: thick atmospheres Earth cooler: oceans form. Still lots of erosion. Venus slow rotator: little erosion

29 (from Bennett et al.)

30 (from Bennett et al.)

31 Impact Craters and Age All planets impacted during heavy bombardment Old surface = high crater density Age ~ 4.4 billion years Maria highlands Lunar highlands maria Age ~ billion years Heavy bombardment ended ~4 billion years ago Impact history on moon applies to other planets Crater counts geological age Apollo 16 image of the moon (mostly far side) (NASA)

32 Geology of the Moon and Mercury Apollo 16 image of the moon (mostly far side) (NASA) MESSENGER image of Mercury &gallery_id=2&image_id=143 Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington Many craters Cooled long ago little recent tectonic/volcanic activity No atmospheres no erosion Ancient volcanic features active when young

33 Terrestrial Planets (from Bennett et al.) Mercury Venus Earth Moon Mars Mass M M 1.0 M M M Radius R R 1.0 R R R Density 5.43 g cm g cm g cm g cm g cm-3

34 Overview of the Moon Smallest of the terrestrial worlds Heavily cratered highlands Smooth maria: lava plains Some tectonic features No erosion Geologically dead today. Crater counts, calibrated on the Moon, allow us to determine geological age Apollo 15 image of Mare Imbrium (NASA;

35 Geology of Moon Highlands: bright, heavily cratered Maria: smooth, dark regions Craters should be roughly uniform what happened in Maria? Lava very runny lack of water/trapped gases (NASA/Apollo 17) (Luc Viatour/Wikipedia)

36 Geology of Moon (from Bennett et al.)

37 Geology of Moon Tectonic features found in maria Contraction during cooling (graben) Apollo 15 image of Mare Imbrium (NASA;

38 High iron content in the Maria Source: Clementine Project database Maria are composed of basalt. Lunar basalt has a higher iron content than on earth.

39 (NASA/LPI) Lunar far side is at higher elevation than Earth-facing side. Why?

40 Today's Moon No geological activity Major impacts infrequent No wind/weather Micrometeorite impacts break up surface rock into powder Apollo 11 footprint. (NASA; Apollo 11, AS )

41 Overview of Mercury Lots of craters (lower density than Moon) Volcanic resurfacing Small lava plains Cliffs and shrinking of planet Geologically dead MESSENGER image of Mercury Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington Scarp radial to Caloris Basin (NASA/Mariner 10)

42 Mercury Innermost planet No activity No atmosphere Hot on day side, cold on night side (100 K) Rotates 3 times for every two orbits Unusually high density Distance from Sun: 0.39 AU radius: 0.38 R mass M average density: 5.43 g cm-3 composition: rocks, metal moons: 0 Mercury as imaged by the MESSENGER spacecraft Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

43 Mercury from Earth

44 Geology of Mercury Lots of impact craters = old surface Lower crater density than some lunar regions Resurfacing by lava flows? Small lava plains Volcanically active when young MESSENGER image of Mercury &gallery_id=2&image_id=143 Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

45 Mare Orientale (Moon) as imaged by NASA's Lunar Orbiter 4 (NASA/Lunar Orbiter 4) Caloris Basin - largest impact crater on Mercury (1300 km) Few craters inside occured near end of heavy bombardment Similar feature seen on the western edge of the Moon Mare Orientale. Caloris Basin (Mercury) imaged by Mariner 10 (NASA)

46 This enhanced-color image of Mercury,... shows the great Caloris impact basin, visible in this image as a large, circular, orange feature in the center of the picture. The contrast between the colors of the Caloris basin floor and those of the surrounding plains indicate that the composition of Mercury s surface is variable. Many additional geological features with intriguing color signatures can be identified in this image. For example, the bright orange spots just inside the rim of Caloris basin are thought to mark the location of volcanic features... Credit: Image produced by NASA/Johns Hopkins University Applied Physics Laboratory/Arizona State University/Carnegie Institution of Washington. Image reproduced courtesy of Science/AAAS.

47 Mercury s Surface Scarp radial to Caloris Basin (NASA/Mariner 10) Jumbled Terrain Opposite Caloris Basin on Mercury (NASA/Mariner 10) Disturbance from Caloris Basin impact propagated throughout Mercury.

48 Mercury s Surface Mercury's surface shows many scarps or cliffs. Unique to Mercury not seen on Moon Some are 3 km in height Associated with shrinking of the crust of Mercury how? A 300 km long scarp or cliff on Mercury's surface (NASA/Mariner 10)

49 Mercury s Surface (from Bennett et al.)

50 Mercury s Volcanism Impact cratered spurred volcanism spreading basalt Smooth plains evidence of later volcanism Young craters stand out because of prominent rays This NAC image... acquired during MESSENGER's third flyby of Mercury... show large areas of Mercury's surface that appear to have been flooded by lava. In this view, craters are visible that have been nearly filled with lava, leaving only traces of their circular rims. MESSENGER images have revealed that the smooth plains in this region of Mercury's surface are quite extensive... After the Mariner 10 mission, there was some controversy concerning the extent to which volcanism had modified Mercury's surface. Now MESSENGER results, including color composite images, evidence for pyroclastic eruptions, and images of vast lava plains (such as shown here) have demonstrated that Mercury was indeed volcanically active in the past. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington php?page=1&gallery_id=2&image_id=372

51 Mercury Today Geologically active longer than Moon Volcanic/tectonic activity ended ~ 1 billion years after formation Dead today

52 Overview of Venus Thick clouds Major volcanic and tectonic activity Relative lack of craters Erosion not important Remains geologically activity (Magellan Spacecraft, Arecibo Radio Telescope, NASA)

53 Venus Earth's twin Rotates opposite direction from orbit Very slow rotation period Thick atmosphere surface hidden Extreme greenhouse effect (average temperature 740 K) Surface shows evidence of activity. Distance from Sun: 0.72 AU radius: 0.95 R mass 0.82 M average density: 5.24 g cm-3 composition: rocks, metal moons: 0 Venus in UV as imaged by the Pioneer Venus Orbiter (NASA)

54 Phases of Venus

55 Missions to Venus Soviet Venera probes explored Venus. Venera 3 6 were descent probes Venera 4: atmosphere CO2 High atmospheric pressure, Failed before reaching surface Venera 7: new design for higher P Confirmed 90 bar surface pressure and 740 K surface temperature Surface of Venus as imaged by Venera 14. The compressibility of the surface was supposed to be measured by a probe extending from the lander, but instead it measured the compressibility of the lens cap. (this copy from y-venus.html)

56 Craters on the Venusian Surface Magellan: ~ 1000 impact craters Sizes: km No really large craters / impact basins Venus surface only ~750 million yr old (heavy bombardment?) No really small craters (why?) The triple crater Stein likely produced by the incoming body breaking apart in the Venusian atmosphere (NASA/Magellan) Magellan radar image of craters Danilova, Aglaonice, and Saskia (NASA)

57 Craters on the Venusian Surface Computer generated 3-dimensional perspective view of the "crater farm" on Venus, consisting of the 37.3 km diameter Saskia in the foreground (28.6S,337.1E), 47.6 km Danilova (26.35S,337.25E) to the left, and 62.7 km Aglaonice to the right (26.5S,340E). The image was created by superimposing Magellan images in topography data, and coloring is based on Venera 13 and 14 Lander images. (Magellan press release P-39146)

58 Volcanic Features Volcanism important Lava plains and volcanic mountains Some steep volcanoes Runny lava Thicker lava Probably still active No eruptions observed Cloud composition Pancake domes stratovolcanoes (thick lava) (NASA/Magellan) Shield volcanoes (thinner lava) tm (NASA/Magelilan)

59 Volcanic Features The hot surface of Venus shows clear signs of ancient lava flows. Evidence of this was bolstered by the robot spacecraft Magellan, which orbited Venus in the early 1990s. Using imaging radar, Magellan was able to peer beneath the thick perpetual clouds that cover Earth's closest planetary neighbor. Picture above, lava apparently flowed down from the top of the image and pooled in the light colored areas visible across the image middle and bottom. The lava cut a channel across the darker ridge that runs horizontally across the image center. The picture covers about 500 kilometers across. (Magellan Project, JPL, NASA)

60 Volcanic Features NASA/JPL-Caltech/ESA... volcanoes on Venus appeared to erupt between a few hundred years to 2.5 million years ago. This suggests the planet may still be geologically active, making Venus one of the few worlds in our solar system that has been volcanically active within the last 3 million years. The evidence comes from the European Space Agency's Venus Express mission, which has been in orbit around the planet since April The science results were laid over topographic data from NASA's Magellan spacecraft. Magellan radar-mapped 98 percent of the surface and collected high-resolution gravity data while orbiting Venus from 1990 to Scientists see compositional differences compared to the surrounding landscape in three volcanic regions. Relatively young lava flows have been identified by the way they emit infrared radiation. These observations suggest Venus is still capable of volcanic eruptions.

61 The Tick Crater (Magellan/NASA) A volcano on the surface of Venus with ridges along its perimeter that give it an insect appearance

62 Tectonics Animation of the surface of Venus based on NASA Magellan data (Calvin J. Hamilton) Magellan probe radar image of Venus. Here we see through the clouds. Red: high elevations (mountains); blue: low elevations (valleys) (Magellan Spacecraft, Arecibo Radio Telescope, NASA) One hemisphere of Venus is dominated by long bands these are compression features in the crust.

63 Tectonic Features of Venus Created via tension or compression in the crust No well-defined tectonic plates. Lakshmi plains: cracks (spaced 1 2 km apart) created by stretching in one direction and compressing in the perpendicular direction. (NASA/Magellan)

64 Coronae Aine Corona (200 km diameter) (NASA/Magellan) Concentric circular/oval features 100s 1000s km across No counterpart on Earth Origin: Mantle plumes Volcanoes found near coronae.

65 Geology of Venus (from Bennett et al.)

66 Erosion on Venus Thick atmosphere, but slow rotation: little erosion Water cannot be in ice or liquid form: little erosion Surface images show little evidence of erosion Venera 13 Lander image of the surface of Venus, showing a composition similar to basalt on Earth (NASA)

67 Plate Tectonics on Venus No evidence for plate tectonics on Venus Surprising (why?) Dominant reshaping force for Earth Venus: surface uniformly repaved ~ 750 million years ago Tectonic/volcanic activity Earth's surface: different ages in different locations Perhaps plate tectonics was active before repaving? Why no plate tectonics? thicker/stronger lithosphere? High surface T water in crust/mantle backed out

68 Overview of Mars All 4 geological processes present Largest volcanoes in SS Large tectonic canyons Impact craters Erosion features Water still in polar ice caps and underground ice. Perhaps liquid water underground? Mosaic of Viking 1 images of Mars (NASA)

69 Mars Much smaller than Earth Two small moons Surface shows ancient volcanoes, large canyon Polar caps of CO2 ice and water ice Evidence that liquid water was once present Atmosphere is very thin Distance from Sun: 1.52 AU radius: 0.53 R mass 0.11 M average density: 3.93 g cm-3 composition: rocks, metal moons: 2 (captured asteroids) Mosaic of Viking 1 images of Mars (NASA)

70 Terrestrial Planets (from Bennett et al.) Mercury Venus Earth Moon Mars Mass M M 1.0 M M M Radius R R 1.0 R R R Density 5.43 g cm g cm g cm g cm g cm-3

71 Map of Martian Surface Topography from the orbiter Mars Global Surveyor. Spatial resolution is 15 km at the equator. (NASA/JPL) Great differences between the Northern and Southern hemispheres remains a mystery North South Primarily low-lands Primarily highlands Relatively low crater count Relatively high crater count Younger surface Older surface

72 Martian Craters 1000s of impact craters Fluidized ejecta craters Viking 1 image of Yuty a flower or rampart crater. The ejecta features here were likely shaped by subsurface ice melted upon impact, giving it the fluidized appearance (NASA; Viking of Arandas a 28-km pancake crater. (NASA)

73 Impact Basins Largest Hellas basin 1800 km in diameter 6 km deep Interior covered by dust (NASA)

74 Martian Volcanoes Volcanism important in shaping surface Dominates in Northern highlands Several large shield volcanoes Olympus Mons largest Base is larger than Arizona 3x higher than Mt. Everest Rimmed by cliff (as high as 6 km) Why can mountains grow so high on Mars? Olympus Mons: the tallest mountain in the solar system (Mars Global Surveyor Project, MSSS, JPL, NASA)

75 Martian Volcanoes Tharsis Bulge Olympus Mons + several other shield volcanoes ~ 4000 km across How did it form? Still active? Interior heat? Impact crater density inactive for 10s of millions of years Martian volcanoes may erupt again Lithosphere thickening, dead within billions of years (Malin Space Science Systems, MGS, JPL, NASA)

76 Tectonics and Valles Marineris Tectonic features (not plate tectonics) Largest: valley system Valles Marineris ~ 1/5th of the planet's equator 4x deeper than Grand Canyon Not caused by water or lava Likely connected to Tharsis Bulge (Image Credit: NASA/MOLA Science Team/O. de Goursac-A. Lark)

77 Water on Mars Erosion features present Some features carved by water. Rain? Underground source of water?... Is there liquid water on the surface of Mars now? Water features? Mars was wet in the past Warmer T, higher P Crater count 2-3 billion years old Martian riverbed imaged by Mars Global Surveyor Credit: MGS Project, JPL, NASA

78 Overview of Earth Plate tectonics Volcanism, tectonics, erosion all important Erosion more important than on any other planet (Apollo 17/NASA) _148_22727.html

79 Earth Only planet known to host life Temperature allows for liquid water Moon is unusually large Distance from Sun: 1.0 AU radius: 1.0 R mass 1.0 M average density: 5.52 g cm-3 composition: rocks, metal moons: 1 Earth and Moon as imaged by the Galileo spacecraft (3.9 million miles away) (NASA)

80 from Astronomy Picture of the Day The Moon transiting Earth as seen from 31 million miles away. This image was taken by the EPOXI mission (Deep Impact spacecraft). (Donald J. Lindler, Sigma Space Corporation, GSFC,Univ. Maryland, EPOCh/DIXI Science Teams)

81 Terrestrial Planets (from Bennett et al.) Mercury Venus Earth Moon Mars Mass M M 1.0 M M M Radius R R 1.0 R R R Density 5.43 g cm g cm g cm g cm g cm-3

82 Geology of Earth Earth exhibits all the processes we've discussed Largest terrestrial planet interior still warm Atmosphere + rotation rate + surface temperature = erosion Volcanic outgassing created atmosphere Surface is shaped primarily by plate tectonics Next chapter: plate tectonics important for climate too. Important for life!

83 Plate Tectonics Lithosphere fractured into plates Plates float on mantle Plates move over, under, around one another ~ few centimeters / year movement (USGS;

84 How Do We Know Earth's Surface Moves? Today: GPS Other evidence: Past continental motion Continents seem to fit together Rocks and fossils similar where Africa and S. America join Sea floor spreading Difference in sea and continental crust (from Bennett et al.) (USGS;

85 Past Continental Motion Mid-ocean ridges indicate seafloor spreading Continents move due to underlying mantle convection Mid-atlantic ridge (NOAA)

86 Seafloor vs. Continental Crust Seafloor crust thinner, denser, younger than continental crust Seafloor crust 5-10 km thick Primarily basalt (high density, runny when molten) < 200 million years old (crater counts/radiometric dating) New seafloor is constantly emerging (from Bennett et al.) Continental crust km thick Lower density rock (granite) Some parts of continental crust ~ 4 billion years old

87 Seafloor Recycling Subduction: seafloor pushed under less dense continental crust Trenches form Subducted seafloor heats up, melts, erupts upward Low density material melts first creates continental crust Stratovolcanoes form (from Bennett et al.)

88 Seafloor Recycling Mid-ocean ridges: new sea floor created by rising mantle About 2 sq. km of new seafloor produced at ridges each year Enough to replace seafloor every 200 million years. (from Bennett et al.)

89 Continent Building Continental crust not recycled built over billions of years Reshaped by volcanism, plate tectonics, erosion Read your book for more specific examples of plate tectonic features on Earth (from Bennett et al.)

90 Earthquakes What about plates sliding sideways? Sideways movement fault Stresses build, produce earthquakes Movement can be dramatic LA and SF together in 20 million years (from Bennett et al.)

91 Hot Spots Volcanic activity outside of plate boundary Mantle plume rises at hot spot (Wikipedia/Demis)

92 The Future Plates move ~ 2 cm per year We can project to the future (from Bennett et al.)

93 How Did We Get Here? Small planets = cool quickly Plate tectonics not only due to size Venus? Size, rotation, distance from Sun seems to predict outcome (from Bennett et al.)

94 Summary Terrestrial planets all looked the same when young Geological differences arouse due to Size Rotation Distance from Sun This understanding allows us to predict what new planets will look like, based on knowing a few physical properties