Planetary Geology Earth and other terrestrial worlds Chapter 9 Planetary Geology • Geology is the science that deal with Earth’s physical structure, it history and processes that act on it. • An extension of this science is Planetary geology, the extension of the study to other solid bodies in the solar system What are the terrestrial planets like inside? Most of the terrestrial planet interiors are divided into three layers: • Core Mainly consist of high density material such as iron and nickel • Mantle This is the layer that surrounds the core. It is rocky material that consist of mineral that contains silicon, oxygen and other elements • Crust This is the outer most layer. It is lowest-density rocky material such as granite and basalt ( a common form of volcanic rock) The interior of the planets are layered because the material was melted. The heavier material sank towards the interior. Gravity was the force that drag the heavy material and left the lighter material to remain at the top. This process is called differentiation All the terrestrial planets went trough the process of differentiation The Earth’s metallic core consist of two distinctive regions: A solid inner core and a molten (liquid) outer core Interior structure of the terrestrial planets Why is the Earth Differentiated? • Begins by accretion 1,500 K – 4.6 Billion years ago (age of Sun) • Differentiation - The Earth melts – 4.5 Billion years ago • Crustal Formation – 3.7 Billion years ago 2,000 – 2,500 K Melt iron 1/3 mass of Earth falls to the center in the form of molten iron 5000 K (molten Earth) What causes differentiation and geological activity • Heat of accretion As planetesimals collide and form a planet, the kinetic energy from the collision is transformed in heat which add to the thermal energy of the planet • Heat from differentiation In the process of differentiation, the dense material sink and the lighter material rises. The gravitational potential energy is transformed into thermal energy by friction • Heat from radioactive decay The material that built the planets contain radioactive isotopes of elements such as uranium, potassium and thorium. These elements decay into lighter elements releasing energetic subatomic particles that collide with the material heating it. Heating mechanisms for a terrestrial planet Cooling mechanisms for the interior of a terrestrial planet Convection. Hot material expands and rises while cooler material contract and falls Conduction. Transfer of heat from hot material to cooler material through contact Radiation. A planet will lose heat to space through thermal radiation. A planet acts like a black body and emit radiation (light) . Because of their low temperature, planets radiate primarily in IR How do we know what is inside a planet? The only planet for which we have information about the interior is the Earth. But the information does not come from direct sampling of the interior of the Earth Earth’s radius is 6,378 km. Drilling only can go not more than a few km (16 km). It is possible to sample only a small upper layer of the crust! For Earth, much of the information about the interior comes from seismic waves (waves generated during an earthquake) Seismic waves have been used to study the interior of the Moon using monitoring stations left by the Apollo astronauts on the surface Seeing inside the Earth • Drill a hole – Petroleum geologists usually drill to ~ 6 km – Deepest hole: Kola Super deep borehole (~ 12 km) – Deepest we can go ~16 km P (pressure) and S (shear) Wave analogy How do we know anything about the interior of the Earth? Seismology! • Earth’s interior structure is probed by studying how seismic waves travel through it (we can only drill so far! – 16 km). • Earthquakes generate seismic waves. •P waves (pressure) can travel through the liquid core but they are deflected by the core •S waves (shear) travel in the mantle but not through the core • Waves are reflected and refracted by different materials and travel through these materials at different speeds (higher density material →faster speed). The Earth interior Using seismic wave and computer models it is possible to model the interior Earth radius = 6,378 km • Mantle - 3000 km thick (80% of planet volume). • Crust - 15 km thick (8 km under ocean - 20-50 km under continents. • Density and temperature increase with depth. • High central density suggests the core is mostly nickel and iron. • There is a “jump” in density between the mantle and the core caused by different materials. • No jump in density between inner and outer core because material is the same and just goes from liquid to solid. • The temperature in the core is about 5,000K and the density about 12 g/cm^3 •(Reference: Density of water is 1 g/cm^3 or 1000 kg/m^3) The surface area-to-volume ratio Mathematical Insight 9.1 For a spherical object (planet), r is the radius of the sphere: Surface area = 4 π r² Volume = 4/3 π r³ Surface area-to-volume ratio = Surface area/ volume = (4 π r²) / (4/3 π r³) = 3/r The radius appears in the denominator. Larger bodies have a lower surface area- to-volume ratios. If two objects start with the same internal temperature, the larger body cool off slower than the smaller body Parameters that affect the geological history of a planet Shaping the planetary surface Process that shape a planetary surface: • Impact cratering Creation of bowl-shape impact craters by asteroids and comets striking the surface • Volcanism Molten rocks or lava coming from the plantet’s interior in an eruption • Tectonics The disruption of the planet’s surface by the internals stresses • Erosion Wearing down or building up of features by effect of wind, rain, water and ice Shaping the crust: Impacts on Earth Barringer meteor Crater Located near Winslow, Arizona The best preserved impact crater Diameter 1.2 km Age ~ 50,000 yr Estimated impactor size 30-50 m Estimated speed of impactor ~ 26,000 miles/hr Shaping the crust: Volcanoes Mt. St Helen before and after 1980 eruption Mauna Loa – 1984 Length ~ 75 miles Covers ~ half Big Island of Hawaii 33 eruptions since 1850s Shaping the crust: Cordon Caulle Volcano (Chile). June, 2011 Shaping the crust: Cordon Caulle volcano (Chile). June 2011 Shaping the Crust: Tectonics • The ocean floors are continually moving, spreading from the center, sinking at the edges, and being regenerated. • Convection currents beneath the plates move the crustal plates in different directions. • The source of heat driving the convection currents is radioactivity deep in the Earths mantle Shaping the Crust: Erosion and Deposition • Erosion (breaking down) and deposition (building up) require the presence of a fluid (gas or liquid) • Water, rain, wind cause erosion Deposition from the Mississippi River erosion created the river created Louisiana Grand Canyon wetlands. Geology of the Moon and Mercury Mercury’s Surface • Surface similar to the moon, large number of impact craters! •Old surface •No indication of plate tectonics •Craters have a flat bottom and have thinner ejecta rims than lunar craters due to higher gravity on Mercury than the moon •Craters not as dense as on the moon - filled by volcanic activity • Not dark features like the “maria” on the Moon • Caloris Basin, evidence of a large impact crater. It is the largest crater on the Mariner 10 image (Flyby in 1974- 1975) planet, about 1/2 Mercury radius Mercury, an image of half of Caloris Basin Image taken by Mariner 10 Mercury’s Surface • Cliffs are seen on the surface •This features are not seen on the moon •They appear to be about 4 billion years old •They are not the result of plate tectonics •Probably the result of the surface cooling, shrinking and splitting at this time •Some are several hundred km long and has high as 3 km high •Cliffs may have formed when tectonic forces compressed the crust Mercury A recent image (false colors) taken by the Messenger spacecraft The Messenger spacecraft is at the present in orbit around Mercury. It went into orbit in 2011 It is the first and only spacecraft to orbit this planet A high resolution image of Mercury taken by the Messenger mission A detailed view of craters in high resolution image of Mercury taken by the Messenger mission Lunar Geological History Moon surface Energy from the formation caused at least the outer few kilometers to melt (deep ocean of molten rock) This took place about 3-4 billions years ago. Molten rocks flooded the largest impact craters. Maria (singular mare) are large impact craters flooded by lava. The dark color comes from dense, iron rich basalt that rose from the lunar mantle Lunar Surface , large scale features Lack of atmosphere and water preserves surface features Maria, singular Mare (younger) – Highlands (older) – crust material mantle material (maria means • Elevated many km above maria “seas’) • Aluminum rich, low density (2,9 • Maria - darker areas resulting g/cm3). from earlier lava flow • Basaltic, relatively iron rich, high density (3,3 g/cm3). Lunar Highlands Highlands - light, rough, heavily cratered Older areas compared to maria Heavy cratered compared with maria Lunar Lowlands Lowlands – dark, smooth Maria Basalt – fine grained dark igneous rock rich in iron and magnesium (stuff that sank in magma ocean) Few hundred meters thick Younger than the high lands Less craters compared with highlands The formation of Mare Humorum Lunar maria and mountain ranges Caucasus Mountain ↓ ← Mare Serenitatis ← Apollo 11 Mare Tranquillitatis Moon Volcanism: Maria (Dark Areas) Mare Imbrium SW Mare Imbrium The volcanism took placeafter the impacts – most 3 – 1 billion years ago Rilles Aristarchus Plateau Marius Hills Mountains Montes Tenerife Montes Appeninus (Mons Huygens 5.5 km) Montes Alpes- Mons Blanc 3.6 km The Alpine valley is the long feature to the left of the image Lunar Erosion Lunar Craters - caused by meteoroid impacts • Pressure to the lunar surface heats the rock and deforms the ground.