Earth Physical Properties of Earth Equatorial Diameter Mass 12,756 km 5.976 10 24 kg Average Density 5.497 g/cm 2 Surface Gravity 9.8 m/s 2 Escape Velocity Surface Temperature 11.2 km/s -50 o C to 50 o C Average Albedo 0.39 1
Earth From Space (Western Hemisphere) Seismology (Pressure) (Shear) S-waves move more slowly than P-waves, but they are more powerful. The difference betweeen the arrival times of the S- and P-waves can be used to determine the location of the earthquake epicenter. 2
Why are seismic waves strongly reflected at distances between 10 and 60 km below the solid surface of the earth? Whenever a wave hits a boundary between two regions of very different density and/or composition, some of the wave is reflected. We conclude from this that earth has a solid crust that varies in thickness from 10 km (under oceans) to 45 km (under continents). Below this crust is a region where the material is neither liquid nor solid. The high pressure and temperature result in a state in which the material is solid over short time intervals but also flows slowly ( plastic ). This region is called the mantle. The map shows lines of constant crust thickness. Within a region centered on a point 180 o away from the epicienter of an earthquake, there is a zone in which S-waves are very weak compared to P-waves. What does this tell us about the internal structure of the earth? S-waves are transverse waves; i.e., their oscillations are perpendicular to their direction of motion. Since a fluid offers little resistance to a transverse motion, an S- wave cannot travel through a liquid. This implies that Earth s outer core is liquid. Temperature and pressure both increase steadily from about 300 K at the surface to about 6000 K at the center. If the outer core is liquid and the inner core is hotter than the outer core, why do we believe that the inner core is solid? The melting point of a substance depends on both pressure and temperature. Knowing the approximate composition of the core, we can calculate the melting point as a function of depth. We find that at the pressure of the inner core, the melting point of the nickel-iron core is higher than its temperature. 3
Internal Structure Earth s surface is shielded from the solar wind by its magnetic field. When charged particles encounter a magnetic field, they experience a force perpendicular to the magnetic field. The Magnetosphere The region where the solar wind (moving at about 400 km/s) is stopped by earth s magnetic field is called the bow shock. The region surrounding earth that is shielded from the solar wind is called the magnetosphere. Why does earth have a relatively strong magnetic field (dynamo effect)? Magnetic fields are a result of electrical currents. Convection and rotation of earth s metallic liquid core results in an electrical current. 4
Barringer, Arizona (49,000 years) How large was the meteorite that produced this crater? 1.19 km Chicxulub, Yucatan Peninsula, Mexico (65.0 million years) Why is it believed that this impact was a significant factor in the extinction of the dinosaurs and many other species? 5
Wolfe Creek, Australia (~ 300,000 years) 0.88 km Tunguska Event What evidence indicates that this was caused by the explosion of a stony asteroid with a diameter of about 30 m? How much energy was released by this impact? Meteorite Impact Calculator 6
Car Hit by Meteorite (Peekskill, New York, 1992) 3 lb meteorite People Hit by Meteorites On Tuesday, September 23,. 2003 at ~4:00 pm, a large meteorite hit a house in downtown New Orleans. It entered through the roof and shattered into pieces after hitting the ground. The mass of the fragments recovered was about 20 kg. This 14-year old girl was hit in the foot by a meteorite in August, 2002. 7
Meteorite Impact Damage Size (meters) Yield (megatons) Interval (years) < 50 < 10 < 1 75 10-100 1000 160 100-1000 5000 350 10 3-10 4 15,000 700 10 4-10 5 63,000 Consequences meteors in upper atmosphere most don't reach surface irons make craters like Meteor Crater; stones produce airbursts like Tunguska; land impacts destroy area the size of a city irons,stones hit ground; comets produce airbursts; land impacts destroy area size of large urban area (New York, Tokyo) land impacts destroy area size of small state; ocean impact produces mild tsunamis land impacts destroy area size of moderate state (Virginia); ocean impact makes big tsunamis 1700 10 5-10 6 land impact raises dust with global implications; 250,000 destroys area size of large state (California, France) http://seds.lpl.arizona.edu/nineplanets/nineplanets/meteorites.html Lunar Surface Lowlands Maria, lightly-cratered and relatively new. Highlands heavily cratered and old. 8
Clementine Discovery of Lunar Ice? Moonquakes Apollo 12, 14, 15, and 16. Gravity Mapping Lunar orbiters Crust thickness average is 70 km, thicker under highlands Core radius ~ between 300 and 450 km. Lunar Interior 9
Tycho Why are most craters round? Why does a crater usually have a mountain peak at its center. Copernicus Why are there light-colored rays extending outward from the crater? 10
Mare Imbrium This arch-shaped structure is called Sinus Iridum (The Bay of Rainbows). What is it? Mercury Old, heavily-cratered surface. Intercrater plains. 2439 km Smooth plains. Radar images of the polar regions of Mercury show some highly reflective material inside some craters that are never exposed to direct sunlight. Some suspect that this might be ice. 11
Internal Structure of Mercury Core (R ~ 0.75 R Mercury) ) Mantle (Thickness ~ 0.25 R) Crust (Thickness ~ 100 km) Density at surface ~ 3.5 g/cm 3. Average density ~ 5.4 g/cm 3. large metallic core. Weak magnetic field part of core is liquid Fe/Ni? Why does Mercury have such a large metallic core? False Color UV Image of Venus 12
False Color Radar Image of Venus Pancake Domes Pancake domes are the result of volcanic eruptions of viscous lava. 250 km 13
Aine Corona with Pancake Domes A corona is a large bulge in Venus surface due to the pressure of magma below the crust. 300 km Arachnoids An arachnoid is a network of lava-filled cracks in Venus crust. The cracks are caused by magma pressure, and their name is due to their resemblance to spider webs. 14
Asymmetrical Impact Crater Why isn t it symmetrical? Mars Valles Marineris 15
Valles Marineris Compared to Grand Canyon Olympus Mons The Largest Shield Volcano in the Solar System What is it about this volcano that demonstrates that Mars has a thicker crust than Earth? Why does it have a thick crust while Earth s crust is relatively thin? 16
Olympus Mons Compared with Mount Everest Why is Olympus Mons so much larger than Mauna Kea? Erosion 17
Outflow Channel Tiu Vallis - Evidence for Water on Mars Source of water seems to be the chaotic terrain shown in the closeup view below. Erosion in Outflow Channel Ares Vallis 18
Mangala Vallis Source Source appears to be a deep crack in the crust. Long, Winding Valley Nirgal Vallis 19