Transcript Lecture17
Finish: survey of terrestrials Titan, Mars, Venus, Earth Review: Io and moons of Jupiter Almathea - Red colour from Io? Europa - Young icy surface Io - Red colour is solid sulfur - White patches SO2. - Surface T~120 K, but volcanoes much hotter Callisto • Heavily cratered Ganymede Titan Surface hidden by thick atmosphere • mostly nitrogen, and some hydrocarbons Titan • First direct images of surface, from Huygens probe in Feb 2005 • Stones in foreground probably water ice Volcanic dome? Mars Google Mars Google Mars Mars: crust thickness • variations in Mars’ gravity field measured by Global Surveyor reveal internal density fluctuations • crust thickness varies from ~20km in north to ~50km in south Mars: Tharsis bulge • Existence of a core consistent with extensive volcanism • Interior is probably still hot and volcanoes are active today • image from Mars Global Surveyor shows two volcanoes in Tharsis region • these “relatively small” Martian volcanoes are similar in size to the big island of Hawaii (from the sea floor) Mars: Valles Marineris • steep cliffs along the edge of Valles Marineris • chasm floor shows evidence of lava flow, water, and wind Mars • Mars Express image above Olympus Mons • caldera is ~100km across; pits 3km deep • compare with Hawaiian calderas at ~18km across; • height ~3x Mount Everest Mars • Mars Express image shows large impact crater Solis Planum • faultlines on crater floor and mountain nearby suggest plate tectonics in the past? Mars: Tectonics • measurements of magnetic field show bands of alternating polarity running east-west • similar to data on Earth’s ocean floor, suggestive of plate tectonics Water on Mars? Mars colour-coded by elevation. Blue areas are low-lying areas and may once have held a vast ocean. Computer-generated perspective of how a Martian valley may form a natural passage between ancient lakes. Water on Mars? • false colour map of energetic neutrons from Mars’ surface • hydrogen near surface will absorb cosmic rays, so may indicate presence of water near surface • results suggest significant amounts of near surface water at all latitudes Water on Mars? • photograph by Mars Express dust covered water ice plates? near to features which could be flow sources low crater surface density indicates surface age here is ~5Myr Mars: surface rocks • red arrows: smooth rocks – sedimentary? • blue arrows: ragged, sharp-edged – volcanic ejecta? • white arrows: uncertain origin – composite history? Mars surface • Erebus Crater • surface is covered not only by dark sand but also light outcrops of rock. • Scattered across the exposed rock are numerous small round pebbles known as blueberries . These unexpected and unusual rocks likely formed by accretion in an ancient wet environment. Water on Mars • example of rock smoothed by water over time or created by water (sedimentary?) • indications that Mars’ surface was wetter and warmer in the past • Flow patterns past crater rims highly suggestive of a liquid past Venus • Little is known about the surface of Venus, at is is shrouded in thick clouds. Venus surface: radar • Red: highest elevation • blue: lowest • Magellan used radar imaging to produce these surface views • surface mostly very low relief Venus • Magellan “image” shows domes probably volcanic in origin, though the precise mechanism is unclear Barren Surface Imaged by Venera: • barren and rocky surface • but visible below Venus’ atmosphere some light reaches the surface through the dense atmosphere Venus: surface features (radar) Volcanic caldera ~300 km Venus: surface features (radar) Rare crater ~100 km Venus surface features (radar) ~200 km • Shield volcano: Gula Mons • Bright central caldera, with prominent lava flow Venus: surface features (radar) • Arachnoids: show many surface cracks 100s of km long Earth • • • • Only planet with liquid surface water Plenty of tectonic and volcanic activity Craters Strongly affected by atmospheric and water erosion. Terrestrial atmospheres General considerations Overview • Most of the planets, and three large moons (Io, Titan and Triton), have atmospheres Mars • • • • Very thin Mostly CO2 Some N2, Ar Winds, dust storms Venus • • • • Very thick Mostly CO2 Some N2 Sulfuric acid clouds Earth • • • • Mostly N2, O2 Some H20, Ar Only 0.03% CO2. Water clouds Secondary atmospheres • Can calculate how many volatiles had to be added to the atmosphere to get present surface conditions Not just current atmosphere content, but also the oceans and CO2 locked up in rocks and shells. • Percent (by weight) added to atmosphere by volcanic outgassing Gas Deep eruptions Continental geysers Required amount H2O 57.8 99.4 92.8 CO2 23.5 0.33 5.1 Cl2 0.1 0.12 1.7 N2 5.7 0.05 0.24 S2 12.6 0.03 0.13 Others <1 <1 <1 Atmospheric compositions • Comparison of total volatile content on Venus, Earth and Mars shows better agreement. Thus difference in atmospheres is due to differences in secondary atmosphere production Table 11.2: mass fraction of volatiles (x109). Volatile Venus Earth Mars H2O Atmosphere 60 3 0.02 Oceans/caps 0 250,000 5000? Crust 160,000? 30,000 10,000? Total 160,000? 280,000 15,000? Atmosphere 100,000 0.4 50 Oceans/caps 0 0 10 Crust 0 100,000 >900? Total 100,000 100,000 >1000? 2,000 2,000 300 4 11 0.5 CO2 N2 Atmosphere 40Ar Atmosphere Physical Structure Use the equation of hydrostatic equilibrium to determine how the pressure and density change with altitude, in an isothermal atmosphere. You may neglect the change in gravitational force with altitude. dP GM 2 g dr r Physical Structure • Pressure decreases with increasing altitude • Atmospheres are compressible, so density decreases with altitude • Compare with the pressure structure of the oceans, where the density remains approximately constant. Physical Structure • Surface temperatures and pressures are very different for the three terrestrial planets But the pressure scale heights are similar Venus Earth Mars Tequil (K) 238 263 222 Tsurf (K) 733 288 215 Psurf (bar) 92 1.013 0.0056 surf (kg/m3) 65 1.2 0.017 H(km) 8.5 18 16 Next Lecture Atmospheres, continued.