Lecture 2: Overview of the Terrestrial PlanetsAstro 202Prof. Jim Bell ([email protected])Spring 2008Planetary Categories!It is convenient and appropriate to divide the major planets into two classes:–Inner Planets•Mercury, Venus, Earth, Mars•"Terrestrial" or Earthlike worlds–Outer Planets•Jupiter, Saturn, Uranus Neptune•"Jovian" or Giant planets!Pluto is anomalous, and defies simple classification, as do many large moons and asteroidsFundamental Differences PROPERTY Inner Planets Outer Planets Size Small Large Composition Rocky Gas/Ice Atmospheres Thin Atmos. Thick Atmos. “ Oxygen-rich Hydrogen-rich Spin Slow Fast Moons Few Many Rings No Yes Internal Heat Little/None LotsThe Terrestrial Planets Mercury Venus Earth Moon Mars D = 0.39 AU D = 0.72 D = 1.0 R = 0.25RE D = 1.5# = 5.4 g/cm3 # = 5.3 # = 5.5 # = 3.3 # = 3.9T = 120/720 K T = 740 T = 290±20 T = 120/383K T = 215g = 3.7 m/sec2 g = 8.9 g = 9.8 g = 1.6 g = 3.7Patm = 0 mbar Patm = 90,000 Patm = 1000 Patm = 0 Patm = 10Mass and Density!Density (#) = Mass/Volume!Provides clues about a planet's interior composition–Water: # = 1 g/cm3 [Lecture 3]–Rock: # ! 2.5 g/cm3 –Metal: # ! 8-10 g/cm3 !Earth: # ! 5.5 g/cm3: implies rock+metal interior!All the inner planets have high densities (3.9-5.5)!The Moon is somewhat less dense (3.3). Why?!Gas Giants are much less dense (0.7-1.6)! Object! % Total Mass! Sun! 99.80! Jupiter! 0.10! All Comets! 0.05(?)! All Other Planets! 0.04! Moons and Rings! 0.00005! All Asteroids! 0.000002! Cosmic Dust! 0.0000001Solar System Distances!The basic unit: Astronomical Unit (AU)–Defined as Earth's average distance from the Sun–1 AU ! 150,000,000 km ! 93,000,000 miles!Can also think of distances in light travel time–Light travels at 300,000 km/sec–1 AU = 150,000,000 km / (300,000 km/sec) = 500 sec–Earth is a little over 8 "light minutes" from the Sun–Pluto is more than 5.5 light HOURS from the Sun!–Nearest star (" Centauri) is ! 4.3 light YEARS away!Overhead Views of the Solar SystemEdge-On Views of the Solar System A Scale Model!Sagan Planetwalk, downtown Ithaca...Inside a Terrestrial PlanetDifferentiation has produceda distinctive onion-skin style of layering inside the planets.How do we know this??How Do Interiors Get Hot?Heat InHeat OutHow Do Interiors Cool Down?Planetary Surface ProcessesFour major geologic processes:Impact CrateringVolcanismTectonismErosion (also sometimes called “Gradation”)The combination of these processes, working over a variety of timescales, is responsible for producing what we see on planetary surfaces today with spacecraft and telescopesImpactsImpact cratering is perhaps the single most important geologic process operating in the solar system, but it was only recently recognized to be so!Impact craters are circular depressions formed when an asteroid or comet crashes into a planetary surface • Typical impactor velocities: 5-20 km/sec• Energy released depends on mass, velocity, strength of impactor and gravity, composition, and atmospheric density of the target bodyImpact Craters:Common geologic landforms on all spatial scales d. Opportunity sol 433Navcam image of a !20 cm diameter impact crater Volcanism Volcanism is the melting of materials within a planet and the transport or eruption of these materials onto a planetary surface On the terrestrial planets, the molten material, or magma, is silicate rock. For some outer planet satellites, the magma is water ice or even molten sulfur!Tectonism Tectonism is the deformation of the surface and interior !of a planet, driven by either internal (volcanic, tidal, !etc.) or external (impact) forces Tectonic processes produce faults, fractures, and folds !with morphologies (shapes) dependent upon the style of local deformationEarthErosionErosion or gradation is the wearing down of high places and the filling in of low places on a planetary surfaceThese erosional and depositional processes are driven by gravity, atmospheric pressure, temperature, and densityThe primary means of moving materials by gradation come from the actions of liquid water, ice, wind, and "mass wasting", or the downslope movement of rocks and debrisGradational processes produce landslides, talus cones, drainage channels, glacial features, dunes, wind streaks...• Erosion is driven by wind, water, gravity, & temperature• Erosion occurs on planets with no atmospheres as wellKnow Thy (Terrestrial)Neighbors...EarthThe MoonMarsVenusMercuryNot to scaleInterior Atmosphere Crust Lithosphere Plate Tectonics LifeEarth as a PlanetJames Lovelock and the “Gaia” hypothesis...Discover article posted on the course web site...Atmosphere Greenhouse Effect Surface (from Radar) Impact Craters Volcanism TectonismAtmosphere Surface Erosion (wind, water) Impact Craters Volcanism TectonismThe Mars rovers Spirit and Opportunity...(part of your Cornell legacy!)Spirit “West Valley” panorama, from atop Home Platetaken on sols 1366-1369 (Nov. 6-9, 2007)http://technology.timesonline.co.uk/tol/news/tech_and_web/specials/space/article3232035.ecehttp://www.cnn.com/video/#/video/bestoftv/2008/01/23/cooper.shot.tuesday.cnnSurface Impact Craters Volcanism Tectonism Samples! Origin?Surface Impact Craters Volcanism Tectonism Interior New Data!MESSENGER missionFlew past Mercury just last week!Ages of Planetary SurfacesRelative ages determined by impact crater density[craters/km2]Absolute ages determined from radioactive decay(More details in Lectures 7, 8, and 9)If these two images are of the same-sized regions on the same planet, which area is older?Why?What assumptions are required?Radioactive dating of rocks from the Moon provides the link between absolute ages and relative ages from crater countingSummaryThe “Terrestrial Planets” are quite diverseMercury, The Moon: Ancient, “dead” worldsEarth, Mars, and Venus: Active surface and atmospheric processesFour main processes modify