Published on January 9, 2008
Can You Estimate & Visualize?: Can You Estimate & Visualize? We know how large the Moon appears in the sky from Earth. How many times larger would the Earth appear to us if we could stand on the Moon? Diameter of Earth = ~13,000 km Diameter of Moon = ~3,500 km About 4x larger: 13000/3500 How many degrees across? Earth’s Magnetic Field Shields Us from the Sun’s charged particles: Earth’s Magnetic Field Shields Us from the Sun’s charged particles Van Allen Radiation Belts Views of Northern Lights From Above and Below.: Views of Northern Lights From Above and Below. What’s “Missing” About the Moon?: What’s “Missing” About the Moon? No magnetic field What’s missing from the recipe? Rapid rotation, liquid core No tectonics Why has Moon cooled off completely? Internal heating depends on volume (radioactivity) Cooling occurs through surface (area) No atmosphere Why? Weak Gravity and low escape velocity No erosion Could a Planet or Moon Lose an Atmosphere?consider gravity & temperature: Could a Planet or Moon Lose an Atmosphere? consider gravity & temperature Conclusions: 1. Heavier atoms move more slowly and are less likely to escape than lighter ones like H and He. 2. Lower temperatures mean all gas atoms are moving more slowly. 3. Higher gravities help retain atmospheres. “Escape Velocity” V2esc = 2GM R If gas atoms in a planet’s atmosphere have enough “kinetic energy” (mv2/2), they can move fast enough to ‘escape’ the planet’s gravity forever. Planet Mercury is Hard To Observe: Planet Mercury is Hard To Observe Mercury’s small orbit causes it always to appear close to the Sun. At best you can see Mercury just after sunset or just before sunrise toward the horizon through a lot of blurring atmosphere. Early astronomers thought Mercury would be in synchronous rotation like our Moon. Thus, the Sun-facing side would always be hot! Early maps showed imagination only! Synchronous Rotation?: Synchronous Rotation? 1962 Radio astronomers found that the backside wasn't cold. 1965 Dyce & Pettengil bounced radar off Mercury and learned its rotation period = 58.7 days Mercury’s year is about 88 days Doppler shifted light A Solar Day on MercuryGiuseppi Columbo (1920-1984): A Solar Day on Mercury Giuseppi Columbo (1920-1984) Mercury’s solar day is 176 days long Sun USUALLY rises in east & sets in west: UNLESS Mercury is near perihelion when it swings by Sun more quickly & Sun seems to reverse directions This “resonance” is caused by tides, just as we discussed for the Moon. Mercury rotates exactly 3 times during two orbits around Sun. RATIO = 87.97/58.65 = 3:2 [“3:2 spin-orbit resonance”] How long is a “day” on Mercury? Mercury rotates on its axis exactly 1.5 times for each revolution around the Sun. One revolution is about 88 days. Mercury: A Planet of Extremesthe least explored “terrestrial planet”: Mercury: A Planet of Extremes the least explored “terrestrial planet” Highest density of any planet. Its ancient surface records processes from the earliest part of planetary formation. Its exotic atmosphere is the thinnest among those of all the terrestrial planets. Mystery: It is the only terrestrial planet besides Earth to possess a global magnetic field. 0.38 AU from Sun; 2.5x closer than Earth Sunlight is (2.5)2x = 6.25x more intense Global Propertiesonly 45% of Mercury’s surface has been seen: Global Properties only 45% of Mercury’s surface has been seen Largest temperature range (~1100 F) in solar system 800 F (700 K) at noon at perihelion (lead would melt!) -280 F (100 K) at midnight albedo = 0.12 mass = 0.055 of Earth’s density = 5.43 gm/cm3 [most iron-rich object in solar system] surface gravity = 0.38 Earth’s [twice the Moon’s] axial tilt = 0 degrees ORBIT: 0.39 AU = semimajor axis 0.207 = eccentricity 7 deg = inclination 88 day = period Mercury’s southern hemisphere Mariner Venus-Mercury Flyby Mission1974-1975: Mariner Venus-Mercury Flyby Mission 1974-1975 The first-ever “gravity-assist” trajectory used Venus’s gravity & orbital motion to help Mariner 10 fall towards Mercury. Such a maneuver avoids the need for large braking rockets. Mariner 10 went into orbit around the Sun and flew by Mercury three times before exhausting its positioning fuel. Remote controlled instruments Mapped 45% of Mercury’s surface Moon or Mercury?Why would craters differ between Mercury & Moon?: Moon or Mercury? Why would craters differ between Mercury & Moon? A large crater is shown at higher resolution. It is ~100 km diameter and shows radial ejecta, secondary craters, central peaks, etc. ~100 km diameter. Consequences of a Gravity Difference Between the Mercury & Moon: Consequences of a Gravity Difference Between the Mercury & Moon Mercury’s Surface Features: Mercury’s Surface Features Transition from heavily cratered area to smooth lava plains. About 490 km across. “Discovery scarp” 350 km long. Intersects two craters with diameters 35 & 55 km. Caloris Basin: 1300 km diameter. Mountain ridges ~2 km high. Mercury’s Atmosphere10-15 of Earth’s pressure: an “exosphere”: Mercury’s Atmosphere 10-15 of Earth’s pressure: an “exosphere” So thin that molecules are more likely to bounce around the surface than to hit each other. Hydrogen & helium come from solar wind. Hydrogen & oxygen may come from comets that evaporated nearby as they approached the Sun. Potassium, sodium, & oxygen may come from surface rocks that have been vaporized by impacts. Escape Velocity Earth = 11.2 km/sec Mercury = 4.3 Mercury’s Magnetic Fieldmore like Earth’s than Venus, Moon, Mars: Mercury’s Magnetic Field more like Earth’s than Venus, Moon, Mars Only other terrestrial planet besides Earth with an overall magnetic field. Strength ~ 1% of Earth’s Could Mercury still have a molten core? a simulated magnetic dipole Water Ice at Mercury’s Poles?“Messenger” mission en route to confirm: Water Ice at Mercury’s Poles? “Messenger” mission en route to confirm Since 1992, radio astronomers have bounced “radar” beams off Mercury and detected possible water ice at its poles and in numerous areas with circular shapes. Quiz #1: Quiz #1 Which of the following two telescope+instrument configurations would give astronomers the longest lookback time? Explain by comparing the two options numerically. a 1-meter telescope with an electronic camera exposing for 10 hrs OR a 10-meter telescope with an electronic camera exposing for 10 min VenusEarth’s “sister planet”: Venus Earth’s “sister planet” 0.72 AU from Sun ~2x farther from Sun than Mercury so ~4x less sunlight ~1.4x closer than Earth so sunlight is (1.4)2 times =~2x more intense Venus approaches closer to Earth than any other planet so we should know a lot about it. However, its surface is invisible underneath a thick atmosphere. “true” color image Venusglobal properties: Venus global properties albedo = 0.65 (Earth is 0.37) mass = 0.815 of Earth’s diameter = 0.95 of Earth's density = 5.3 gm/cm3 surface gravity = 0.91 Earth’s axial tilt = 177.3 deg. ORBIT: 0.72 AU = semi. axis 0.0068 = eccentricity 3.4 deg = inclination 224.7 day = period “false color” image in UV Would you expect Venus to have seasons and a magnetic field? Discovery of “Greenhouse Effect”Venus is the hottest planet and has the densest atmosphere: Discovery of “Greenhouse Effect” Venus is the hottest planet and has the densest atmosphere 1950's: radio wave emission surface temperature: 750 K = 900 F Venus’ thick CO2 atmosphere keeps the surface hot. Surface pressure is 100x Earth’s How many pounds per square-inch? [same as ~3000 feet under water] Sulfuric acid clouds “runaway greenhouse effect” Slide22: 1962: Carpenter & Goldstein (US) used radar "echoes“ to study Venus’ surface "double" inverse square problem RADAR: “Radio Detection and Ranging” Arecibo Observatory in Puerto Rico Mapping the Surface via RadarVenus is the most thoroughly mapped of any planet: Mapping the Surface via Radar Venus is the most thoroughly mapped of any planet movie 98% of the surface has been mapped to 100 meters resolution Over 80% of Venus’ surface lies within 1 km of its mean radius Surface “Flights” computer generated images from radar data: Surface “Flights” computer generated images from radar data Alpha Regio movie Vertical scale is exaggerated for visibility Impact Craters: Impact Craters About 1000 craters are located randomly around Venus. Venus’s atmosphere is a shield very few craters <35 km diameter Craters show little modification by tectonism or volcanism. Few craters suggests the surface is relatively young. ~30 km diameter Largest crater is Mead ~280 km diameter with a double-ring. Effects of a Hot Surface: Effects of a Hot Surface Impact crater Addams, 90 km diameter. The surface is so hot that impacts more easily melt the surface than on other planets. Outflow of lava extends 600 km away. Volcanoes & Lava Flows over 100,00 small shield volcanoes: Volcanoes & Lava Flows over 100,00 small shield volcanoes Gula Mons movie 3 km tall volcano Maat Mons 8 km tall volcano with lava flows extending hundreds of km 1962-1994: Spacecraft to Venus: 1962-1994: Spacecraft to Venus Unmanned Spacecraft US: Mariner 2, 5, 10 flybys Pioneer Venus orbiter & probes (1st radar from space) Magellan (high resolution radar) Galileo (flybys enroute to Jupiter) Soviet: Venera 4-16 (#7,8; 9,10; 13,14 landed) VEGA 1, 2 (landers & balloons) Surface map from 10 years of radar data Lava Channels1-2 km wide and at least 100’s of km long: Lava Channels 1-2 km wide and at least 100’s of km long 12 km diameter crater Baltis Vallis longest channel in solar system 6800 km long and 1.8 km wide Lo Chen Valles How Does Venus Release Its Heat?volcanic plumes instead of plate tectonics?: How Does Venus Release Its Heat? volcanic plumes instead of plate tectonics? Arachnoids Circular features with concentric rings & fractures with diameters 50-230 km Coronae Circular troughs around volcanic plateaus Could the troughs be subduction zones? Artemis Corona 2000 km diameter 6 km from rim to trough 528 km across The Surface as Imaged by Venera 14 Composition: volcanic basalt: The Surface as Imaged by Venera 14 Composition: volcanic basalt These surface probes typically survived about one hour Atmosphere96.5% CO2, 3.5% N2 -- no O2 or H2Oalso acids: hydrochloric, hydrofluoric & sulfuric: Atmosphere 96.5% CO2, 3.5% N2 -- no O2 or H2O also acids: hydrochloric, hydrofluoric & sulfuric Dark areas are locations of the sulfuric acid clouds. Bright areas are gaps between clouds. Clouds near equator are fluffy but those at higher latitudes are stretched by high winds. H2SO4 (sulfuric acid) clouds may come from SO2 outgassing from active volcanoes Nighttime side of Venus in false-color image from a near-infrared camera (2.3mm) on the Galileo spacecraft No Magnetic Field: No Magnetic Field 25,000 x weaker than Earth’s Too slow a rotator to generate an internal dynamo? Perhaps Venus’ lack of crustal plates inhibits internal convection currents of molten rocks.