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Lecture 21: Venus Lecture 21: Venus 1 Venus Terrestrial Planets Animation Venus •The orbit of Venus is almost circular, with eccentricity e = 0.0068 •The average Sun-Venus distance is 0.72 AU (108,491,000 km) •Like Mercury, Venus always appears close to the Sun in the sky Venus 0.72 AU 47o 1 AU Sun Earth 2 Venus •Venus is visible for no more than about three hours •The Earth rotates 360o in 24 hours, or o o 360 = 15 24 hr hr •Since the maximum elongation of Venus is 47o, the maximum time for the Sun to rise after Venus is 47o ∆ t = ≈ 3 hours 15o / hr Venus •The albedo of an object is the fraction of the incident light that is reflected Albedo = 0.1 for Mercury Albedo = 0.1 for Moon Albedo = 0.4 for Earth Albedo = 0.7 for Venus •Venus is the third brightest object in the sky (Sun, Moon, Venus) •It is very bright because it is Close to the Sun Fairly large (about Earth size) Highly reflective (large albedo) Venus •Where in its orbit does Venus appears brightest as viewed from Earth? •There are two competing effects: Venus appears larger when closer The phase of Venus changes along its orbit •Maximum brightness occurs at elongation angle 39o 3 Venus •Since Venus is closer to the Sun than the Earth is, it’s apparent motion can be retrograde •Transits occur when Venus passes in front of the solar disk as viewed from Earth •This happens about once every 100 years (next one is in 2004) Venus (2 hour increments) Orbit of Venus •The semi-major axis of the orbit of Venus is a = 0.72 AU Venus Sun a •Kepler’s third law relates the semi-major axis to the orbital period 4 Orbit of Venus •Kepler’s third law relates the semi-major axis a to the orbital period P 2 3 ⎛ P ⎞ = ⎛ a ⎞ ⎜ ⎟ ⎜ ⎟ ⎝ years⎠ ⎝ AU ⎠ •Solving for the period P yields 3/ 2 ⎛ P ⎞ = ⎛ a ⎞ ⎜ ⎟ ⎜ ⎟ ⎝ years⎠ ⎝ AU ⎠ •Since a = 0.72 AU for Venus, we obtain P = 0.611 Earth years or P = 225 Earth days Bulk Properties of Venus •We have for the radius and mass of Venus Rvenus = 6,052 km = 0.95 Rearth 27 Mvenus = 4.9 x 10 g = 0.82 Mearth •The volume of Venus is therefore given by 4 V = π R 3 venus 3 venus 26 3 •Hence Vvenus = 9.3 x 10 cm Bulk Properties of Venus •The average density of Venus is therefore × 27 ρ = M venus = 4.9 10 g venus × 26 3 Vvenus 9.3 10 cm •We obtain ρ = −3 venus 5.2 g cm •This is similar to the average density of the Earth’s •Hence Venus probably contains lot of iron and a large, dense core 5 Surface Gravity •We can compute the surface acceleration on a planet or moon using Newton’s laws of motion and gravitation: - GMm F = mA = R 2 •Solving for the surface acceleration A yields - GM A = R 2 Surface Gravity •Using values for the Earth, Moon, Mercury, and Venus, we obtain for the surface accelerations = - GMearth = −2 Aearth 2 9.8ms R earth = − GMmoon = −2 Amoon 2 1.7 ms R moon - GM = mercury = −2 Amercury 2 3.7 ms R mercury = - GM venus = −2 Avenus 2 8.9ms R venus •The acceleration on Venus is about 91% of the acceleration on the Earth Rotation of Venus •The surface of Venus is shrouded in thick clouds (in visible light) •Based on observations of the motions of the cloud tops, the rotation period of Venus was thought to be about 24 hours •However, as in the case of Mercury, radar observations of Venus made in the 1960’s proved surprising… Mariner 10 (1974) Galileo (1990) 6 Rotation of Venus •One sidereal day on Venus is takes 243 Earth days •Unexpectedly, the rotation is retrograde (i.e., clockwise as viewed from the north celestial pole) •The time between noons is 117 Earth days = 1 solar day •The sidereal period of the orbit of Venus is 225 Earth days Rotation of Venus sidereal orbital period is 225 Earth days 7 Rotation of Venus Sidereal vs. Synodic Orbital Periods A: Inferior Conjunction t = 0 Sun •The synodic orbital period is longer than the sidereal orbital period •This is reversed in the outer planets Venus •Note that the synodic orbital period of 584 Earth days is almost exactly 5 times longer than the solar day on Venus: synodic period 584 = = 4.99 solar day 117 •This may represent a spin-orbit resonance between Venus and the Earth, but the truth is unclear •We are not sure why the spin of Venus is slow and retrograde •This may be due to a massive impact in the distant past of Venus 8 Venus •This may be due to a massive impact in the distant past of Venus •Telescopic observations of Venus reveal: Lots of CO2 in clouds Temperature of 240 K at cloud tops Venus •This led astronomers to believe that the surface temperature of Venus was probably only slightly higher than that of the Earth •Radio observations of the blackbody (Planck) spectrum radiated by the surface of Venus displayed a Wien spectrum with temperature T = 600 K •Venus has been explored by 20 different spacecraft, both American and Soviet Mariner 2, 5, 10 (radar mapping from orbit) Venera 4 – 12 (landed on surface) •The earliest Venera spacecraft were crushed by the high pressure in the atmosphere before reaching the surface! Venus •Atmospheric composition: Clouds are mostly sulfuric acid “Air” is mostly CO2 Carbon Dioxide: 96.5% Nitrogen: 3.5% Trace gases: < 0.01% Practically zero water Almost no oxygen •Why is the composition of the atmosphere of Venus so different from Earth’s? •The primary atmosphere of Venus (mostly hydrogen) was lost into space due to the high temperature and low mass of Venus Venus Express Mission 9 10 Runaway Greenhouse Effect •When CO2 and H2O were originally out-gassed from volcanoes to form the secondary atmosphere, the temperature was too high for the them to be absorbed by rocks •Precipitation (rain) never formed, therefore no oceans or lakes were produced •Instead, a heavy “blanket” of greenhouse gases surrounded the planet: Runaway Greenhouse Effect •Due to the greenhouse-gas blanket, the equilibrium temperature was very high: T = 1,500 K •The water vapor rose to very high altitudes, where it was split apart by solar UV radiation: •The hydrogen escaped into space, and the free oxygen reacted with sulfur and carbon to form CO2 and SO2 Runaway Greenhouse Effect •The planet ended up hot and dry as a result •This is an irreversible process, called the “Runaway Greenhouse Effect” •Hopefully this will never happen to the Earth, but who knows for sure? •The future for Earth probably depends on how well we can manage the production of greenhouse gases •Already, we see strong evidence for global warming caused by man’s pollution… 11 Surface of Venus •Radar observations from orbit indicate that Venus is mostly smooth •There are only 2 or 3 continent-sized features comparable in elevation to Earth’s continents •Many craters are observed •Most of the craters are volcanic in origin, but some are due to asteroid impacts Venus Topography Animation 12 Surface of Venus •Some of the craters are lava-filled due to violent impacts and volcanic activity • Over 85% of the surface is covered with volcanic rock •There is no evidence for “seafloor spreading” and no plate tectonic activity, although we don’t understand why •Volcanism “resurfaces” the planet every few million years •This process fills in craters Surface of Venus •Volcanism is probably ongoing •Radio flares suggest lightning discharges occurring near active volcanoes •Fluctuations are observed in the concentration of SO2 above clouds •However, no erupting volcanoes have ever been seen! Rotating Venus Movie (Magellan data) 13 Interior of Venus •Venus is probably differentiated, but has only one plate •The crust is silicon- rich, like the Earth’s •The core of Venus is probably molten, like the Earth’s •Venus has no magnetic field, probably due to the very slow rotation 14.
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