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Astro 101 Fall 2013 Lecture 6

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Astro 101 Fall 2013 Lecture 6 Astro 101 Fall 2013 Lecture 6 T. Howard Solar System Perspective Sun, Planets and Moon to scale (Jupiter’s faint rings not shown) Two Kinds of Planets "Terrestrial" "Jovian" Mercury, Venus, Jupiter, Saturn, Earth, Mars Uranus, Neptune Close to the Sun Far from the Sun Small Large Mostly Rocky Mostly Gaseous High Density (3.9 -5.3 g/cm3) Low Density (0.7 -1.6 g/cm3) Slow Rotation (1 - 243 days) Fast Rotation (0.41 - 0.72 days) Few Moons Many Moons No Rings Rings Main Elements Fe, Si, C, O, N Main Elements H, He initial gas and dust nebula dust grains grow by accreting gas, colliding and sticking continued growth of clumps of matter, producing planetesimals planetesimals collide and stick, enhanced by their gravity Hubble observation of disk around young star with ring structure. Unseen planet a few large planets sweeping out gap? result Terrestrial - Jovian Distinction Outer parts of disk cooler: ices form (but still much gas), also ice "mantles" on dust grains => much solid material for accretion => larger planetesimals => more gravity => even more growth. Jovian solid cores ~ 10-15 MEarth . Strong gravity => swept up and retained large gas envelopes of mostly H, He. Inner parts hotter (due to forming Sun): mostly gas. Accretion of gas atoms onto dust grains relatively inefficient. Composition of Terrestrial planets reflects that of initial dust – not representative of Solar System, or Milky Way, or Universe. The Jovian Planets Jupiter Saturn (from Cassini probe) Uranus Neptune (roughly to scale) Discoveries Jupiter and Saturn known to ancient astronomers. Uranus discovered in 1781 by William Herschel. Neptune discovered in 1845 by Johann Galle. Predicted to exist by John Adams and Urbain Leverrier because of irregularities in Uranus' orbit. Basic Properties Mass Radius Orbit semi-major Orbital Period (MEarth) (REarth) axis (years) (AU) Jupiter 318 11 5.2 11.9 (0.001 M ) Sun Saturn 95 9.5 9.5 29.4 Uranus 15 4 19.2 84 Neptune 17 3.9 30.1 164 Chemical Compositions of Outer Planet Atmospheres Element / Jupiter Saturn Uranus Neptune Molecule H2 86% 96% 83% 80% He 13.6 % 3.3% 15% 19% CH4 < 1% < 1% 2.3 % 1 – 2 % NH3 < 0.1 % < 0.1 % < 100 ppb < 1 ppm H2O ~ 600 ppm 20 ppb ~ 10 ppb few ppb ppm = parts per million (0.0001%) ; ppb = parts per billion (0.001 ppm) CH4 = methane; NH3 = ammonia Jupiter has rings: photo from Voyager 2, May 1995 Galileo spacecraft photo Jupiter's Atmosphere and Bands Whiteish "zones" and brownish "belts". Optical – colors dictated by how Infrared - traces heat in molecules reflect sunlight atmosphere, therefore depth So white colors from cooler, higher clouds, brown from warmer, lower clouds. Great Red Spot – highest. Other Jovian planets: banded structure and colors More uniform haze layer makes bands less visible. Reason: weaker gravity allows clouds to rise higher and spread out to create more uniform layer Blue/green of Uranus and blue of Neptune due to methane. Colder than Jupiter and Saturn, their ammonia has frozen and sunk lower. Methane still in gas form. It absorbs red light and reflects blue. - Zones and belts mark a convection cycle. Zones higher up than belts. -- Zones were thought to be where warm gas rises, belts where cooled gas sinks. Now less clear after Cassini, which found rising gas only in the belts! - Jupiter's rapid rotation stretches them horizontally around the entire planet. - Winds flow in opposite directions in zones vs. belts. Differences are hundreds of km/hr. Storms on Jovian Planets Jupiter's Great Red Spot: A hurricane twice the size of Earth. Has persisted for at least 340 years. Reaches highest altitudes. New storm “Oval BA” "white ovals" - may last decades "brown ovals" - only seen near 20° N latitude. Not known why. May last years or decades Neptune's Great Dark Spot: Discovered by Voyager 2 in 1989. But had disappeared by 1994 Hubble observations. About Earth-sized. Why do these storms last so long? Jupiter's Internal Structure Can't observe directly. No seismic information. Must rely on physical reasoning and connection to observable phenomena. (acts like a liquid metal, Core thought to be molten or partially molten rock, conducts electricity) 3 maybe 25 g/cm , and of mass about 10-15 MEarth . Other Jovians similar. Interior temperatures, pressures and densities less extreme. Rapid rotation causes Jupiter and Saturn to bulge: Gravity Gravity without with rotation rotation Jupiter and Saturn rotate every ~10 hours. Radius at equator several % larger due to bulge. Differential Rotation Rotation period is shorter closer to the equator: Near poles At equator Jupiter 9h 56m 9h 50m Saturn 10h 40m 10h 14m Uranus 16h 30m 14h 12m How do we know? Tracking storms at various latitudes, or using Spectroscopy and Doppler shift. Uranus' rotation axis is tilted by 98o Why? Unknown. Perhaps an early, grazing collision with another large body. Moons of Jovian Planets The Galilean Moons of Jupiter (sizes to scale) Io Europa Ganymede Callisto Closest to Jupiter Furthest from Jupiter Radii: 1570 km (Europa, slightly smaller than our Moon), to 2630 km (Ganymede - largest moon in Solar System). Orbital periods: 1.77 days (Io) to 16.7 days (Callisto). The closer to Jupiter, the higher the moon density: from 3.5 g/cm3 (Io) to 1.8 g/cm3 (Callisto). Higher density indicates higher rock/ice fraction. Io's Volcanism More than 80 have been observed. Can last months or years. Ejecta speeds up to 1000 m/s. Each volcano ejects about 10,000 tons/s Rich in S, SO2. S can be yellow, orange, red, black depending on temperature. Frozen SO2 snowflakes are white. Activity causes surface to slowly change over the years: Voyager 2 (1979) Galileo (1996) Volcanic activity requires internal heat. Io is a small body. Should be cold and geologically dead by now. What is source of heat? First, Io and Europa are in a "resonance orbit": Day 0 Jupiter Europa “pulls Io Europa Io outward” here. Jupiter Day 1.77 Io Europa Day 3.55 Jupiter The periodic pull on Io by Europa makes Io's orbit elliptical. Europa Io orbital orbital speed Io speed slower faster (exaggerated ellipse) - Io “tidally locked” like our Moon. Tidal bulge always points to Jupiter. So angle of bulge changes faster when Io is closer to Jupiter. -But Io rotates on its axis at a constant rate, so cannot keep bulge exactly -pointed at Jupiter at all times during orbit. - So bulge moves back and forth across surface => stresses => heat => volcanoes Europa may have Warm Water Ocean beneath Icy Surface Fissures suggest tidal stresses. Hardly any impact craters. 860 km Dark deposits along cracks suggest eruptions of water with dust/rock mixed in (Europa’s density => 90% 42 km rock, 10% ice). Icebergs or "ice rafts" suggest broken and reassembled chunks. Io pulls Europa inward here. Ganymede pulls Europa outward here. What is source of heat? Same as Io: resonant orbits with Ganymede and Io make Europa's orbit elliptical => varying tidal stresses from Jupiter => heat. Warm ocean => life? Further down: rocky/metallic layers Saturn's Titan: A Moon with a Thick Atmosphere Taken during Huygens’ descent From Cassini- Surface from Huygens mission Huygens probe Surface pressure is 1.6 times Earth’s, T=94 K. Atmosphere 98% Nitrogen, also methane, ethane, benzene, propane, etc. Evidence for methane rain, a few lakes of methane/ ethane, drainage channels, liquid-eroded rocks, an icy volcano (active? replenishing the methane?). Mostly dry now – rain and liquid flow may be episodic (centuries?). Origin of atmosphere: internal heat from natural radioactivity may escape surface through volcanoes. Atmosphere trapped by Titan’s cold temperature and relatively high gravity. Interior: rocky core and water mantle. Bizarre Orbits of some of Saturn's Moons Tethys Janus and Epimethius Telesto and Calypso share orbit with Tethys, Janus and Epimethius are in close orbits. and are always 60 deg. ahead and behind it! When the approach each other, they switch They stay there because of combined orbits! gravity of Saturn and Tethys. Saturn's Rings (all Jovians have ring systems) - Inner radius 60,000 km, outer radius 300,000 km. Thickness ~100 m! - Composition: icy chunks, <1 mm to >10m in diameter. Most a few cm. - A few rings and divisions distinguishable from Earth. Please read how the gaps form. Voyager probes found that rings divide into 10,000's of ringlets. Structure at this level keeps changing. Waves of matter move like ripples on a pond. Origin of Cassini Division: another resonance orbit Approximate radius of Mimas' orbit Mimas' orbital period is twice that of particles in Cassini division. Makes their orbits elliptical. They collide with other particles and end up in new circular orbits at other radii. Cassini division nearly swept clean. Other gaps have similar origins. Origin of Saturn's Rings: Unclear. Total mass of ring pieces equivalent to 250 km moon. Perhaps leftover debris from moon building? A shattering collision? A captured object? Regardless, a large moon could not survive so close to Saturn: If a large moon, held together by gravity, gets too close to Saturn, tidal force breaks it into pieces, at a radius called the Roche Limit. Rings inside Roche Limit => pieces can’t reassemble into moon. Not clear whether rings are as old as Saturn or much younger (about 50 million years). Rings of other Jovian Planets The rings of Uranus. Discovered by "stellar occultation". Jupiter, Uranus, Neptune rings much thinner, much less material. Formed by breakup of smaller bodies? Also maybe "sandblasting" of material off moon surfaces by impacts. Given rings have short lifetime and all Jovian planets have them, their formation must be common.
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