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The Jovian Planets Saturn (from Cassini probe) Jupiter Uranus Earth (roughly to scale) Neptune 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 (MEarth) Radius Orbit semi-major Orbital Period (REarth) axis (years) (AU) Jupiter 318 11 5.2 11.9 Saturn 95 9.5 9.5 29.4 Uranus 15 4 19.2 84 Neptune 17 3.9 30.1 164 (0.001 MSun) Jupiter's Atmosphere and Bands Whiteish "zones" and brownish "belts". Optical – colors dictated by how molecules reflect sunlight Infrared - traces heat in 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 Jupiter’s Composition: mostly H, some He, traces of other elements (true for all Jovians). Gravity strong enough to retain even light elements. Mostly molecular. We only see the upper regions of the atmosphere. Spectroscopy of reflected sunlight reveals which molecules present. We find Hydrogen, Helium, Methane, Ammonia, some water, a few others. All of these molecules should produce white clouds. The molecules responsible for the colors we see in the bands and spots are not known. Internal Structure Can't observe directly. No seismic information. Must rely on physical reasoning and connection to observable phenomena. Core thought to be molten or partially molten rock, maybe 25 g/cm3, and of mass about 10-15 MEarth . 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. Moons of Jovian Planets The Galilean Moons of Jupiter (sizes to scale) Io Closest to Jupiter Europa Ganymede Callisto 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": Jupiter Day 0 Europa Europa “pulls Io outward” here. Io Jupiter Day 1.77 Europa Io Jupiter Day 3.55 Europa The periodic pull on Io by Europa makes Io's orbit elliptical. Io orbital speed slower Io orbital speed 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 42 km Icebergs or "ice rafts" suggest broken and reassembled chunks. Dark deposits along cracks suggest eruptions of water with dust/rock mixed in (Europa’s density => 90% rock, 10% ice). 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 CassiniHuygens mission Surface from 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. 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. Neptune's moon Triton is spiraling in to the planet and should produce spectacular ring system in 100 million years. The Jovian Planets Saturn (from Cassini probe) Jupiter Uranus Neptune (roughly to scale) Pluto Predicted to exist by remaining irregularities in Uranus' orbit. Discovered in 1930 by Clyde Tombaugh (1905-1997). Irregularities later found to be incorrect! Model created from Hubble images. This is the most detail we have. Two more moons found in 2005 with the Hubble. Discovery image of Pluto's moon Charon (1978) Basic Properties of Pluto Mass 0.0025 MEarth or 0.2 x mass of Moon Radius 1150 km or 0.2 REarth Density 2.0 g/cm3 (between Terrestrial and Jovian densities. More like a Jovian moon) Icy/rocky composition Moons: Charon: radius about 590 km or 0.1 REarth . Pluto and Charon tidally locked. Nix and Hydra about 30-100 km. Origin of Pluto Now known to be just the largest known of a class of objects in the outer reaches of the Solar System. These objects are Kuiper Belt Objects. The Kuiper Belt Objects Over 1000 found since 1992. Probably 10,000's bigger than 100 km exist. Icy/rocky. Orbits tend to be more tilted, like Pluto's. Leftover planetesimals from Solar System formation? Oort Cloud Oort Cloud is a postulated huge, roughly spherical reservoir of comets surrounding the Solar System. ~108 objects? Ejected planetesimals. A passing star may dislodge Oort cloud objects, plunging them into Solar System, where they become long-period comets. If a Kuiper Belt object's orbit takes it close to, e.g., Neptune, its orbit may be changed and it may plunge towards the inner Solar System and become a short-period comet. The New “Dwarf Planet” Eris Radius 1200 ± 50 km or at least as big as Pluto. Icy/rocky composition, like Pluto. It too has a moon, Dysnomia (Keck telescope) Asteroids Rocky fragments ranging from 940 km across (Ceres) to < 0.1 km. 100,000 known. Most in Asteroid Belt, at about 2-3 AU, between Mars and Jupiter. The Trojan asteroids orbit 60 o ahead of and behind Jupiter. Some asteroids cross Earth's orbit. Their orbits were probably disrupted by Jupiter's gravity. Total mass of Asteroid Belt only 0.0008 MEarth or 0.07 Mmoon. So it is not debris of a planet. Probably a planet was trying to form there, but almost all of the planetesimals were ejected from Solar System due to encounters with Jupiter. Giant planets may be effective vacuum cleaners for Solar Systems. Gaspra Ida and Dactyl The Sun The Sun is a star: a shining ball of gas powered by nuclear fusion. Mass of Sun = 2 x 1033 g = 330,000 MEarth = 1 MSun Radius of Sun = 7 x 105 km = 109 REarth = 1 RSun Luminosity of Sun = 4 x 1033 erg/s = 1 LSun (amount of energy put out each second in form of radiation, = 1025 40W light bulbs) The Sun in X-rays over several years Temperature at surface = 5800 K => yellow (Wien’s Law) Temperature at center = 15,000,000 K Average density = 1.4 g/cm3 Density at center = 160 g/cm3 Composition: 74% of mass is H 25% He 1% the rest Rotation period = 27 days at equator 31 days at poles The Interior Structure of the Sun (not to scale) Let's focus on the core, where the Sun's energy is Review of Atoms and Nuclei Hydrogen atom: electron Helium atom: _ _ + proton + + _ The proton is the nucleus The nucleus is 2 protons + 2 neutrons What binds the nuclear particles? The “strong” nuclear force. Number of protons uniquely identifies element. Isotopes differ in number of neutrons. Helium example: 4He: 2p + 2n. 3He: 2p + 1n Review of Ionization Radiative ionization of H _ + Energetic UV Photon "Collisional Ionization" of H _ _ + + Core of Sun is hot: gas is completely ionized by energetic collisions What Powers the Sun Nuclear Fusion: An event where nuclei of two atoms join together. Need high temperatures. Energy is produced. Elements can be made. nuc. 1 + nuc. 2 → nuc. 3 + energy (radiation) Mass of nuc. 3 is slightly less than mass of (nuc. 1 + nuc. 2). The lost mass is converted to energy. Why? Einstein's conservation of mass and energy, E = mc2. Sum of mass and energy always conserved in reactions. Fusion reactions power stars. Chain of nuclear reactions called "proton-proton chain" or p-p chain occurs in Sun's core, and powers the Sun. In the Sun's Core... neutrino (weird particle) proton deuteron (proton + neutron bound together) positron (identical to electron but positively charged) proton photon proton + proton → proton+neutron { 1) + neutrino + positron (deuteron) + energy (photon) 2) deuteron + proton → 3He + energy He nucleus, only 1 neutron 3) 3He + 3He → 4He + proton + proton + energy Net result: 4 protons → 4He + neutrinos + energy Mass of end products is less than mass of 4 protons by 0.7%. Mass converted to energy. 600 million tons per second fused. Takes billions of years to convert p's to 4He in Sun's core. Process sets lifetime of stars. Hydrostatic Equilibrium: pressure from fusion reactions balances gravity, allows Sun to be stable. How does energy get from core to surface? photon path core "radiative zone": photons scatter off nuclei and electrons, slowly drift outwards: "diffusion". "convection zone" "surface" or photosphere: gas density low enough so photons can escape into space. some electrons bound to nuclei => radiation can't get through => heats gas, hot gas rises, cool gas falls Can see rising and falling convection cells in photosphere => granulation. Bright granules hotter and rising, dark ones cooler and falling. (Remember convection in Earth's atmosphere, interior and Jupiter). Granules about 1000 km across Why are cooler granules dark? Stefan's Law: brightness T4 The (Visible) Solar Spectrum Spectrum of the Sun shows: 1) The Black-body radiation 2) Absorption lines (atoms/ions absorbing photons at specific wavelengths). 10,000's of lines from 67 elements, in various excited or ionized states. Again, this radiation comes from photosphere, the visible surface of the Sun. Elements weren’t made in Sun, but in previous stellar generations. Sunspots They are darker because they are cooler (4500 K vs. 5800 K). Related to loops of the Sun's magnetic field. • Roughly Earth-sized • Individual spots last ~2 months • Usually occur in pairs • Follow solar rotation radiation from hot gas flowing along magnetic field loop at limb of Sun. Objects Seen in Transit Transit of Venus 6/8/04 Photo: J. Lodriguss Venus transit with bird, 4-frame composite Rafael Navarro and Ismael Cid Tres Cantos, Madrid, Spain 44 http://www.vt-2004.org/photos/vt-photostop01.html#iss 45 Transit of ISS and Shuttle Atlantis, 50 min after undocking, September 17th 2006 at 13h 38min 50s UT. Taken from the ground at Mamers (Normandy) France. Takahashi TOA150 refractor (diameter 150mm, final focal 2300mm), Baader helioscope and Canon 5D. Exposure of 1/8000s at 50 ISO, extracted from a series of 14 images (3 images/s) started 2s before the predicted time. Image copyright Thierry Legault. Transit of ISS and Shuttle Atlantis, 50 min after undocking, September 17th 2006 at 13h 38min 50s UT. 46 • Sunspot numbers vary on a 11 year cycle. • Sun's magnetic field changes direction (flips) every 11 years. • Maximum sunspot activity occurs about halfway between reversals. • We just passed through a sunspot minimum (c. 2009). Sunspot activity now on the rise again. • High levels of sunspot activity correlate with other active Sun behavior -- flares, coronal mass ejections (CMEs), prominences. • Solar flares can disrupt radio communications on Earth, are hazardous to astronauts in space (high levels of radiation), and can even permanently damage spacecraft in orbit. Above the photosphere, there is the chromosphere and... The Corona Best viewed during eclipses. T= 6 10 K Density = 10-15 3 g/cm only! We expect X-rays from gas at this temperature. Yohkoh X-ray satellite X-ray brightness varies over 11-year Solar Cycle: coronal activity and sunspot activity go together. The Solar Wind At top of corona, typical gas speeds are close to escape speed => Sun losing gas in a solar wind. Wind escapes from "coronal holes", seen in X-ray images. Wind speed 500 km/sec (takes a few days to reach Earth). 106 tons/s lost. But Sun has lost only 0.1% of its mass from solar wind. Active Regions Prominences: Loops of gas ejected from surface. Anchored in sunspot pairs. Last for hours to weeks. Flares: A more energetic eruption. Lasts for minutes. Less well understood. Prominences and flares occur most often at maximum of Solar Cycle.