2000 February 29, SPS 1020 (Introduction to Space Sciences) TODAY IS SPECIAL: February 29, 2000. - Read TNSS Chs. 17 and 18 (Io and Europa) for Thursday, March 2. After Spring Break: Chapters 19 and 20, PBD Ch. 7 (Ganymede, Callisto, and Titan) --------------- Planetary magnetospheres (continued) ------------------------ Effects of radiation, from solar flares: - Makes aurorae (air glows, like fluorescent lamps) - Health danger to astronauts - Ages spacecraft electronics (2 satellites failed after 1989 March flare) - Charges up spacecraft, causes electical problems - Charges up high-voltage power lines, caused Hydro Quebec blackout (1989) Types of radiation: - charged particles (protons, electrons, alpha particles (helium nuclei) - neutrons - ionizing electromagnetic radiation: gamma rays and X-rays All so energetic, they ionize matter, e.g. your cell membranes. Radiation doses: 0.14 rem/day: normal sea-level dose, from cosmic rays and natural radioactivity in rocks 50 rem: typical dose for a 2.5-year Mars expedition, outside of Earth's magnetosphere. But the body can heal itself over time: this level of radiation elevates the risk of cancer of about 1% per year, about as much as smoking for this length of time. 75-200 rem within 30 days: Body can't repair cell damage fast enough. Radiation sickness (vomiting, fatigue, hair loss; birth defects in later children, cancer in later life.) 500 rem, promptly: fatal dose Large solar flares (e.g., the one that hit 2 weeks after the Apollo 16 mission) can have > 2000 rems/hour. => Definitely want a "storm shelter" inside Mars-bound spacecraft! Best shielding: water! (Little secondary radiation.) Therefore, build the wash-water tank with a cavity in the middle, so you can duck inside it! Jupiter's magnetosphere: ------------------------ Largest "object" in Solar System; would have a 6-degrees angular diameter, if you could see it. (Angular diameter of Earth's Moon, from Earth, is 0.5 degrees.) - Known since 1940s from radio emission (Jupiter among brightest objects in radio sky) - Trapped radiation belts: Pioneers 10 and 11 soaked up 1000s of times more than a lethal dose, for humans. => Electronics did fail; camera optics darkened - Magnetotail: can _wag_ by 45 degrees, extends past Saturn! - Io plasma torus: volcanoes on Io contribute sodium and sulfur ions, make current, affect aurorae Ganymede: a magnetosphere within a magnetosphere! No bow shock around Ganymede: particles in Jovian magnetosphere are subsonic, unlike Solar wind Saturn's magnetosphere: also trapped radiation belts, decametric radio emission, aurorae. Interaction with Titan's atmosphere? Uranus and Neptune: both magnetospheres asymmetrical and corkscrew-shaped, since tipped sideways! Magnetic declinations very large for both (59 degrees Uranus, 47 degrees Neptune), much more than any other Solar System objects (< 15 degrees); unknown why. The heliopause: the "edge" of the Solar System (or heliosphere), where -------------- the solar wind meets the interstellar medium. Indirect detection by Voyager 1 in 1992, from radio emission: at about 150 AU. (Pluto at 40 AU.) The Jovian Planets (TNSS Chs. 14 and 15): ----------------------------------------- Exploration of the Outer Solar System (beyond the main-belt asteroids): A simple, mostly American story (these are *all* the spacecraft). Most used "gravitational slingshot" flyby maneuvers, to pick up speed, or change direction. 1) Pioneer 10 to Jupiter (launched 1972 March, flyby 1973 December) 2) Pioneer 11 to Jupiter (1974) and Saturn (1979) - relatively small precursors Needed to demonstrate feasibility for more-sophisticated Voyager: - Passage through asteroid belt (found surprisingly *little* dust, vs. sci-fi) - Passage through Jovian magnetosphere, trapped radiation belts 3) Voyager 1 to Jupiter (1979), Saturn (1980), targeted to Titan and so flung away from ecliptic 4) Voyager 2 to Jupiter (1979), Saturn (1981), Uranus (1986), Neptune (1989) - Wow! THAT was exploration at its best! 5) Ulysses (probe launched by NASA, but built by ESA, the European Space Agency, so the Outer Solar Systems _isn't_ an all-American Story): Jupiter flyby (1992): loaded with instruments for space physics 6) Galileo: Jupter orbiter (arrived 1995 December, still going). Dropped probe into Jupiter's atmosphere. 7) Cassini: Saturn orbiter (launched 1997, to arrive 2004, 4-year nominal mission) To drop Huygens probe into Titan's atmosphere. Also radar mapping for Titan Jovian planets fundamentally differ from terrestrial planets in several fundamental ways: Really a dichotomy: two sets of two bodies, Jupiter & Saturn versus Uranus & Neptune - Masses, M(Earth): Jupiter: 318, Saturn: 95, Uranus: 14.5, Neptune: 17.2 - Compositions: moslty H_2, and He; < 1% H_2 O, CH_4, NH_3, etc - None have solid surfaces (all have densities too low to be mostly solid) - All have rings, although Saturn's by far most prominent - All have large retinues of satellites. Can be both large and small: Jupiter: the Galilean satellites, Io, Europa, Ganymede, Callisto Saturn: Titan Neptune: Triton Satellites have spacings similar to Bode's Law: really miniature Solar Systems. - All have fierce, vicious magnetospheres! Doses: Callisto: 0.01 rem/day Ganymede: 8 rem/day Europa: 540 rem/day Io: 3600 rem/day! Thebe and inner satellites: 18,000 rem/day! (cf. Entering Space by Robert Zubrin (1999), Table 8.5 on page 167.) - All rotate rapidly: smears clouds into bands Jupiter: 9 h 56 m, Saturn: 10 h 39 m, Uranus: 17 h , Neptune: 16 h This is differential rotation: shearing motion, faster at equator than at poles, like the Sun. _Not_ like a rigid body, e.g. a terrestrial planet - All produce more heat from their interiors than they receive from the Sun (exception: Uranus) Produced by Kelvin-Helmholtz contraction: Essentially, PV = nRT, so T still high, since still contracting => More like small stars, than terrestrial planets! Interiors: All based on _models_. Constrained by gravity fields (from spacecraft tracking), magnetic fields, heat flow, but uncertain and non-unique. Depends on high-pressure laboratory experiments. See TNSS, p. 197, Figure 7: Jupiter: ------- Contains 2/3 of Solar System mass, outside Sun, and nearly all of the Solar System's angular momentum R = 11 Earth radii Density = 1.3 x liquid water (1 gm/cm^3): can't be all solid Albedo = 51%: covered in clouds Atmosphere: series of parallel stripes, visible even in binoculars Equatorial, temperate belts, marked with (mostly white) spots: followed by amateur astronomers Zones (lighter, rising clouds), separated by belts (darker, falling clouds) Great Red Spot: discovered in 1600s, still around A great cyclonic storm, like a hurricane Changes color: greyish & indistinct in the early 1980s 3 layers of clouds (all in troposphere), different chemical compositions: Top: ammonia crystals Middle: ammonium hydrosulfide (NH_4 SH) Bottom: water clouds Galileo probe fell through hole in clouds, saw only wisps of top, and bottom layer! - *Did* confirm less-than-solar abundance of He: rainout into interior. - Found lots of neon: did Jupiter form where Neptune is now, and move? Rich organic chemistry: hydrocarbons (H-C molecules) including ethane, acetylene, propane (C_3 H_8); also phosphine (PH_3), germane (GeH_4) Collision of Comet Shoemaker-Levy 9 (in 1994): dredged up H_2 S from deep interior; made organic "tarry gunk" Young Carl Sagan: floating life? (Gasbags and predators.) Probably not, though, because of: Atmospheric circulation: Can generally be mimicked by speeding up rotation, in a model of Earth's atmosphere. Convective downdrafts would bring complex molecules to hot interior and dissociate, but still... Lightning: from shearing atmosphere, as expected (as with cumulus clouds, in Florida). Mega-bolts! Saturn: ------ Atmospheric features similar to Jupiter's but less clear because of haze layer, because it's colder Small spots, like Jupiter Sudden, violent, planet-wide storms: Hubble in 1990 Fastest planetary wind speeds known: 500 m/s (= 1800 km/hr = 1100 miles/hr) Uranus: ------ Discovered by William Herschel, 1781: first planet discovered since ancient times (with a 6-inch Newtonian reflecting telescope) Planet tipped on its side Voyager 2 saw haze: have to look *hard* to see atmospheric features But not now: Hubble finding clouds, changes Should see He rainout, but don't; don't see heat from interior: ? Uranus and Neptune: both blue, from methane clouds (colder) Neptune: ------- Predicted by calculations of gravitational perturbation of Uranus: Adams and Leverrier, 1846 G. B. Airy: said don't bother to look Johann Galle & Heinrich d'Arrest: found it in 10 minutes! Surprise: Great Dark Spot found by Voyager 2 (1989) => Another surprise: wasn't there in 1998! (Hubble Space Telescope imaging) => Uranus and Neptune have dynamic atmospheres - surprisingly! Planetary Rings and Dynamics (TNSS Ch. 16): ---------------------------- All Jovian planets (Jupiter, Saturn, Uranus, Neptune) have extensive satellite systems and rings. All ring systems are *highly individualistic*: radii, masses, albedoes, particle sizes, compositions, structures all are substantially different in all four systems. Another Solar System connection to the astrophysical Universe: rings and disks are common, all over the Universe, e.g.: - accretion disks and rings around young stars; - binary star systems; - disks extruded by rapidly rotating Be stars; - disks and rings around the central engines (black holes) in active galactic nuclei and quasars. Discovery: --------- - Saturn: Galileo saw it wasn't round (1610), was baffled by mid-plane crossing (1612). (Earth crosses Saturn's ring plane every 15 years; last time was 1995.) Not resolved into rings until observed by Christian Huygens (1659). In a modern telescope, a truly unearthly sight! - 1977 - Uranus rings discovered, during stellar occultation (moved in front of star, which winked off and on) (Occcultation: whenever one heavenly body moves in front of another) - 1984: Neptune rings found by stellar occultations- but occulted 10% of the time => ring arcs Ring arcs confirmed by Voyager 2 imaging, 1989 - 1979 - Jupiter rings discoved by Voyager 1 close-up spacecraft imaging 1997: discerned from ground-based infrared imaging Structure: see TNSS, pp. 226. All distinctively different! ----------- Saturn's rings: -------------- Composition: mostly water ice; some dust Known from albedo = 0.2 = 0.8; also radio science, spectra. Extensively imaged, probed by radio science from Voyager 1 and 2. WONDERFUL case of an unexpected discovery! > 5000 *ringlets*, more than number of grooves on an LP record Thickness: at most (in A and B rings), < 100 m thick! (20,000 km wide) Why so flat? => Collisions (and near-collisions: grav. interaction) average out up-and-down velocities; settle into plane Particle size (also from radio science, scattering of radio waves): cm to ~ 5 m Rings have little total mass, 10^18 - 10^19 kg: a mountain is about 10^15 kg => Would make a satellite with r < 100 km, if compressed together Resonances between moons and rings: sweep gaps clean Cassini division: at radius of 2:1 resonance of satellite, Mimas Encke's division: cleared by satellites Atlas & Pan Density waves - like galactic spiral arms: fluctuations in particle-density, propagating outward What kind of waves are these? F ring: braided, by shepherding satellites Pandora and Prometheus (& others?) (vertical) Bending waves - train of vertical corrugations, propagating inward What kind of waves are these? Spokes: (relatively dark) dust particles entrained in magnetic field, corotating about planet Jupiter's rings: --------------- Composition: micron-sized dust albedo 5%, appearance changes with Sun angle; maybe silicate or carbonaceous - inside halo: unique 10^4 km vertical extent, probably levitated by electromagnetic forces in magnetosphere - Main ring: shepherded by satellites Metis and Adrastea - outer Gossamer rings: have gaps cleared by satellites Amalthea and Thebe Uranus's rings: -------------- Composition: probably carbon-rich: soot? cm - m size Ten narrow (and very dark (1.5% albedo) rings; Dust pervades ring plane, but relatively absent from the ten (optically thick) rings. Visibility _greatly_ depends on Sun angle. The thickest, the epsilon ring, is shepherded by two tiny (r < 15 km) satellites, Ophelia and Cordelia Neptune's rings: --------------- Composition: dust, cm - m size - probably carbonaceous Adams ring (thickest): clumped into ring arcs, Liberte', Egalite', Fraternite' - shepherded by satellite Galatea Origin and future of planetary rings: ------------------------------------ Roche limit: radius inside which a fluid (not solid) moon will be sheared apart, by planet's tides. All known rings lie inside planets' Roche radii => Accretion of planetesimals into larger moons can't occur (Solid bodies can still exist inside Roche radii, because of their material strength.) Stability: computer simulations show most ring systems are stable only over about 100 million years! Also, angular momentum transferred by density waves should clear them in this much time. => Will Saturn's rings not always be there, until the next comet comes too close?