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페이지를 파싱하기 어렵습니다.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?