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Admin. 9/29/16 1. Class website http://www.astro.ufl.edu/~jt/teaching/ast1002/ 2. Optional Discussion sections: Tue. ~11.30am (period 5), Bryant 3; Thur. ~12.35pm (end of period 5 and period 6), start in Pugh 170, then Bryant 3 3. Office hr: Tuesday 12.30-1pm; Wed. 12.30-1.00pm, Bryant 302 (but email me if coming on Wed.). 4. Homework 5: is due Mon. Oct 3rd 11.59pm via Canvas e-learning under “Quizzes” 5. Reading this week: Chapters 0, 1, 2.1-2.4, 4.1, 5, 6, 7, 8, 4.2, 4.3 6. Midterm 1: was on Tue. Sept. 27th: results via Canvas e-learning soon. 7. Observing project deadline: Thursday Oct. 27th 2016, however, you are strongly advised to complete observing by Fri. Oct. 7th. 8. Email me Astro-news, jokes, tunes, images: [email protected] 9. Printed class notes? Name tags? Key Concepts: Lecture 16: Jovian Planets: Rings and Moons Rings and the Roche Limit Galilean Moons of Jupiter Titan and other moons of Saturn Overview • All Jovian Planets have –Rings –many satellites • Rings & satellites orbit planet like mini-solar system • Composition of rings & satellites differs from objects in inner solar system –because temperatures in outer solar system are much colder Orbits • Most satellites of Jovian planets follow “direct” or “regular” orbits: counter-clockwise if viewed from above the north pole. • Ring systems are in regular orbits • There are some “irregular” satellites – retrograde orbits (clockwise) – or have highly elliptical orbits – or have highly inclined orbits Orbits of Jupiter’s moons • Orbits give us information about the formation of the rings & satellites Moons and Rings of Uranus and Neptune • Objects in regular orbits – probably formed at the same time as the planet – probably formed by processes similar to those that formed the planets • Object in irregular orbits – probably formed far away and were captured later Rings • Rings consist of billions of small particles or moonlets orbiting close to their planet –size of particles range from grain of sand to house-sized boulders Roche Limit – Ring Formation A moon is pulled apart by TIDAL FORCES (i.e. gravity stronger on near side than far side) if it approaches inside about 2.5 planetary radii. • Particles follow Kepler’s laws – inner particles orbit faster than those farther out – ring not rotating as a solid body, rather individual moonlets are orbiting • if ring particles widely spaced - move independently • if particles are close - interact gravitationally • moons clear gaps in rings Origin of Rings • Breakup of shattered satellite • Or, remains of particles that were unable to come together and form satellite • Gravity plays important role Saturn’s rings –differential force of gravity -- i.e. tidal forces • tear bodies apart • inhibit loose particles from coming together Much larger ring system - maybe formed quite recently? Rings of Saturn The Galilean Moons • Rings are thin - only about 20 m thick! • 3 broad, bright rings – A, B, C • handful of narrow rings of which F is most substantial • Gaps between rings • Ring particles made up of ices and rock • Small “shepherd” moons can create very narrow rings Io • Innermost Galilean moon The Jupiter System [You do not need to remember precise number of moons around the giant planets] • 16+ moons –4 large satellites : Galilean satellites Io Europa • Most volcanically active place in solar system – Voyager I - 8 volcanoes erupting – Voyager II (4 months later) - 6 active • Io - size of Earth’s Moon • Europa - size of Earth’s Moon • Ganymede - bigger that Mercury • Callisto - bigger than Mercury –12+ much smaller moons • 3 groups of 4 moons – 4 orbiting inside the orbit of Io - regular orbits – 4 outer moons in direct, but inclined orbits – 4 outermost moons in retrograde orbits • Faint ring • Similar size & density as our Moon • Surface is constantly changing Ganymede Callisto • Volcanic eruptions mainly composed of sulfur & sulfur dioxide • Lava lakes made of liquid sulfur • Why so much volcanic activity ? • Tidal heating by Jupiter. Other satellites disturb Io’s orbit, preventing it from becoming tidally locked to Jupiter. Thus Io is continually stretched and pulled: leads to heating. Volcanoes on Io Europa Europa • Size & density similar to our Moon • mainly rocky composition – heat from Jupiter during early evolution evaporated much of the ice • smooth surface • few impact craters • Surface is ice covered • extensive & complex network of cracks in icy crust – internal geological activity • Liquid water is rare in the Solar system, but likely present on Europa • Europa is good place to look for extraterrestrial life Ganymede & Callisto Ganymede: - largest satellite in solar system - third of surface heavily cratered & dark - older - tectonic activity & volcanism: created mountains, cracked crust, ice-mountain ridges, cracks & craters flooded with water - dark material is organic (carbon based) Callisto: - Outermost Galilean satellite - orbital period = 17 days = period of rotation (tidal lock) - large icy moon size of Mercury, but 1/3 of mass: less dense, less rocky, mostly ice Titan Galilean moons • As we move away from Jupiter, the composition of the moons become more icy, less rocky, less dense. (Similar to planets as we move away from Sun). The Saturn System • Largest satellite of Saturn • similar size, mass & density as Callisto • composition - rock & ice • Substantial atmosphere • Active atmospheric chemistry • Mostly nitrogen – Also some carbon monoxide, methane, ethane, hydrogen cyanide, cyanogen • Thick - atmospheric surface pressure higher than Earth • multiple cloud layers – methane • due to low temperatures - may be liquid methane oceans. HST -IR Huygens Probe Visits Titan Jan 2005 • 19+ satellites –7 satellites in regular orbits • Titan is the largest • Titan only one with atmosphere –Several small moons orbit near rings –2 distant irregular satellites in retrograde orbit • Beautiful set of rings –broad & flat –huge collection of icy pieces in orbit around equator River channels Surface view Water ice & Methane springs Titan Uranus: moons and rings • Earth-like geology • chemistry involved is quite different • liquid methane instead of water • frozen water ice instead of silicate rock • hydrocarbon particles instead of dirt • volcanoes spew very cold ice & ammonia instead of lava • Ring & satellite system are tilted at 98o • 15 regular satellites –5 largest are comparable in size to moons of Saturn • 11 rings –narrow ribbons of material with broad gaps Some other moons of Saturn Neptune: moons and rings • 8 satellites – 6 regular satellites close to planet – 2 irregular satellites farther out – Triton Mimas • large • retrograde orbit - probably captured • has an atmosphere • active volcanic eruptions • Rings Tethys – narrow & faint