Astronomy 6 PDF

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Astronomy 6...

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Astronomy Chapter 6 • Jupiter & Saturn are made almost entirely of hydrogen & helium. • Uranus & Neptune are made primarily of hydrogen compounds such as water (H20), methane (CH4) & ammonia (NH3). ! • Jovian planets formed in the outer solar system beyond the frost line (right outside asteroid belt) • Jupiter is sometimes referred to as a “failed star”. Becomes a star if 75 times more massive ! • All 4 Jovian planets grown from icy planetesimals of about the same mass (10 MEarth) • The icy planetesimal of Jupiter first to grew large enough to draw in gas, followed by Saturn, Uranus & Neptune. • All planets stopped accreting gas at the same time when solar wind blew remaining gas into space. • Particles in the solar nebula were more spread out at greater distances (from the Sun), so that accretion took longer and there was less time to pull in gas before the solar wind cleared the nebula • Jupiter is 3 times more massive than Saturn but is only slightly larger than Saturn in radius. JUPITER • Gaseous hydrogen in outer layer is ~10% of radius. Jupiter’s atmosphere ! • Liquid hydrogen occupies next 10% of Jupiter’s interior. Temperature exceeds 2,000K !pressure exceeds 500,000 times Earth’s surface pressure

• Metallic hydrogen in most of the rest of Jupiter can conduct electricity. ! • extreme temperature & pressure forced hydrogen into compact metallic form generates Jupiter’s magnetic field • Jupiter’s magnetic field, due to metallic hydrogen, is 20,000 times of Earth’s • Enormous magnetosphere deflects solar wind & results in auroras ! COMPARING JOVIAN PLANETS • Saturn’s 4 interior layers differ from Jupiter due to its lower mass & weaker gravity. • phase change occurs deeper in Saturn thicker gaseous layer & thinner & deeply buried metallic hydrogen

• Pressure on Uranus & Neptune not high enough to form liquid or metallic hydrogen at all. JOVIAN MOONS • Jovian planets has numerous moons • Contain large amount of ice, besides metal & rock • We know of 150 moons orbiting the Jovian planets. Jupiter has the most (>60 moons) • Galilean moons would count as planets if they orbited the Sun. • Io is the most volcanically active body in the solar system.

• Europa has one of the smoothest surface in the solar system. • Ganymede is the only satellite in the solar system known to possess a magnetosphere. • Callisto is a heavily cratered ice ball.! JUPITER’S LARGEST 4 MOONS IO – Most Volcanically Active World • Io’s entire surface is covered with large active volcanoes & eruptions are so frequent that virtually every crater is buried. • Io must be quite hot inside but it should also be geologically dead è Same size as our Moon should have lost internal heat long ago • Huge tidal forces exerted by Jupiter & Io’s slightly elliptical orbit causes tidal heating on Io (Io is also tidally locked to Jupiter) • Io continuously flexed in different directions, generating friction inside it tremendous heat generated on Io. EUROPA – The Water World • covered by water ice.! • It has fractured, frozen surface that must hide an interior made hot by tidal heating (weaker than on Io due to farther distance from Jupiter) GANYMEDE – Largest Moon In The Solar System • have water ice surfaces & possibly subsurface ocean of liquid water. • Some regions of its surface are dark & densely cratered • Other regions are light- coloured with very few craters.

• liquid water must have recently erupted & refrozen • Ganymede has some tidal heating but not strong enough to account for an ocean CALLISTO • looks like a heavily cratered ice ball. • ridges & crests are bright white • Callisto also have water ice surfaces. • Magnetic field data also suggest that Callisto could hide a subsurface ocean. • origin of interior heat is unknown as it does not have tidal heating SATURN’S MAIN MOON TITAN • Thick atmosphere which hides surface from view • 90% nitrogen & the rest are argon, methane, ethane & other hydrogen compounds ! • Appreciable greenhouse effect due to methane & ethane. • Surface temperature of -180 C due to great distance from Sun • Conditions right for methane/ethane rain create rivers, lakes, oceans • Titan appears to have an ocean below its icy crust.

RINGS ON JOVIAN PLANETS

• Rings formed from dust created in impacts on moons orbiting those planets. • cannot be left-over from planet formation as the particles are too small to have survived this long • there must be a continuous replacement of tiny particles • Jovian planets all have rings because they possess many small moonlets close-in. • Tiny impacts gradually grind away these small moons large enough to still exist after 4.5b years of such sandblasting

ASTEROIDS AND METEORITES • Asteroids are rocky leftover planetesimals orbiting the Sun that never become part of a planet. • A meteorite is a rock from space that falls through Earth’s atmosphere. ! • Meteor refers to the bright trail left by a meteorite. ! • Most meteorites are pieces of asteroids colliding with Earth. ! • objects smaller than 10m are called meteoroids METEORITES • Primitive – unchanged in composition since they first formed 4.6b years ago • Processed – younger & are pieces from core, mantle or crust of a shattered asteroid. Some experienced processes like volcanism or differentiation • Primitive meteorites represent first materials to condense & accrete in the solar nebula. • Processed meteorites represent samples of shattered worlds. • some asteroids must have active volcanoes when young

COMETS • Comets are any leftover ice- rich planetesimals formed beyond the frost line. • regardless of its size or whether it has a tail, or where it resides or come from • Comet’s tail: nearly stationary relative to stars ! • rise & set like stars while gradually moving relative to constellations usually visible for several weeks or more, until it fades from view • Vast majority of comets do not have a tails & are never visible in our skies. • Comets are heated by the Sun when they enter the inner solar system. • The nucleus is a “dirty snowball” & the source for comet’s tail. • Coma is the dusty atmosphere that comes from heated nucleus. • Plasma tail is ionized gas (by solar UV) escaping from coma, pushed by solar wind.

• Dust tail is dust-size particles pushed by photons (radiation pressure).

CRITICAL THINKING • NOT true: Jovian planets è they all have the same exact set of internal layers, though these layers differ in size (Jupiter and Saturn and different from Uranus and Neptune) • Typical wind speeds in Jupiter's atmosphere compare to typical wind speeds on Earth: They are much faster than hurricane winds on Earth • The Great Red Spot is a long-lived, high-pressure storm on Jupiter • Which moon has a thick atmosphere? Titan • Among all the large jovian moons, one is thought to have been captured into its present orbit rather than having formed in the "miniature solar nebula" that formed its planet. Which one? Triton • NOT true: Saturn's rings formed along with its moons 4.6 billion years ago.

• Jupiter and the other jovian planets are sometimes called "gas giants." In what sense is this term misleading? They really contain relatively little material in a gaseous state. Much more of their mass is liquid, metallic, or in strange high-pressure states that we don't naturally find on Earth. • why did Uranus and Neptune end up to be much less massive than Jupiter and Saturn? Particles in the solar nebula were more spread out at greater distances, so that accretion took longer and there was less time to pull in gas before the solar wind cleared the nebula. • Which of the following best describes what is going on in the alternating dark and light "stripes" that we see in photographs of Jupiter and Saturn. The light stripes are regions of high clouds, and the dark stripes are regions where we can see down to deeper, darker clouds. • Which Jovian planet should have the most extreme seasonal changes? Uranus • Why do astronomers believe that Triton is a captured moon? Triton orbits Neptune in a direction opposite that of Neptune's rotation. According to current understanding, which of the following is required in order for a planet to have rings? The planet must have many small moons that orbit relatively close to the planet in its equatorial plane....