1Announcements• Homework 6 is up onOWL. Due on Monday,December 1, beforemidnight.• Left: impact crater inNorthern Arizonaproduced by an asteroidroughly 50 meters acrossNov. 20, 2008Asteroids vs. Comets• Both are relatively smallobjects that orbit the Sun inhuge numbers• Asteroids are small androcky; largest ones are afew hundred km across• Roughly 10,000 asteroidshave been cataloged, butwe only see the big oneseasily (most shooting starsare small, uncatalogedasteroids)This asteroid was roughly 50 meters acrossThe Layout of the Solar System:The Asteroid Belt• Most of the asteroids are located in theasteroid belt between Mars and Jupiter.• The Trojan asteroids are Jupiter’s groupies;they cluster in the vicinity of Jupiter andfollow it aroundComets• Comets are also relatively small butotherwise are quite different from asteroids• Comets have rocky cores but are largelycomposed of ices (water ice, methane ice, andammonia ice)• Comets are mostly located much farther outin the Solar System• There are approximately a trillion cometsout in the fringes of our Solar SystemThe Layout of the Solar System:Asteroid & Comet Summary1. Asteroid belt:disk-like distribution ofasteroids between Mars andJupiter. Asteroids all orbit inthe same direction.2. Kuiper belt:disk-like distribution ofcomets from foundbetween 30 and 100 AU.All orbit in same direction.3. Oort cloud:spherical distribution ofcomets from foundup to 50,000 AU away (i.e.,1/4 the distance to the neareststar). Orbit are inclined in alldirections.The Layout of the Solar System:The Kuiper Belt and the Oort CloudIn many respects, Pluto is more like a Kuiper Belt object than a planet.Indeed, astronomershave now found objects comparable toor bigger than Pluto,but farther out.The Oort cloudis huge!2Extent of the Oort CloudMercury(Text Unit 38)• No real atmosphere leads tolong hot days (T ≈ 800 F) and…• Long cold nights (T ≈ —300 F)• Days and nights last for 3months• Huge cliffs - hundreds ofkilometers long and several kmhigh. For some reason,Mercury’s crust has undergonetitanic shifts• Ancient, long-dead volcanicflows• Most metal-rich planet: mostlycomposed of ironLeast studied of the terrestrial worlds...Visited by the NASA Mariner 10mission in 1974The NASA MESSENGERSpacecraft: a new missionto study Mercury• Image of Mercuryrecorded on Oct. 6,2008• Close-up view:Venus(Text Unit 39)••Intensively studied with theIntensively studied with theMagellan spacecraftMagellan spacecraft••Covered by a thick atmosphereCovered by a thick atmosphereand layers of cloudsand layers of clouds••Super-harsh greenhouse effectSuper-harsh greenhouse effect••Greenhouse effect keeps theGreenhouse effect keeps thetemperature at ~850 F all day andtemperature at ~850 F all day andall nightall nightMercury and the Moon• Both have similar sizes• Neither has an atmosphere• Tides have locked both intoorbital resonances (the Moon hasa 1:1 resonance with the Earth,and Mercury has a 3:2 resonancewith the Sun)• Day vs. night much moredramatic on MercuryMercuryVenusSurface geology• Both have heavily crateredregions• Both have smoother regionsindicating lava flowsThe Moon’s geology: evidence ofthe “heavy bombardment” period• Apollo astronauts brought backrocks from the highlands and mare• The highlands rocks are mucholder than the maria rocks• Maria formed by large impactsthat melted the surface• After the maria cooled, impactswere much more rare• “Heavy bombardment” - earlyperiod of the solar system whenimpacts were much more commonHeavily cratered “highlands”Lightly cratered “maria”3Nov. 13, 2008 NASA press release:Hubble Space Telescope DirectlyObserves Planet Orbiting FomalhautFomalhaut here (lightblocked using a specialtechnique)Ring of dust and asteroidssurrounding FomalhautNov. 13, 2008 NASA press release:Hubble Space Telescope DirectlyObserves Planet Orbiting FomalhautRing of dust and asteroidssurrounding FomalhautArtist’s conception of the planetorbiting in the disk aroundFomalhaut• Oxygen for breathing• Ozone layer to blockdangerous ultravioletlight from the Sun• Magnetic field todeflect the “Solar wind”Earth. The only world with:Mars(Unit 40)Olympus Mons, thelargest volcano in theSolar System: 600 kmacross (the size of AZ)and 3 times higher thanMt. Everest!Valles Marineris, as long as theUnited States is wide and 4 times deeper than the Grand CanyonSpace missions to MarsWhy not just go to Nevadaand be done with it?4Schiaparelli/Lowell “canals”Lowell commissioned construction of Lowell Observatory in northern AZ for the study of Mars.Actual image of Mars ofquality attainable in Lowell’stimePercival Lowell sketch(partially correct, partiallydeluded) Mars• Two tiny moons -- notimportant (asteroidscaptured in gravitationalencounters)• Ancient extinct volcanos• Huge canyons• Polar caps - (mostly)carbon dioxide and (a bitof) water frost• Dramatic dust storms• An abundance of evidenceindicating that Mars oncehad water• Water still present insufficient quantities tosupport life??Jupiter••Largest and most massive planet - 300 timesLargest and most massive planet - 300 timesthe mass of the Earth and 1000 times thethe mass of the Earth and 1000 times thevolumevolume••Hydrogen, helium, and a few hydrogenHydrogen, helium, and a few hydrogencompoundscompounds••Before Voyager 1 visited Jupiter in 1979, it wasBefore Voyager 1 visited Jupiter in 1979, it wasbelieved to have 13 moons (4 seen bybelieved to have 13 moons (4 seen byGalileo). Now, 61 moons are known, many onlyGalileo). Now, 61 moons are known, many onlyrecently discoveredrecently discoveredSaturn••Second-largest planetSecond-largest planet••Hydrogen, helium, andHydrogen, helium, anda few hydrogena few hydrogencompoundscompounds••At least 30 moonsAt least 30 moons••Titan is enshroudedTitan is enshroudedby a thick atmosphereby a thick atmosphere••How about those rings!How about those rings!Uranus••Much smaller than Jupiter & SaturnMuch smaller than Jupiter & Saturn••Hydrogen, helium, and a few hydrogenHydrogen, helium, and a few hydrogencompoundscompounds••21 moons; rings21 moons; rings••Entire system of moons and rings is highlyEntire system of moons and rings is highlytilted - probably indicates that there wastilted - probably indicates that there wassome type of spectacular collision in thissome type of spectacular collision in