1	Astr 1050 Fri., Dec. 13, 2002 Today: Astronomy Articles Homework #11 Finish Ch. 18, Chapter 19, Pluto and “Debris”
2	Homework #11 Q1: If you were an astronomer in the Alpha Centauri system (assume 4.2 light years from Earth) looking toward the solar system, what would be the maximum angular separation between Jupiter and the sun you could ever see? (Hint: 1 ly equals 63,000 AU). Small Angle formula, Jupter 5.2 AU from sun, yields 4 arcseconds Q2: If the Atlantic Seafloor is spreading at 30 mm/year and is now 6400 km wide, how long ago were the continents in contact? 6400 km/30 mm/year = 210 million years. Q3: Which effect is the most important for clearing the remaining gas and dust in the solar nebula following planet formation? Radiation pressure. Q4: Europa has few craters because It has erased craters nearly as fast as they form. Q5: A meteor shower is produced when Earth passes through the orbital path of a comet. Q6: An asteroid has an orbital period around the sun of 5.2 years. How far from the sun is this asteroid? Kepler’s Law. P² = a³. P = 5.2 years, so a in AU is the cube root of 5.2 squared, which is 27, making a = 3 AU.
3	Comparison of Jovian Planets Variation in distance presumably ultimate causes other effects P: Kepler’s third law T: Falloff mostly just result of falling solar energy But Neptune hotter because more internal heat M: Clue to details of solar nebula mode Less material in outer solar system – or perhaps less efficient capture r: Should drop with mass because less compression Works for Saturn vs. Jupiter Increase for Uranus, Neptune indicates less H, He and more heavy material
4	Effects of T (and E) on Atmospheres Saturn’s bands much less distinct than Jupiter’s Temp. lower on Saturn Þ cloud condense lower Deeper clouds Þ markings less visible Differences at Uranus and Neptune Even colder Þ clouds even deeper So cold CH₄ can condense Little solar energy to drive weather Uranus has strange seasons – tipped on its side Neptune has strong internal heat source, so it still can have weather Large amounts of heavy elements compared to amount of H, He on Jupiter, Saturn Large amounts of CH₄ gas absorb red, make planets appear blue
5	Implications of M, r for nebula Relative amount of H, He (compared to heavy elements) drop for Saturn then drop dramatically at Uranus and Neptune Why were these outer planets so less efficient at capturing H, He? Their mass is still great enough to do this, especially given low temperatures in the outer solar system May be a problem of timing Accretion takes longer in the outer solar system because The velocities of all objects there are much less The distances between objects are greater This is the same reason the periods of the orbits are so long Uranus and Neptune may have only started to grow to critical size by the time the H, He gas was being driven out of the solar system
6	The Jovian planets are miniature solar systems Because planets captured gas as they were forming they had small “solar” nebulae Þ tests of nebula theory Regular satellite systems Large moons in direct (i.e. counterclockwise), equatorial orbits Preserve solid material from time of formation Jupiter shows solar-system like density gradients Rings Moons unusual and interesting objects in their own right Io – most volcanically active body in solar system Europa – also active, may have liquid water ocean (and life!) Titan – only satellite with a thick atmosphere Many properties are due to tidal forces and resonances
7	The Roche Limit When can tides tear a moon apart? As a planetary body get close to another object, tidal forces distort the body more and more. Remember, Earth raises tides on the Moon just like it raises tides on the Earth If the distortion gets large enough, the moon will be pulled apart Happens at “Roche Limit” when moon is ~2.44 ´ radius of planet away At that point, tidal force pulling up on surface of moon is greater than moon’s gravity pulling down Only matters for objects held together by gravity Astronaut in orbit will not be pulled apart Is held together by much stronger chemical forces Astronaut standing on the outside of the shuttle, hoping the shuttle’s gravity would hold her there, will be pulled away from the shuttle
8	Saturn as seen by the Hubble Space Telescope
9	Rings are individual particles all orbiting separately Each particle – dust to golf ball to boulder size – is really a separate moon on its own orbit Orbit with Keplerian velocities: high in close, slow farther out Nearby relative velocities are low – so particles just gently bump into each other – slowly grinding themselves up Structure in rings largely caused by gravity of moons
10	Resonances: Properly timed gravitational “pushes” Like someone pushing kid on a swing Timing of pushes just as important as force used Pushing at random times has little effect Pushing at just right point in each cycle can produce big effect
11	Cassini division at 1:2 resonance with Mimas
12	Comparison of Rings All within Roche limit Details controlled by Resonances and Shepard Satellites
13	Jupiter as a miniature solar system Four large moons (Io, Europa, Ganymede, Callisto) Regular (equatorial, circular) orbits Pattern of changing density and composition with distance Inner two (Io, Europa) mostly rocky Outer two (Ganymede, Callisto) more icy
14	Io, Europa break rules about activity Io most volcanically active body in solar system Europa shows new icy surface with few craters
15	Tidal heating explains activity Large tides from Jupiter flex satellites Friction from flexing heats interiors Important for Io, Europa, some other outer solar system satellites
16	Probable H₂O ocean on Europa Tidal heating may keep H₂O liquid under ice cover Perhaps a location where life could evolve “Europa Orbiter” Mission being planned to determine if ocean exists
17	Callisto not active
18	Comparison of Satellites
19	Titan Largest moon of Saturn Has thick atmosphere Pressure ~ 1 earth atmosphere Mostly N₂, some CH₄ Gas held because of low T UV acting on CH₄ Þ smog Ethane produced – Lakes? Can “see” surface only in IR Cassini will drop probe in Fall 2004 “Code of the Lifemaker” by James P. Hogan, good sf
20	Triton Largest moon of Neptune In unusual retrograde orbit Probably captured after it formed Tides during capture may have caused heating Does have thin atmosphere Shows recent “activity” Not volcanic – rather volatile related Ices migrate with seasons “Geysers” caused by heated ices
21	Chapter 19: Meteorites, Asteroids, Comets Small bodies are not geologically active They provide “fossil” record of early solar system Asteroids Mostly from region between Mars and Jupiter Left over small debris from accretion, never assembled into a large planet Meteorites come mostly from asteroids Comets “Stored” on large elliptical orbits beyond planets Thought to be “planetesimals” from Jovian planet region, almost ejected from solar system in its early history Meteorites provide only samples besides Apollo With sample in hand, can perform very detailed analysis: detailed chemistry; radioisotope age; other isotope info
22	Asteroids Most located between Mars and Jupiter Largest is Ceres 1/3 diameter of moon Most much smaller >8,000 known Total mass << Earth A few make it to earth source of the meteorites
23	Meteorites from Asteroids If meteorite speed and direction is observed as it enters Earth’s atmosphere, you can work backwards to find its orbit. Almost all of the meteorites with well determined orbits have most distant part of orbit ellipse within the asteroid belt.
24	The larger asteroids
25	Asteroid Belt Structure As Jupiter formed it stirred up velocities in what would become the asteroid belt Higher velocities meant planetesimals destroying each other rather than accreting Gaps and concentrations occur at resonances with Jupiter
26	Are Asteroids Primitive? Ida (56 km diam.) and its moon Dactyl (1.5 km diam.) Colors have been “stretched” to show subtle differences Imaged by Galileo on its way out to Jupiter
27	Another Galileo Asteroid: Gaspra
28	Phobos & Deimos: Two “misplaced” asteroids? Phobos and Diemos are small (~25 km and ~15 km diam.) moons of Mars Look like captured asteroids rather than moons formed in place Are “C” class – i.e. dark “Carbonaceous” type “asteroids”
29	Clues from Meteorites Three main kinds of meteorites Carbonaceous chondrites: Most primitive material – dark because of C Stones Similar to igneous rocks Irons Metallic iron – with peculiarities Why do we have different kinds? How are the main types of meteorites related to the asteroids?
30	Types of asteroids observed Simple classification by albedo and color Three main types C (carbonaceous?) S (stones?) M (metals?) Finer classification by spectra
31	Origin of different asteroid types Carbonaceous = undifferentiated? Stones and Metals from differentiated planetesimals? S = mantles M = cores Try to sort out using meteorite samples
32	Meteors vs. Meteorites Meteor is seen as streak in sky Meteorite is a rock on the ground Meteoroid is a rock in space Meteor showers (related to comet orbits) rarely produce meteorites Apparently most comet debris is small and doesn’t survive reentry Meteorites can be “finds” or “falls” For a fall – descent actually observed and sometimes orbit computed Most have orbits with aphelion in asteroid belt
33	Large Meteor over the Tetons (1972)
34	The Leonids 2001 APOD site: Picture by Chen Huang-Ming
35	Meteor Showers and Comets Meteor showers caused by large amount of small debris spread out along comet orbits Almost none makes it to the ground – no meteorites Occur each year as earth passes through orbit of comet Appears to come from “radiant point” in sky Leonids: Mid November
36	Comets: Hale-Bopp in April 1997
37	Comet characteristics Most on long elliptical orbits Short period comets – go to outer solar system “Jupiter family” still ~ in plane of ecliptic “Halley family” are highly inclined to ecliptic Longer period ones go out thousands of AU Most of these are highly inclined to ecliptic Become active only in inner solar system Made of volatile ices and dust Sun heats and vaporizes ice, releasing dust “Dirty snowball” model
38	Comet structure Gas sublimates from nucleus Dense coma surrounds nucleus Ion tail is ionized gas points directly away from sun shows emission spectrum ions swept up in solar wind Dust tail curves slightly outward from orbit shows reflected sunlight solar radiation pressure gently pushes dust out of orbit
39	Hale-Bopp clearly shows components
40	Where do comets come from? Long period comets: The Oort Cloud Most (original) orbits have aphelions of >1000 AU Need ~6 trillion comets out there to produce number seen in here Total mass of 38 M_Earth Passing stars deflect comets in from the cloud
41	Formation of Oort cloud comets Composition indicates formation in region between Jupiter and Neptune Ejected to the Oort cloud by near collisions as Jovian planets formed Most probably lost from solar system – a few have just barely closed orbits Occasional passing stars perturb more comets into orbits passing in close to sun
42	Where do the Jupiter family comets come from?: The recently discovered Kuiper Belt Material beyond Neptune never ejected into the Oort cloud Pluto and Charon the biggest members – now also Quarar Very hard to detect because very faint far from the sun so little illumination comets not active at that distance Hubble and new large telescopes have recently detected ~100
43	Importance of comets Evidence of solar nebula Source of H₂O and CO₂ for earth Impacts continue Impacts on Earth Extinction of the dinosaurs SL-9 impact on Jupiter
44	Chapter 16-19 Review Solar Nebula Terrestrial Planets Properties of Earth Greenhouse Effect (cf. Venus, Mars) Cratering, origin of moon Jovian Planets Properties of Jupiter, composition, atmosphere Rings “Debris” Asteroids and Comets
45	Chapter 16-19 Review We’ve covered this material fast – exam will not cover subtle concepts or obscure facts. Very basic information and only the most fundamental ideas. Things you should know include: Order of planets in solar system, general sizes of orbits, sizes and compositions of the planets (also asteroids and comets in general, notable moons). How these items fit into the solar nebula picture.
46	Chapter 16-19 Review Example questions: True/False: Jupiter was probably influential in preventing the formation of a planet at the location of the asteroid belt. The dirty snowball theory suggests that the head of a comet is composed of ices. Jupiter radiates more heat than it absorbs from the sun. Venus is very hot because its atmosphere is rich in CO₂. The Greenhouse effect occurs because gases like carbon monoxide are opaque to IR radiation. The Jovian planets have lower densities than the terrestrial planets.
47	Chapter 16-19 Review Example questions: True/False: Meteorites appear to be composed of material similar to that found in comets. Jupiter’s interior is mostly liquid helium. Saturn’s rings are composed of metallic dust grains. Flow channels on Venus suggest it was once rich in water. The oxygen in Earth’s atmosphere was outgassed by volcanic explosions. Mars is the third rock from the sun.
48	Chapter 16-19 Review Example questions: Multiple choice: On a photograph of the moon, the moon measures 30 cm in diameter and a small crater measures 0.2 cm. The moon’s physical diameter is 1738 km. What is the physical diameter of the small crater? About 1738 km About 12 km About 520 km About 350 km About 3.5 km
49	Chapter 16-19 Review Example questions: Multiple choice: Though Titan is small, it is able to retain an atmosphere because? It is very cold. It is very dense. It rotates very slowly. It attracts gas from the solar wind. It has a very strong magnetic field.
50	Final Exam 30 Multiple Choice questions, 15 true/false, 3 essay/written questions, plus 1 follow-up extra credit problem (computational and meant to be challenging). About 1/3 of the questions the same as or slightly modified from previous exams About 1/3 of the questions covering the solar system About 1/3 of the questions completely new but covering old material Questions mostly cover the basics and are not intended to be subtle or tricky.
51	Final Exam List of possible topics for essay questions: Dark Matter Cosmic Microwave Background Radiation The Evolution of the Sun on the H-R diagram The Solar Nebula Extrasolar Planets Comparative Planetology of Venus, Earth, and Mars The Seasons Phases of the Moon The Distances to Astronomical Objects (Distance Ladder)