1. The planets

2. Midterm! ► In next week Part I (take home exam, including 10 points from Mastering Astronomy, 50 pts) will be available, due October 26 th, noon ► In 2 weeks, Part II (in class exam, 50 pts.)  Taken in 3 rd hour (week of 10/22 to 10/25)  Bring SCANTRON (882 form) and #2 pencil  Based on “Review Questions” handout, available now! ► Also: 10 of the 25 extra credit points are due by October 26 th, noon.

3. © Sierra College Astronomy Department3 Lecture 7: A Planetary Overview Comparative Planetology ► In this chapter we wish to look at the solar system as whole and compare the worlds to each other, seeking to understand their similarities and differences – comparative planetology ► Planetology broadly includes moons, asteroids, and comets as well as the planets. ► We can see that the solar system is not a random collection of worlds

4. © Sierra College Astronomy Department4 Lecture 7: A Planetary Overview Comparative Planetology ► We see several values to comparative planetology (CP):  CP has revealed similarities and difference in the planets have helped guide the development of our theory of solar system formation  CP gives us deep new insights into the physical processes that have shaped the Earth and other worlds  CP allows us to apply lessons from our own solar system to the study of other solar systems.

5. © Sierra College Astronomy Department5 Lecture 7: A Planetary Overview Solar System Roll Call ► The Sun the is largest and brightest object in the solar system ► The Sun is hot (5800 K on surface) ► The Sun is gaseous and converts matter into energy in core ► The Sun has the greatest influence on the rest of the solar system (light, solar wind…) Sun

6. © Sierra College Astronomy Department6 Lecture 7: A Planetary Overview Solar System Roll Call ► Mercury is the smallest planet in the solar system ► It rotates every 58.6 days and revolves every 88 days and is tidally locked to the Sun ► The produces 88 days of daylight and 88 days of night, making temperatures extreme (425°C to -150°C; 800°F to -240°F). ► One spacecraft has visited Mercury and another is on its way on its wayon its way Mercury Tidal

7. © Sierra College Astronomy Department7 Lecture 7: A Planetary Overview Solar System Roll Call ► Venus is often called Earth’s “twin” because it is nearly the same size as the Earth. But it’s nothing like the Earth… ► It rotates backwards (or upside down) very slowly ► It is covered with an atmosphere of mostly CO 2 which allows a runaway greenhouse effect to occur raising the temperature to 470°C (880°F) planetwide ► Its surface pressure in 90 times greater than the Earth and there are clouds of sulfuric acid near the surface of the planet ► The Venus Express currently orbits Venus Venus Express Venus Express Venus

8. © Sierra College Astronomy Department8 Lecture 7: A Planetary Overview Solar System Roll Call ► Earth is only world that we know of that has or had life on it ► It is the only world with a significant amount of oxygen in the atmosphere ► It is the only world with significant amounts of liquid water ► It is the closest planet to the Sun to have a moon and our Moon is quite large compared to the Earth Earth

9. © Sierra College Astronomy Department9 Lecture 7: A Planetary Overview Solar System Roll Call ► Mars may bear the closest resemblance to the Earth ► It has a thin atmosphere of mostly CO 2 ► It has polar caps made of CO 2 and water-ice ► In the past, water very likely flowed on the surface ► It has great geological wonders such as a great canyon and the largest volcano in the solar system ► It has two tiny moons ► It is the most studied extraterrestrial planet and has several spacecraft present and proposed to land or orbit Mars. present and proposed present and proposed Mars moons

10. © Sierra College Astronomy Department10 Lecture 7: A Planetary Overview Solar System Roll Call ► Jupiter is largest planet in the solar system and is made mostly of gas with a Earth sized rocky-ice core in the center ► It has more than 300 times the diameter and 1000 times the volume of the Earth ► Its atmosphere has many storms many of which have lasted for hundreds of years ► Its four largest moons (of 63) have interesting properties too (active volcanoes, subsurface water, magnetic fields) Jupiter Jupiter2 Gal Moons

11. © Sierra College Astronomy Department11 Lecture 7: A Planetary Overview Solar System Roll Call ► Saturn is another gaseous giant planet with a spectacular ring system ► The ring system is made of millions of ice- dust chunks orbiting around the planet ► Saturn has 60 moons, a few of them midsize moons and one large one, Titan, which has a significant atmosphere. ► Currently, Cassini is orbiting around Saturn Cassini Cassini Saturn

12. © Sierra College Astronomy Department12 Lecture 7: A Planetary Overview Solar System Roll Call ► Uranus (YUR-uh-nus) is a smaller gas giant with a green- blue color due to methane ► It has several dozen moons a few of which are midsize ► The entire system (planet, rings, moons) is tilted on their side ► Neptune is just a bit smaller than Uranus and bluer in color ► It has about a dozen moons, one of which is large (Triton). Triton is the largest moon to go backward (retrograde) around the planet ► Both Neptune and Uranus has been visited by only one spacecraft (Voyager 2) Neptune Uranus Neptune2Uranus2 Neptune3 Uranus3

13. © Sierra College Astronomy Department13 Lecture 7: A Planetary Overview Solar System Roll Call ► Pluto (and the other Dwarf Planets) are round objects which orbit around the Sun ► Pluto was discovered as a planet in 1930, but was an oddball world. One of its 3 moons is half its size (Charon). It will be visited by spacecraft in 2015. by spacecraft in 2015by spacecraft in 2015 ► Soon in the 1990s other objects out where Pluto lived were being discovered. One of these, Eris, was found to be a little larger than Pluto ► In 2006, the phrase “dwarf planet” was defined for these objects and asteroids (like Ceres) which were round but were found “nearby” other solar system objects Pluto Eris Ceres

14. © Sierra College Astronomy Department14 Lecture 7: A Planetary Overview Solar System featurs ► Looking at the general characteristics, there are 4 features which stand out: 1.Patterns of motion among large bodies 2.Two major types of planets 3.Asteroids and comets 4.Exceptions to the rules Stat Sheet Stat Sheet 2

15. © Sierra College Astronomy Department15 Lecture 7: A Planetary Overview Distances In The Solar System Measuring Distances in the Solar System ► Copernicus used geometry to determine relative distances to the planets. ► Today we measure planetary distances using radar. ► Average distances to the planets from the Sun range from.387 AU for Mercury to 39.53 AU for Pluto.

16. © Sierra College Astronomy Department16 Lecture 7: A Planetary Overview Feature 1: Patterns of Motion ► All planetary orbits are ellipses, but all are nearly circular. ► Each of the planets revolves around the Sun in the same direction. ► All planets - except Venus, Uranus - rotate in a counterclockwise direction. ► Most of the satellites revolving around planets also move in a counterclockwise direction, though there are some exceptions. orbits Stat Sheet Stat Sheet 2

17. © Sierra College Astronomy Department17 Lecture 7: A Planetary Overview Feature 1: Patterns of Motion ► Inclination of a planet’s orbit is the angle between the plane of a planet’s orbit and the ecliptic plane (the plane of the Earth’s orbit). ► The elliptical paths of all the planets are very nearly in the same plane (inclination about 0°), though Mercury’s orbit is inclined at 7° and Pluto’s at 17°. tilts

18. © Sierra College Astronomy Department18 Lecture 7: A Planetary Overview Planet Diameters Diameters of Non-Earth Planets ► Diameters are determined from distances (from the Earth to the planet) and the planet’s angular size via the small angle formula (Mathematical Insight 2.1) ► Diameter of Sun (1.39 × 10 6 km) is over 100 times that of Earth (1.3 × 10 4 km). ► Jupiter’s diameter is 11 times that of Earth. ► Pluto’s diameter is 1/5 that of Earth.

19. © Sierra College Astronomy Department19 Lecture 7: A Planetary Overview Planet Masses Mass of the Planets ► Kepler’s third law was reformulated by Newton to include masses (Mathematical Insight 4.3): a 3 /p 2 = K (M 1 + M 2 ) ► Newton’s statement of Kepler’s third law allows us to calculate the mass of the Sun. ► Consider the orbits of planets around the Sun. Since one of the masses to the Sun (the other being a planet), the sum of the two is essentially equal to the mass of the Sun, and the equation can be rewritten as: a 3 /p 2 = KM

20. © Sierra College Astronomy Department20 Lecture 7: A Planetary Overview Planet Masses ► We can do the same sort of calculation for planets as long as they have satellites orbiting them ► The masses of 7 of the 9 known planets can be calculated based on the distances and periods of revolution of these planets’ natural satellites. ► For Mercury and Venus, which do not possess any natural satellites, accurate determinations of their respective masses had to await orbiting or flyby space probes.

21. © Sierra College Astronomy Department21 Lecture 7: A Planetary Overview Feature 2: Classifying the Planets ► The planets (except Pluto) fit into two groups: the inner terrestrial planets and the outer Jovian planets. Size, Mass, and Density ► The Jovian planets have much bigger diameters and even larger masses than the terrestrial planets. ► Terrestrial planets are more dense, however. ► Earth is the densest planet of them all. Stat Sheet Inside the planets Stat Sheet 2

22. © Sierra College Astronomy Department22 Lecture 7: A Planetary Overview Classifying the Planets Satellites and Rings ► The Jovian planets have more satellites than the terrestrials. ► 4 Jovian planets: 163 total satellites as of September 2007 (63 for Jupiter, 60 for Saturn, 27 for Uranus, and 13 for Neptune). ► 4 terrestrial planets: 3 total satellites. ► Pluto has 3 satellites. Eris has one satellite. ► Each Jovian planet has a ring or ring system. None of the terrestrial planets do.

23. © Sierra College Astronomy Department23 Terrestrial Jovian Near the Sun Far from the Sun SmallLarge Mostly solid Mostly liquid & gas Low mass Great mass Slow rotation Fast rotation No rings Rings High density Low density Thin atmosphere Dense atmosphere Few moons Many moons A comparison of planetary characteristics Stat Sheet Stat Sheet 2

24. © Sierra College Astronomy Department24 Lecture 7: A Planetary Overview Feature 3: Asteroids and Comets Asteroids ► These rocky bodies orbit the Sun, but are much smaller than planets. Most lie between Mars and Jupiter Comets ► Small icy (water, ammonia, methane) objects which occasionally visit the inner solar system and become visible ► Comets originate from two regions: the Kuiper Belt and the Öort Cloud Asteroids comets

25. © Sierra College Astronomy Department25 Lecture 7: A Planetary Overview Feature 4: Exceptions to the Rules ► There are objects in the solar system that are unusual or have characteristics which are unusual as compared to the rest of the solar system. Some examples:  Venus and Uranus rotate differently (backwards and on its side, respectively)  Small moons of Jupiter and Saturn and the large moon Triton (around Neptune) revolve in the opposite direction of the rotation of the host planet.  While other terrestrial planets have no moons (Mercury, Venus) or tiny moons (Mars) The Earth’s moon is large compared to the Earth. Asteroids comets

26. © Sierra College Astronomy Department26 Lecture 7: A Planetary Overview Spacecraft Exploration of the Solar System ► Our knowledge of the solar system has been dramatically increased by telescopic observations ► We gone to the Moon to directly explore the surface and bring back moon rock samples ► Other samples of the solar system have come to use via meteorites ► But most of our recent knowledge of the solar system has come from robotic spacecraft

27. © Sierra College Astronomy Department27 Lecture 7: A Planetary Overview Spacecraft Exploration of the Solar System There are 4 broad categories in which robotic mission may be classified: ► Flyby: spacecraft goes by planet once ► Orbiter: spacecraft orbits planet allowing longer term study ► Lander or Probe: spacecraft lands on planet (or sends a probe to explore the planet), some may have rovers for mobile surveys of the planet ► Sample return mission: spacecraft lands gets a sample of the surface and takes off to Earth All these mission carry some sort of radio for communication to and from the Earth

28. © Sierra College Astronomy Department28 Lecture 7: A Planetary Overview Spacecraft Exploration of the Solar System Flybys: ► Generally, the cheapest of the spacecraft types ► Fuel is only used to change the course of the spacecraft ► Some spacecraft such a Voyager 2 use gravity assists (gravity slingshots) to help the spacecraft change direction and increase speed ► They carry telescopes, cameras and spectroscopes. Voy 2

29. © Sierra College Astronomy Department29 Lecture 7: A Planetary Overview Spacecraft Exploration of the Solar System Orbiters: ► More expensive than flyby mission because they must carry more fuel so they can get into an orbit ► Sometimes an orbit may be very large and elliptical and must be changed to get it more circular and closer to the planet ► Some spacecraft have used a technique of aerobraking or skimming the atmosphere to shrink the orbit to a smaller size ► Orbiters have equipment like flyby spacecraft plus instruments to detect magnetic fields and radar to measure precise altitudes ► Orbiters have been sent to the Moon, Venus, Mars, Jupiter, Saturn and to the asteroid Eros

30. © Sierra College Astronomy Department30 Lecture 7: A Planetary Overview Spacecraft Exploration of the Solar System Landers or Probes: ► One can get the closest to a planet by landing on it or sending a probe through the atmosphere. ► Galileo sent a probe through Jupiter’s atmosphere sending information about temperature, pressure, composition, and radiation before the signal was too faint to be detected. It presumably was crushed by the high pressure atmosphere. ► Planets with solid surfaces, landers can provide close up views and local weather monitoring. Some landers may have rovers which can venture across the surface (like Spirit and Opportunity). ► Landers require more fuel since they must land softly on the surface, but the spacecraft which brought the rovers “crashed-landed” on the surface using protective airbags. ► Landers have been sent to the Moon, Venus, Mars. A probe went into Jupiter, and on Titan. Sprit landing

31. © Sierra College Astronomy Department31 Lecture 7: A Planetary Overview Spacecraft Exploration of the Solar System Sample Return Mission: ► The hardest mission of them all is to land on a surface, gather sample and return home. ► The only sample return mission are the Apollo mission to the moon, the Soviet robotic mission to the moon in the early 1970s ► A slight variation to this occurred with Stardust which collected comet dust and returned to the Earth ► There are plans to have a sample return mission to Mars

32. © Sierra College Astronomy Department32 Lecture 7: A Planetary Overview Spacecraft Exploration of the Solar System Combination spacecraft: ► Many mission have combined more than one type of spacecraft. ► The Viking missions of the 1970s had an orbiter and two landers ► The Galileo mission had an orbiter and a probe that went into the Jupiter atmosphere ► The Cassini mission had an orbiter and a probe (Huygens) that went to Titan List of selected robotic missions List of selected robotic missions Cassini

33. © Sierra College Astronomy Department 33 The End