A Planetary Overview

1. A Planetary Overview

2. A Planetary OverviewComparative 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 © Sierra College Astronomy Department

3. A Planetary OverviewComparative 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. © Sierra College Astronomy Department

4. A Planetary OverviewSolar System Roll Call • The Sun theis 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…) © Sierra College Astronomy Department

5. A Planetary OverviewSolar 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 has flown by and will orbit Mercury © Sierra College Astronomy Department

6. A Planetary OverviewSolar 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 CO2 which allows a runaway greenhouse effect to occur raising the temperature to 470°C (880°F) planet-wide • Its surface pressure in 90 times greater than the Earth and there are clouds of sulfuric acid near the surface of the planet • TheVenus Express currently orbits Venus © Sierra College Astronomy Department

7. A Planetary OverviewSolar 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 © Sierra College Astronomy Department

8. A Planetary OverviewSolar System Roll Call • Mars may bear the closest resemblance to the Earth • It has a thin atmosphere of mostly CO2 • It has polar caps made of CO2 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. © Sierra College Astronomy Department

9. A Planetary OverviewSolar 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) © Sierra College Astronomy Department

10. A Planetary OverviewSolar 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 © Sierra College Astronomy Department

11. A Planetary OverviewSolar 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) © Sierra College Astronomy Department

12. A Planetary OverviewSolar 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. • 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 © Sierra College Astronomy Department

13. A Planetary OverviewSolar System Features • Looking at the general characteristics, there are 4 features which stand out: • Patterns of motion among large bodies • Two major types of planets • Asteroids and comets • Exceptions to the rules © Sierra College Astronomy Department

14. A Planetary OverviewDistances 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. © Sierra College Astronomy Department

15. A Planetary OverviewFeature 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. © Sierra College Astronomy Department

16. A Planetary OverviewFeature 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°. © Sierra College Astronomy Department

17. A Planetary OverviewPlanet 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 × 106 km) is over 100 times that of Earth (1.3 × 104 km). • Jupiter’s diameter is 11 times that of Earth. • Pluto’s diameter is 1/5 that of Earth. © Sierra College Astronomy Department

18. A Planetary OverviewPlanet Masses Mass of the Planets • Kepler’s third law was reformulated by Newton to include masses (Mathematical Insight 4.3): a3/p2 = K (M1 + M2) • 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: a3/p2 = KM © Sierra College Astronomy Department

19. A Planetary OverviewPlanet 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. © Sierra College Astronomy Department

20. A Planetary OverviewFeature 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. © Sierra College Astronomy Department

21. A Planetary OverviewClassifying the Planets Satellites and Rings • The Jovian planets have more satellites than the terrestrials. • 4 Jovian planets: 164 total satellites as of March 2009 (63 for Jupiter, 61 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. © Sierra College Astronomy Department

22. A comparison of planetary characteristics © Sierra College Astronomy Department

23. A Planetary OverviewFeature 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 © Sierra College Astronomy Department

24. A Planetary OverviewFeature 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. © Sierra College Astronomy Department

25. A Planetary OverviewSpacecraft 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 © Sierra College Astronomy Department

26. A Planetary OverviewSpacecraft 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 © Sierra College Astronomy Department

27. A Planetary OverviewSpacecraft 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. © Sierra College Astronomy Department

28. A Planetary OverviewSpacecraft 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 © Sierra College Astronomy Department

29. A Planetary OverviewSpacecraft 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. © Sierra College Astronomy Department

30. A Planetary OverviewSpacecraft 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 © Sierra College Astronomy Department

31. A Planetary OverviewSpacecraft 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 © Sierra College Astronomy Department