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A Planetary Overview. 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

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A Planetary Overview

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A Planetary Overview


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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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


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A comparison of planetary characteristics

© Sierra College Astronomy Department


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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


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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


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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


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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


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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


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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


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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


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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


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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


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