1AST 2010: Chapter 61 February 2005

Introduction to Introduction to the Solar the Solar

SystemSystem

1 February 2005 AST 2010: Chapter 6 2

Some Basics Facts

The solar system consists of The SunNine planetsMore than a hundred (100) satellites of the planetsA large number of smaller bodies

Asteroids and comets

Cosmic dustCountless grains of broken rock

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Planetary Orbits in the Solar System

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The Sun and the four planets closest to it belong to what is called the inner solar systemThus the inner planets are

Mercury VenusEarthMars

These 4 inner planets are also called terrestrial planets

The Earth and the other three are similar in that they are composed mainly of rocks and metals

Inner Solar System

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Outer Solar SystemThe part of the solar system outside the inner solar system is called the outer solar system Thus, the planets in the outer solar system are

Jupiter Saturn UranusNeptunePluto

Jupiter, Saturn, Uranus, and Neptune are several times larger than the Earth and hence are called the giant planets

They are also called the jovian planets

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Some More Basic Facts

Sun 99.80Jupiter 0.10Comets 0.05

All other planets 0.04

Satellites & rings 0.00005

Asteroids 0.000002

Cosmic dust 0.0000001

ObjectPercentage of solar system’s

total mass

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The Nine Planets (1)

The planets’ sizes are not to scale

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Inner planets

The Nine Planets (2)

Outer planets

The planets’ sizes are to scale

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Mercury 0.39 0.24 4,878 3.3 5.4

Venus 0.72 0.62 12,102 48.7 5.3Earth 1.00 1.00 12,756 59.8 5.5Mars 1.52 1.88 6,787 6.4 3.9

Jupiter 5.20 11.86 142,984 18,991 1.3

Saturn 9.54 29.46 120,536 5,686 0.7

Uranus 19.18 84.07 51,118 866 1.2

Neptune 30.06 164.82 49,660 1,030 1.6

Pluto 39.44 248.60 2,200 0.01 2.1

NameDistance from Sun

(AU)

Revolution Period(Years)

Diameter(km)

Mass(1023 kg)

Density(g/cm3)

Main Characteristics of the Planets

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(Natural) SatellitesOnly Mercury and Venus do not have a moon or (natural) satelliteThere are more than 100 satellites known today

More are being discovered on a regular basis

The largest of the satellites are as big as a small planet, including

our Moon the four largest moons of Jupiter

called the Galilean satellites after their discoverer

the largest moons of Saturn and Neptune respectively called Titan and Triton

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Rings (1)Each of the jovian planets has rings made up of countless small bodies

They range in size from grains of dust to terrestrial mountains

Each ring system orbits its planet at its equator

The rings of Saturn are the best known, the widest, and by far the easiest to see

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Rings (2)

Recent missions near the jovian planets showed that the rings

have complex shapesare influenced by the pull of the planets' satellites

Jupiter’s rings Uranus’ rings Neptune’s rings

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AsteroidsThese are

rocky and metallic objects, quite modest in sizeimportant members of the solar system found in great number between the orbits of Mars and Jupiternow believed to be remnants of the initial population of the solar system

Some of the smallest satellites of the planets are believed to be asteroids captured by the gravitational pull of the planets

The two moons of Mars are nowadays thought to have such an origin

Asteroids Mathilde, Gaspra, and Ida

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Comets

These are another class of small bodiesThey are composed in part of ice, made of frozen gases such as water, carbon dioxide, carbon monoxide, and methyl alcohol

Comets are believed to be remnants from the formation of the solar systemWith rare exceptions, comets orbit the Sun in distant, cooler regions

Their orbits are large and very eccentric

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Cosmic DustSolar system contains countless grains of broken rock which one refers to simply as cosmic dust    Comic-dust particles constantly collide with the planets and become trapped by their gravitational pull

Million of these hit Earth's atmosphere every dayWhen these particles enter the atmosphere, they heat up quickly, burning and producing brief flashes of light that we see as shooting stars or meteors

Occasionally some of the larger chunks survive the passage through the atmosphere and land on Earth to become meteorites

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Terrestrial or Rocky PlanetsMercury, Venus, Earth, and Mars are called terrestrial or rocky planets They

are composed primarily of rocks and metalshave relatively high densitieshave relatively slow rotationhave solid surfaceshave no rings and few satelliteshave oxidized chemistry

because they are largely composed of oxygen compounds

Venus has an unusual direction of rotation about its axis, compared to the other planets

It spins on its axis backwards

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Jovian or Giant Planets

Jupiter, Saturn, Uranus, and Neptune are called jovian or giant planetsThey

are composed primarily of hydrogen and heliumgenerally have low densitieshave relatively rapid rotationhave deep atmosphereshave extensive ring system and many satelliteshave reduced chemistry

because they are largely composed of hydrogen and its compounds

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Trends in TemperaturePlanets and satellites

do not generate their own heat as the Sun doesare heated by the radiant energy of the Sun

Mathematically, the temperatures decrease approximately in proportion to the square root of the distance from the Sun Thus, the farther a planet/satellite is from the Sun, the cooler it is

Mercury has a surface temperature of 500 K

Pluto’s temperature is only about 50 K, which is colder than water’s freezing point (273 K)

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Earth’s Uniqueness for Life Support

Because of the strong dependence of temperature on distance from the Sun, Earth is the only planet where

the surface temperatures lie between the freezing and boiling points of waterwater can be liquid on the planet’s surface

Therefore, Earth is the only planet that can support life — at least life as we know it

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Physical Appearances

The external appearance of a planet is determined by

its surface compositionexternal bombardments on itgeological activity on the planet

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Planets under Attack!!

Planets are bombarded with a slow rain of projectiles from space

The impact of the projectiles on the surface of the planet leaves craters of all sizesThe amount of craters a planet has on its surface may provide clues about its history

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Shoemaker-Levy 9

A dramatic example: impacts of large pieces of Comet Shoemaker-Levy 9 with Jupiter in summer 1994

Sequence of images taken by Hubble Space Telescope

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Geological Activity (1)

The geological activity on terrestrial planets involves the internal forces that constantly reshape their surfaces by

buckling and twisting their crustsbuilding up mountain rangescreating and erupting volcanoes

Geological activity results from a hot interiorMountains/volcanoes arise from the buildup and release of heat escaping from a planet’s core

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Geological Activity (2)Each of the planets is believed to be heated at the time of its birth

This heat initially powered extensive volcanic activity

Small bodies, such as our Moon, soon cooled offThe larger the body, the more likely it is to retain its internal heat

Thus, it is more likely to see surface evidence of geological activity on the larger planets/satellitesThe inner planets appear to conform to this expectation

Mercury and our Moon are geologically deadEarth and Venus are still geologically activeMars represents an intermediate case

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The Dating GameTwo popular methods for estimating the age of a planet or satellite or its surface

Crater countingRadioactivity dating

The crater counting method is based on the assumptions that

the rate at which asteroids or comets bombard a planet’s surface is roughly constant for a long timethe planet’s surface has not been reshaped, and the craters have never been erased, since it experienced a major change

If these assumptions are satisfied, then the number of craters will be proportional to the length of time the surface has been exposed

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Counting Craters

The number of craters on the surface of a planet provides a clue about the length of time the surface has undergone cosmic bombardment since it solidified or geological activity stopped

Mercury

Moon

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Radioactive Decays (1)

Radioactivity is a natural phenomenon that was discovered at the beginning of the 20th century

Some atomic nuclei are not stable, but can spontaneously split apart, or decay, into smaller nucleiSuch nuclei are said to be radioactiveFor any one radioactive nucleus, the decay process happens randomly — it is not possible to predict when the decay will occurRadioactive decays involve the emission of particles, such as electrons, or of radiation in the form of gamma rays

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Radioactive Decays (2)The rate at which nuclei decay is most easily expressed in terms of their half-life

The half-life is a specific time period during which the chances are fifty-fifty that decay will occur for any one of a large number of radioactive nucleiFor example, if you had 1 gram of pure radioactive nuclei of one type whose half-life is 10 years, then after 10 years you would have ½ gram, after 20 years ¼ gram, after 30 years 1/8 gram, and so on

After many half-lives, the original radioactive nuclei do not disappear, but instead are replaced by their decay products

Sometimes the original nuclei are called parents and the decay products are called daughters

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Radioactive Decays (3)The number of radioactive nuclei in a sample decreases exponentially

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Radioactive Elements as Clocks

Radioactive elements can serve as natural clocks if one can determine how many of the initial radioactive parents have been replaced with their daughters

By comparing how much of a radioactive parent material is left in a rock with how much of its daughter material has accumulated, one can learn how long the decay process has been going on and hence how long ago the rock formedThis method involves the assumption that the rock being studied has been isolated since its formation — none of the parents and daughters in the rock have leaked out of it or been polluted by outside contaminants

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Radioactive Decay Reactions Used to Date Rocks

Parent Nucleus Daughter Nucleus Half-Life (billion year)

Samarium (147Sa) Neodymium (143Nd) 106

Rubidium (87Ru) Strontium (87Sr) 48.8

Thorium (232Th) Lead (208Pb) 14.0

Uranium (238U) Lead (206Pb) 4.47

Potassium (40K) Argon (40Ar) 1.31

Common Dating Elements

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Ages of the Moon and EarthAstronauts that visited the Moon brought back lunar rock for radioactive age dating

Counting craters had given different ages for different parts of the Moon’s surface

Samples brought back enabled an accurate dating of the lunar surface which showed that the Moon is an ancient, geologically dead bodyRadioactive dating of rocks brought back by the Apollo mission and of rocks here on Earth indicated that the two bodies have similar ages

According to this dating method, both were formed some 4.5 billion years ago

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Origin of the Solar System

An observation of the Sun and the nine planets of reveals some patterns

The planets all revolve around the Sun in the same directionAlso, the planets lie in nearly the same flat planeThe Sun spins in the same direction about its own axis

Astronomers regard these facts as evidence that the Sun and the planets formed together as a spinning system of gas and dust, referred to as the solar nebula

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Origin and Composition (1)

The composition of planets provides another clue about the origin of the solar system

Spectroscopic analysis allows a determination of what chemical elements are present in the Sun and the planetsSuch analysis shows that the Sun has the same hydrogen-dominated composition as Jupiter and Saturn

This suggests that these three bodies were formed from the same reservoir of material

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Origin and Composition (2)By contrast, spectroscopic analysis shows that the terrestrial planets and satellites

are relatively deficient in the light gases and the various ices composed of the common elements oxygen, carbon, and nitrogencontain mostly heavy elements like iron and silicon — these are rare on the Sun and the giant planets 

This pattern suggests that the processes which led to the formation of the inner planets must have somehow rejected much of the lighter materials

These appear to have escaped, leaving behind just a residue of heavy stuff

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Other Planetary Systems (1)An additional approach to understanding the origin of the solar system is to look outside the solar system for evidence

Many stars is space are much younger than the Sun, and planet formation might be occuring in some of these star systems that could be accessible to direct observation

Astronomers have observed other “solar nebulas” or circumstellar disks — flattened spinning clouds of gas and dust surrounding young stars

These are believed to be examples of what our solar system may have looked like when it started to form

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Other Planetary Systems (2)Unfortunately, because of the enormous distance involved, and the fact that planets forming are not likely to reflect light very efficiently, astronomers have yet to observe the actual formation of planets in distant solar nebulasAstronomers have now developed tools that enable the observation of planets orbiting distant stars

The technique is still limited to the detection of giant planets only

Observations nonetheless indicate clearly that other solar systems have configurations that vastly differ from the configuration of our solar system

Some have planets with very elliptical orbitsIn other cases, giant planets are very near their stars