General Properties of the Solar System … continued

PTYS/ASTR 206 Solar System2/8/07

General Properties of the Solar System …

continued


Announcements

• Reading for next class– 8-4, 8-5, 8-6 (pp. 171-180)

• Quiz today– Closed book, closed note, no electronic devices (like it will be for the

exam)

• First Exam Next Thursday (2/15)– Brief review and discussion of the exam format on Tuesday– Come prepared with questions– Make use of study groups, instructor and TA office hours to help

you prepare


Next Week’s Preceptor-led Study Group

• Monday – 10:30AM-12:00PMPreceptors: Chris Dockins, Maggie Jahn, Katie Landon, and Jared MosleyRoom 330 of Kuiper Space Sciences

– We encourage you to attend and study for the exam with a group of students from the class


• Most massive object in the solar system

• Formed at about the same time as all of the planets, and from the same material

• The source of energy that keeps is shining for billions of years is thermonuclear fusion

Solar-system inventory continued…The Sun


Solar-system inventory continued… The Solar Wind

• The solar corona is in a constant state of expansion and continues off into space, creating the Solar Wind

• The Solar Wind is a plasma – the 4th state of matter (solid, liquid, and gas are the other 3)

• Its existence was predicted based on observations of comet tails (the blue ion tail in the picture is directed along the solar wind)


Solar-system inventory continued…Small chunks of rock and ice also orbit the Sun

• Asteroids are small, rocky objects, while comets and Kuiper-belt objects are made of dirty ice (or icy dirt?)

• All are remnants left over from the formation of the planets

• Some of them contain the primordial material from which the solar system is made


• Asteroid belt

– Between the orbits of Mars and Jupiter

– Probable origin of Near-Earth objects

• Kuiper Belt Objects– Beyond the orbit of Neptune– Distributed loosely along the

ecliptic plane– Pluto is a large KBO


Solar-system inventory continued… The outer reaches of the Solar System

• The Heliosphere– The cavern carved out of the

interstellar gas by the solar wind

• The Oort Cloud– contains billions of comet nuclei in a

spherical distribution that extends out to 50,000 AU from the Sun

– Intermediate period and long-period comets are thought to originate in the Oort cloud

– As yet no objects in the Oort cloud have been detected directly


Structure of a Terrestrial Planet

• Metallic core in center

• Rocky mantle

• Crust of some sort?

• All are differentiated– But the proportions of the

core, mantle, crust, differ

Look up its definition !


Will a planet have active volcanoes?

• Requires Heat – After the planets formed, they

were very hot– Big planets cool slower – Small planets cool more rapidly

• Big terrestrial planets are active longer– Fewer craters– More likely to have active

volcanoes• Earth and Venus for example –

both of these worlds also have very few visible craters


• Another important tool for “probing” the interior of a planet

• Magnetic fields of terrestrial planets are produced by metals such as iron in the liquid state (molten core) and in motion (dynamo action) – moving electrically conducting material

• The stronger fields of the Jovian planets are generated by liquid metallic hydrogen or by water with ionized molecules dissolved in it

• Earth, Mercury, and all Gas Giants have magnetic fields – Mars and Venus do not

Planetary Magnetic Fields


Impact Cratering

• When an asteroid or comet strikes the surface of a terrestrial planet or moon, the result is an impact crater

• Geologic activity renews the surface and erases craters, so a terrestrial world with extensive cratering has an old surface and little or no geologic activity

• Because geological activity is powered by internal heat, and smaller worlds lose heat less rapidly than larger ones … as a loose general rule … the

smaller a world is, the more heavily cratered it will be


Will a planet have an atmosphere?

• Requires a gas– The gas must be cool

enough to not escape– The planet must have

enough gravity to prevent the escape of gasses

• Big, cool, planets are more likely to have atmosphere


To understand the retention of an atmosphere, we need to understand the motion of particles in a gas

• Kinetic Energy associated with an object of mass m in motion with a speed v

• SI unit of energy– Joule (kg m2/s2)


Kinetic Energy and Temperature

• Kinetic Energy of a gas with temperature T

k = Boltzmann constant

= 1.38 x 10-23 J/K


Average speed of atoms in a gas

• Equate kinetic energy of motion to that of the gas at a given temperature, and solve for the velocity, v

This is the AVERAGE SPEED of atoms in a gas having a

temperature T


To understand whether the gas is gravitationally bound to a planet, we need to understand the

concept of Escape velocity

• The speed that an object must have in order to escape the pull of gravity of a planet of mass M and radius R is:


As a loose, general rule of thumb:

• A Planet can retain a gas if the escape speed is at least 6 times greater than the average speed of molecules in the gas


Tableobject Escape

speed (km/s)

Avg. temp. (K)

Oxygen speed (km/s)

Hydrogen speed (km/s)

Sun 618 5800 - 12

Earth 11.2 293 0.5 1.9

Mars 5.0 240 0.4 1.7

Jupiter 59.5 125 0.3 1.2

Pluto 1.3 40 0.2 0.8

Atmosphere?

Yes / expanding

yes

yes - thin

yes

yes/no(comes and

goes)


The diversity of the solar system is a resultof its origin and evolution

• The planets, satellites, comets, asteroids, and the Sun itself formed from the same cloud of interstellar gas and dust

• This material came from cosmic processes that took place within stars that died long before our solar system was formed

• Different planets formed in different environments depending largely on their distance from the Sun


How Old is the Solar System ?

• How can we determine this ?– Radioactive dating– Need to find the right material to

date !– Because of plate tectonics and

geological activity, Earth rocks are not a good indicator of the age of the Solar System

– Meteorites!


Today’s quiz• Be sure to fill in the ovals for your name

(last name first!!!)

• Closed book, closed notes, no electronic devices

• The quiz has 15 questions (front and back)– Fill in the oval corresponding to your answer on the scantron sheet

using a #2 pencil

• Only turn in the scantron sheet – you may take the quiz itself with you when you leave

• You may leave when you are finished – but please do so as quietly as possible and leave through the North Entrance (upper right door)

Documents

General Properties of the Solar System … continued