Transcript Lec9-020807

General Properties of
the Solar System …
continued
PTYS/ASTR 206
Solar System
2/8/07
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
PTYS/ASTR 206
Solar System
2/8/07
Next Week’s Preceptor-led Study Group
• Monday – 10:30AM-12:00PM
Preceptors: Chris Dockins, Maggie Jahn, Katie Landon,
and Jared Mosley
Room 330 of Kuiper Space Sciences
– We encourage you to attend and study for the exam
with a group of students from the class
PTYS/ASTR 206
Solar System
2/8/07
Solar-system inventory continued…
The Sun
• 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
PTYS/ASTR 206
Solar System
2/8/07
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)
PTYS/ASTR 206
Solar System
2/8/07
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
PTYS/ASTR 206
Solar System
2/8/07
• Asteroid belt
– Between the orbits of Mars
and Jupiter
– Probable origin of NearEarth objects
PTYS/ASTR 206
• Kuiper Belt Objects
– Beyond the orbit of Neptune
– Distributed loosely along the
ecliptic plane
– Pluto is a large KBO
Solar System
2/8/07
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
PTYS/ASTR 206
Solar System
2/8/07
Structure of a Terrestrial Planet
• Metallic core in center
• Rocky mantle
• Crust of some sort?
• All are differentiated
 Look up its definition !
– But the proportions of the
core, mantle, crust, differ
PTYS/ASTR 206
Solar System
2/8/07
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
PTYS/ASTR 206
Solar System
2/8/07
Planetary Magnetic Fields
• 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
PTYS/ASTR 206
Solar System
2/8/07
• 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
will206
be
PTYS/ASTR
Solar System
2/8/07
Impact Cratering
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
PTYS/ASTR 206
Solar System
2/8/07
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)
PTYS/ASTR 206
Solar System
2/8/07
Kinetic Energy and Temperature
• Kinetic Energy of a gas with
temperature T
k = Boltzmann constant
= 1.38 x 10-23 J/K
PTYS/ASTR 206
Solar System
2/8/07
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
PTYS/ASTR 206
Solar System
2/8/07
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:
PTYS/ASTR 206
Solar System
2/8/07
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
PTYS/ASTR 206
Solar System
2/8/07
Table
object
Escape
speed
(km/s)
Avg.
temp.
(K)
Oxygen
speed
(km/s)
Hydrogen
speed
(km/s)
Sun
618
5800
-
12
Yes / expanding
Earth
11.2
293
0.5
1.9
yes
Mars
5.0
240
0.4
1.7
yes - thin
Jupiter 59.5
125
0.3
1.2
yes
Pluto
40
0.2
0.8
yes/no
(comes and
goes)
PTYS/ASTR 206
1.3
Solar System
2/8/07
Atmosphere
?
The diversity of the solar system is a result
of 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
PTYS/ASTR
206
Solar System
2/8/07
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!
PTYS/ASTR 206
Solar System
2/8/07
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
PTYS/ASTR 206
Solar System
(upper right door)
2/8/07