Download October 3

Solar System Formation
Age of the Solar System
The oldest rocks found on Earth are about
4.55 billion years old, not native but
meteorites which fall from space.
The oldest native Earth rocks are 3.85 billion
years old.
Radioactive dating
Unstable parent isotopes decay at a constant
rate to stable daughter isotopes. By measuring
how much of the parent isotope is still present,
and how much of the daughter isotope there
is, we can calculate the age of the rock.
parent
daughter
½ life (millions of years)
Rubidium 187
Strontium 87
48800
Uranium 238
Lead 206
4470
Plutonium 244
Thorium 232
83
Iodine 129
Xenon 129
16
Manganese 53
Chromium 53
3.7
Aluminum 26
Magnesium 26 0.72
Discussion
The age of a rock given by radiometric dating
is the time since the rock last solidified.
What happens to the lighter daughter
elements when the rock is in a molten or
gaseous phase?
Formation of the Solar System
The nebular hypothesis
The Sun and planets formed from the
gravitational collapse (possibly triggered by
nearby supernova) of a single, spherical,
slowly rotating cloud of cold interstellar gas
and dust.
Discussion
Why does the gas cloud need to be cold?
Consequence
Planet formation is a natural outcome of
star formation.
Planetary systems should be common.
Dynamics of the Planets
1. The planets revolve counterclockwise
around the Sun as viewed from above the
Sun’s north pole, the same direction that
the Sun rotates on its axis.
Discussion
Why can’t all the planets just orbit in any
arbitrary direction? Why should they all go
around the Sun in the same way?
2. The major planets have orbital
planes that are only slightly inclined
with the plane of the equator of the
Sun’s rotation, i.e. the orbits are
coplanar.
Orbital inclination
Orbital Inclination
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
7.004
3.394
0
1.85
1.308
2.488
0.774
1.774
Discussion
What does this mean for the paths of the
planets through the sky?
Discussion
If the solar nebula started as a spherical cloud
why do all the planets lie in a plane above the
Sun’s equator. Shouldn’t they be spherically
distributed about the Sun?
Gravity can collapse
a rotating cloud only
along the axis of
rotation.
Discussion
What two changes take place as the solar
nebula collapses due to gravity?
3. The planets move in elliptical orbits
that are very nearly circular.
Eccentricities
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
0.206
.007
.017
.093
.048
.056
.046
.010
Discussion
Why are all the orbits nearly circular? What
happens to planets that formed with
highly eccentric (very elliptical) orbits?
4. The planets rotate counterclockwise as
viewed from above the north pole, the
same direction as they revolve, except for
Venus and Uranus.
Rotation of the Planets
Period (days)
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
58.6
-243.0
0.997
1.026
0.41
0.43
-0.72
0.67
Axis tilt
0.0
177.4
23.5
25.2
3.1
26.7
97.9
29.0
Discussion
Why do you think the planets rotate in the
same direction and why is this direction in
the same sense as the planets orbit the
Sun?
Discussion
Unlike all the other planets Venus rotates
backward. How would the diurnal and
yearly motion of the Sun differ on Venus
compared to that of the Earth?
Discussion
Is the solar day longer or shorter than
the sidereal day on Venus?
Discussion
Which planet will have the most extreme
seasons?
5. The Planets’ orbital distance
from the Sun follows a regular
spacing.
Titius-Bode rule
Write down 0, 3, 6, 12, … each number, after
the first, being double the previous value. Add
4 to each and divide by 10.
Titius-Bode Rule and Distance
Distance AU
Mercury
Venus
Earth
Mars
(Ceres)
Jupiter
Saturn
Uranus
Neptune
0.39
.72
1.0
1.52
2.77
5.2
9.54
19.18
30.06
T-B distance AU
0.4
0.7
1.0
1.6
2.8
5.2
10.0
19.6
38.8
A packed Solar System?
The solar system may be as densely packed as
possible. There do not appear to be any
orbits stable over the lifetime of the solar
system between the current planets.
6. Most satellites revolve in the
same direction as their parent
planet’s rotation and lie close to
their parent planet’s equatorial
plane
An exception is Neptune’s Triton
Discussion
How would you explain this observation with
our formation theory of the Solar System?
7. The Sun contains 99.8% of the
solar systems mass but only 0.5%
of the angular momentum
Discussion
If the Sun formed from a single spherical
rotating cloud, wouldn’t you expect that all
the pieces would have the same angular
momentum as the original cloud? How
must the solar system have changed since
the time of its formation that this is no
longer the case?
Discussion
Either the Sun’s rotation rate has slowed over
time, or the planet’s have been spun up in
their orbits. How could we decide between
these two possibilities?
The Sun rotates once every 33 days, but
should rotate once in about 2 hours if angular
momentum were distributed evenly.
This two hour rotation rate is common among
other young solar mass stars elsewhere in the
galaxy as well as higher mass stars.
Slowing the Sun’s rotation
Magnetic breaking – The Sun’s magnetic
field might interact with the early solar
nebula to slow the Sun’s rotation.
Strong solar winds early in the history of the
Sun might have carried the extra angular
momentum away.
8. Long period comets come
from all directions and orbital
inclinations in contrast to the
coplanar orbits of the planets.
Discussion
If the long period comets can have any
inclination, what does this tell you about
their distribution around the Sun?
2 types of planets
Terrestrial planets – iron-nickel cores and silicate
mantles
Jovian planets – silicate/hydrogen compound
(methane, ammonia, water) cores and mostly H
and He mantles
Terrestrial & Jovian planets
Density
Mass of the planet divided by the volume
of the planet.
Higher density implies a larger
percentage of high density materials,
such as iron and nickel, lower density
implies more silicates.
Mean density of planets
Earth
Mercury
Venus
Mars
Moon
5.5 g/cm3
5.4 g/cm3
5.2 g/cm3
3.9 g/cm3
3.4 g/cm3
Neptune
Sun
Jupiter
Uranus
Saturn
1.6 g/cm3
1.4 g/cm3
1.3 g/cm3
1.3 g/cm3
0.7 g/cm3
9. Most solid planetary surfaces
are heavily cratered
Meteor Crater (1.2 km)
Arizona from the Shuttle
10. All the Jovian planets have
rings and a large number of
moons
11. All the Jovian planets have a
core of icy/rocky material with
between 10-15 times the mass of
the Earth
12. All planets are enriched with
heavier elements in comparison
with the solar abundances
The Planetesimal Hypothesis
Fluffy dust grains condensing out of the solar
nebula stick together as a result of low-speed
collisions, building up to small bodies called
planetesimals.
Protoplanets
As the protoplanets grow by accretion of
planetesimals, their gravity increases
spurring more accretion.
Simulation
Solar nebula composition
We expect that the solar nebula from which
the Sun formed, had the same composition as
the current solar surface.
98% hydrogen and helium
1.4% hydrogen compounds – CH4, NH3, H2O
0.4% silicate rocks
0.2% metals
Discussion
If the planets and Sun all formed from the
same nebula, why don’t all the planets and the
Sun of the same chemical composition?
The outer planets have about the same
composition as the Sun but the terrestrial
planets do not. Why?