Age of the Solar System

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Age of the Solar System
The oldest rocks on Earth are meteorites which
fall from space and are about 4.55 billion years
old. The oldest native Earth rocks are 3.85
billion years old.
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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.
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parent
daughter
½ life (millions of years)
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Radioactive dating gives the time since the rock
solidified. In the molten state daughter
isotopes, which are lighter, can escape.
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Formation of the Solar System
The Sun and planets formed from the gravitational
collapse of a single, spherical, slowly rotating
cloud of cold interstellar gas and dust. This is
called the nebular hypothesis.
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Consequence
Planet formation is a natural outcome of star
formation. Thus planetary systems should be
common.
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Dynamics of the Planets

• The planets revolve counterclockwise around the
  Sun as viewed from above the Suns north pole,
  the same direction that the Sun rotates on its
  axis.

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Discussion
Why cant the all the planets just orbit in any
arbitrary direction? Why should they all go
around the Sun in the same way?
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2. The major planets have orbital planes that
are only slightly inclined with the plane of the
equator of the Suns rotation, i.e. the orbits
are coplanar.
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Discussion
What does this mean for the paths of the planets
through the sky?
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Orbital Inclination
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Discussion
If the solar nebula started as a spherical
cloud why is most of the mass, other than that
which is in the Sun, lie in a plane above the
Suns equator.
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3. The planets move in elliptical orbits that are
very nearly circular.
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Discussion
How would you describe the eccentricity of an
ellipse? What is the eccentricity of a circle?

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Eccentricities
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Discussion
Why are all the orbits nearly circular? What
happens to planets that formed with highly
eccentric (very elliptical) orbits?
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4. The planets rotate counterclockwise as viewed
from above the north pole, the same direction as
they revolve, except for Venus, Uranus and Pluto.
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Discussion
Why do you think all the planets rotate in the
same direction and why is this direction in the
same sense as the planets orbit the Sun?
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Rotation of the Planets
Period (days)
Axis tilt
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Discussion
Unlike all the other planets Venus rotates
backward. How would the diurnal and yearly
motion of the Sun differ on Venus than on the
Earth?
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Discussion
Is the solar day longer or shorter than the
sidereal day on Venus?
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Discussion
Which planet will have the most extreme seasons?
Which planets will experience almost no seasonal
changes?
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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.
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Titius-Bode Rule and Distance
Distance AU
T-B distance AU
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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.
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6. Most satellites revolve in the same direction
as their parent planets rotation and lie close
to their parent planets equatorial plane
An exception is Neptunes Triton
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Discussion
How would you explain this observation with our
formation theory of the Solar System?
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7. The Sun contains 99.8 of the solar systems
mass but only 0.5 of the angular momentum
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Discussion
If the Sun formed from a single spherical
rotating cloud, wouldnt you expect that all the
pieces would have the same angular momentum as
the original cloud? What does this mean for the
evolution of the Solar System after formation?
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Discussion
Either the Suns rotation rate has slowed over
time, or the planets have been spun up in the
orbits. How could we decide between these two
possibilities?
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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.
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Slowing the Suns rotation
Magnetic breaking The Suns magnetic field
might interact with the early solar nebula to
slow the Suns rotation. Strong solar winds
early in the history of the Sun might have
carried the extra angular momentum away.
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8. Long period comets come from all directions
and angles in contrast to the coplanar orbits of
the planets.
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Discussion
If the long period comets can have any
inclination, what does this tell you about their
distribution around the Sun?
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9. All the Jovian planets have rings and a large
number of moons
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10. All the Jovian planets have a core of
icy/rocky material with between 10-15 times the
mass of the Earth
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Discussion
Why do you think these cores are all about the
same size?
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Jovian Planets
Once at protoplanet reaches a mass of about 10
times that of the Earth, it can capture large
amounts of gas directly from the solar nebular,
becoming a Jovian planet.
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Discussion
Why do you think Uranus and Neptune didnt get as
big as Jupiter and Saturn?
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11. Most solid planetary surfaces are heavily
cratered
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Meteor Crater (1.2 km)
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Arizona from the Shuttle
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12. All planets are enriched with high Z elements
in comparison with the solar abundances
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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.
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Protoplanets
As the protoplanets grow by accretion of
planetesimals, their gravity increases spurring
more accretion.
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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
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Discussion
If the planets and Sun all formed from the same
nebula, why dont 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?
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