Lecture 7 Age and Formation of
the Earth, Sun and Solar System
Focus Question-How do we know
about early stages of the Earth and the solar system?
1.
The Sun,
Earth and solar system are related in their timing of origin and in their
composition. To see that
relatedness, we use three sources of information: Light, Elements, and Isotopes.
2.
Light:
Light can help us understand the composition of stars, and especially our
own star, the sun.
a.
Stars have a life cycle.
From interstellar dust and gas and nebula they form, and spend most of
their lives burning hydrogen to produce helium in a nuclear reaction.
This time spent burning hydrogen is called the Main Sequence.
Once the hydrogen is used up, helium will be burned, and in a series of
steps, heavier elements are produced, just as helium was produced from burning
hydrogen. If large enough they
will enter the Red Giant phase, where they will continue to produce elements
through to iron. Once iron is
exhausted, a supernova explosion may occur,in which elements heavier than iron
are produced. New stars will
then form from this interstellar medium, and those stars may have recycled
elements in them from previous supernovas.
b.
Visible
light can be broken into distinct wavelengths (ROYGBIV-the visible light
spectrum) as it passes through a prism.
When the spectrum from starlight is examined, dark lines (absorbtion
lines) are visible, indicating the presence of specific elements in the stars.
So, a main sequence star, which is only burning hydrogen to produce
helium, may have other elements in its spectrum, evidence that the star formed
from dust and nebula from previous supernovas.
c.
The Sun is a main sequence star.
It burns hydrogen to produce helium.
Its spectrum shows
absorbtion lines of other elements, indicating that it formed from previous dust
and nebula of past supernovas.
3.
Meteorites can also tell us about the composition
of the solar system, and its age.
a.
Iron and
Stony Iron meteorites-these are composed of iron or heavy metals, very like the
cores and mantles of rocky inner planets-the rocky inner planets are Mercury,
Venus, Earth and Mars.
b.
Carbonaceous chondrites-these are meteorites that
are high in carbon, not heavy metals.
c.
Radiometric dating shows that the meteorites
formed around 4.5 billion years ago.
4.
Moon
Rocks-these also have dates ranging from 3.8 to 4.5 billion years, so about the
same age as the meteorites.
a.
Moon origin-based on the composition and age of
the rocks, the current hypothesis is that the Moon formed from debris following
a collision between Earth and an asteroid about the size of Mars.
b.
The asteroid impact is inferred from 1) the
Moon’s mantle being of slightly different composition than Earth’s, 2) the small
core, and 3) the scarcity of water in the minerals, all likely outcomes of being
made from asteroid-earth impact debris.
5.
Earth’s
oldest rocks are 3.8 to 4.2 billion years old (Greenland) but the oldest
minerals, which eroded from rocks now gone, are the Jack Hills zircons of
Australia. They are about 4.2-4.4
billion years old.
6.
Earth’s layers (core, mantle, crust) formed
early, and based on studies of the composition of the shields, observations
indicate
a. Crust
was thin, early Earth was hot, but early oceans were present
b.Water in
those oceans came from Earth’s interior, asteroid impacts
c.Dissolved
minerals in those early oceans came from the Earth’s interior, and included lots
of metals, such as iron and manganese
d. High temperature experiments show that granitoid continental crust formed from ocean basalt crust.
7. So we can answer this focus question: We know about the early stages of the Earth and solar system because we can compare the ages and composition of meteorites, moon rocks, and the oldest rocks and minerals on Earth, and we can study the composition of the Sun from the spectrum of light coming from the Sun. These separate lines of evidence indicate that the Earth, Sun, and solar system are all about the same age and compositionally related.
Next Lecture: Early Life: Archean and Proterozoic Eons