ORIGINS: HOW DID IT ALL BEGIN?
ORIGINS: HOW DID IT ALL BEGIN?
• The topic of Origins is perhaps the largest of all
imaginable topics of scientific research, as it includes
everything (both matter and energy) that we know, or
expect, that exists in our universe.
• The following topics, in order of both size and time,
are included in the broader topic of Origins:
1. Origin and Evolution of the Universe
2. The Origin of the Solar System
3. Formation and Early History of Earth
4. Geologic Time and the Rock Record
5. The Chemistry and Origin of Life
IN THE BEGINNING:
THE ORIGIN AND EVOLUTION OF THE UNIVERSE
• The age of the Universe was not well established until
recently, but is now thought to be about 13.7 billion
years, or about 3 times the age of our Solar System.
• The Universe is thought to have originated in a point-like
concentration, which began expanding outward in a “Big
Bang” at the speed of light.
• At the initial temperature and pressure of the early
Universe, matter existed only as elementary particles.
Only after a million years or so of expansion, did the
matter cool enough to form atoms as we know them
today (mostly hydrogen and helium).
IN THE BEGINNING:
THE ORIGIN AND EVOLUTION OF THE UNIVERSE
• As the Universe expanded and cooled, the material
concentrated in individual clumps which formed protogalaxies.
• Within the proto-galaxies, the gas further congregated into
many smaller and denser clumps which in turn condensed
to form stars.
• These original stars, like our Sun and other stars today,
generated heat and light by the fusion of hydrogen
atoms to form helium, and in turn, still heavier elements.
The above diagram is intended only to show, in much simplified
form, the expansion of the Universe, at a maximum speed equal to
that of light, since its origin about 13.7 billion years ago.
In actuality, we cannot define an “outer edge” of the Universe. No
matter where we are in the Universe, it will appear that we are at the
center of the expansion- the “cosmological principle”.
IN THE BEGINNING:
THE ORIGIN AND EVOLUTION OF THE UNIVERSE
• The first generation of stars, made up of almost pure
hydrogen and helium, were much more massive, on the
average, than stars forming at the present time.
• These very massive first-generation stars evolved much
more rapidly than the average of new stars today, and at
the ends of their lives formed heavier elements (such as
carbon, oxygen, silicon, and iron), which were then
returned to the interstellar medium in supernova
explosions.
• Following generations of stars were formed from
interstellar material increasingly richer in these heavy
elements.
THE ENERGY SOURCES OF STARS
• Stars, including our Sun, derive their energy from a source
much more efficient than ordinary chemical reactions (such
as combustion): namely, thermonuclear fusion of the nuclei
of light atoms to form nuclei of heavier ones.
• The primary source of energy through the normal lifetimes of
stars is the fusion of four hydrogen atom nuclei (protons) to
form the nucleus of one helium atom (two protons and two
neutrons).
• Albert Einstein postulated, as part of his Theory of Relativity,
that matter can be converted to energy (and vice versa) by
the relationship, E = mc2 (where c = the speed of light =
300,000 kilometers per second).
• The helium nucleus is only slightly less massive than the four
protons combined to make it, but this difference results in the
release of an enormous amount of energy!
THE ENERGY SOURCES OF STARS
• The mass of a Hydrogen Atom is 1.007825 AMU (Atomic
Mass Units)
• The mass of a Neutron is 1.008665 AMU
• The mass of 2 H + 2 n is 4.032980 AMU
• The mass of a Helium Atom is only 4.002603 AMU. We
are missing 0.030377 AMU!
• According to Einstein, the energy equivalent of matter is:
Energy = Mass x (Speed of Light)2, or E = mc2.
• Therefore, 1 AMU = 931.5 MeV (million electron volts) and
the mass deficiency of 4H
He = 28.3 MeV, compared
with 2 H2 + O2
2 H2O = 5 eV!
THE ENERGY SOURCES OF THE SUN AND STARS
• Hydrogen constitutes about 90% of all the atoms in the Sun
and in newly forming stars.
• Helium is the next most abundant (about 9%); all heaver
elements constitute about 1% (by number of atoms).
• The Sun’s original supply of hydrogen is sufficient to supply its
energy output for 10 billion years, or more than double its
current age of 4.6 billion years.
• When stars deplete their supplies of hydrogen (in their central
cores), helium is fused to form still heavier elements - but,
this process is much less efficient than hydrogen fusion, and
so this energy source lasts for only a relatively short time.
• Stars end their lives in a variety of ways, depending on their
initial masses, which return much of the star’s mass to the
interstellar medium, enriching it in heavier elements.
COSMIC ABUNDANCES OF THE ELEMENTS
(By Number of Atoms)
Hydrogen
Helium
Oxygen
Neon
Carbon
Nitrogen
Silicon
H
He
O
Ne
C
N
Si
106
105
890
500
400
110
32
Iron
Magnesium
Sulfur
Argon
Sodium
Aluminum
Calcium
Fe
Mg
S
Ar
Na
Al
Ca
20-30
25
22
7.8
2
1.7
1.6
THE ORIGIN OF THE SOLAR SYSTEM
• The age of the solar system is believed, from several lines of
evidence, to be about 4.6 billion years.
• The solar system was created by gravitational collapse of a
cloud of interstellar material, consisting of gas and solid dust
particles.
• Observations with the Hubble Space Telescope and other spaceand ground-based instruments have shown that this process of
star (and planet) formation is still going on at the present time
elsewhere in our Galaxy.
• The heavy elements in Earth and the other members of the
solar system were manufactured in previous generations of
stars, which exploded as supernovas at the ends of their lives.
• Observations of regions of star and planetary system formation
occurring presently, elsewhere in our galaxy, and recent
detections of planets orbiting other stars, have added greatly to
our understanding of how our own solar system was created,
and its early evolution.
THE ORIGIN OF THE SOLAR SYSTEM
Conservation of Angular Momentum
• A basic principle of physics is the law of Conservation of
Momentum, = mass x velocity (M =mv).
• Momentum of an object cannot change unless it is subjected to
an Impulse, = Force x Time: Change of Momentum mv = Ft.
• Also conserved is the
Angular Momentum: L  mv  r  mvr
• where m = mass,v = velocity of motion around a center (such
as the motion of a planet around the sun), and r = radius of the
orbit (or other path) around the center. ( vis the component of
the total vector velocity, v, which is perpendicular to r.)
• A consequence of this latter law is that if a large, slowly rotating,
spherical gas cloud contracts under its own gravitational force,
its rotational velocity increases as it gets smaller, and it tends to
change shape from a sphere to a flat disk.
ORIGINS OF STARS AND PLANETS
• In recent times, we have been able to detect new stars and
planetary systems in the process of formation.
• The Hubble Space Telescope, in particular, has observed new
stars and planetary systems in the early stages of formation,
in the region of the Orion Nebula (in the “sword” of the
constellation Orion).
• The observations of these newly forming planetary systems
are consistent with the processes illustrated in the previous
slides.
• In addition, observations elsewhere in the sky have confirmed
that other stars have planets or planetary systems.
• Most of these are much larger and more massive than our
Earth (or, in many cases, even Jupiter), but improvements in
instrumentation technologies are expected to eventually
reveal Earth-like planets as well.
The Orion Nebula
(Ground-Based Telescopic Image)
HST View of the Orion Nebula Central Region
Protoplanetary disks in the Orion Nebula, observed with the Hubble Space
Telescope’s Wide Field/Planetary Camera-2.
THE ORIGIN OF THE SOLAR SYSTEM
• This process of star and planet formation is a continuing
process, in our own and other galaxies, even at the
present time.
• Observations of star and planet formation elsewhere at
present, helps us to understand the creation and early
evolution of our solar system.
• Also, study and exploration of our Moon, the other
planets, comets, and asteroids in our solar system (which
were all formed at the same time as our Earth), help us to
better understand the origin and evolution of Earth.
• In particular, the comets and the giant outer planets are
more representative of the original materials from which
the planets were formed, than are the inner planets such
as our Earth.
Model of the Origin of the Solar System
From “How Comets are Made”, Joseph H. Nuth III, American Scientist, May-June 2001
FORMATION AND EARLY HISTORY OF THE
SOLAR SYSTEM
• The Earth and the other planets of our solar system are believed
to have all formed about the same time as the Sun itself, from
the leftover portions of the cloud of gas and dust from which the
Sun formed.
• The conditions in the leftover material were a strong function of
distance from the newly forming Sun, because the temperature
in the gas cloud determined the extent to which volatile
materials, such as water, were able to condense.
• The sizes of the originally forming protoplanets also affected the
degree to which each could capture non-condensible materials,
such as hydrogen and helium.
• As a result, as observed in our present-day solar system, the
compositions of the planets and their satellites depend both on
their sizes and their distances from the Sun (as well as other
factors, some of which are still poorly known).
FORMATION AND EARLY HISTORY OF THE SOLAR
SYSTEM
• It is believed that the formation of planets began with
accumulations of solid particles (such as water ice, and dust
particles made up of elements such as carbon, silicon, and
iron), which are known to exist today in dense regions of the
interstellar medium.
• Collisions between these particles built up larger “dust balls”
which in turn collided with each other to build up ever larger
accumulations.
• When these protoplanetary objects reached a size that their
own gravitational force was significant, they became more
efficient at collecting additional dust particles and by colliding
with each other, building up ever larger objects.
• Sufficiently massive protoplanets, forming in the cooler, outer
regions of the solar system, could also accrete gaseous
material from the space within the solar system.
How a planet forms. The process begins when dust grains collide and stick together, forming larger and larger clumps (A).
The clumps move toward the central plane of the nebula (B) and collect into bodies the size of asteroids (C). These bodies
gravitate into clusters (D), collide (E), and form the nucleus of a planet (F). The planet grows (G), and as its mass increases,
it may attract gas from the nebula (H). If the planet is large enough, it may attract so much gas and draw it in so closely that
the gas forms a dense shell representing most of the planetary mass (I). (From A. G. W. Cameron, “The Origin and Evolution
of the Solar System”, Scientific American, 1975. From Stanley, Earth and Life through Time, 1989
FORMATION OF THE OUTER PLANETS
• The outer planets (Jupiter, Saturn, Uranus, and Neptune) are
quite different in composition from the inner, terrestrial planets
(Mercury, Venus, Earth, and Mars), mainly because they formed
in a region of space that was cool enough for water vapor and
other hydrogen compounds to condense.
• This allowed these planets to grow to a greater extent, and to
have a much larger percentage of hydrogen, the hydrogen
compounds, and helium, than the inner, terrestrial planets.
• The two largest planets, Jupiter and Saturn, are much more
similar in composition to the Sun, than to the terrestrial planets.
• The two outer giant planets, Uranus and Neptune, are of
intermediate composition, vs. the two largest planets and the
terrestrial planets.
• These differences can, in principle, be accounted for by a
combination of local temperatures (distance from the Sun),
initial density of the gaseous material vs. distance from the Sun,
and the masses of the original heavy-element cores upon the
lower density material was collected.
FORMATION AND EARLY HISTORY OF EARTH
• The Earth, and other inner (terrestrial) planets of our Solar
System, are made up primarily of heavy elements (such as
oxygen, silicon, and iron).
• This was due to their lower initial masses, and higher
temperatures, which made them unable to incorporate the
light gases, hydrogen and helium.
• These heavy elements formed the solid particle (dust)
component of the interstellar material from which the solar
system was created.
• The inner planets were built up by the accumulation of dust
into planetesimals, which in turn combined with each other to
build up the planets.
• Therefore, the composition of our Earth is quite different from
those of the Sun, the giant outer planets, and the interstellar
medium.
RELATIVE ABUNDANCES OF THE ELEMENTS
IN THE WHOLE EARTH
(Percent by Number of Atoms)
Oxygen
Iron
Silicon
Magnesium
Sulfur
Nickel
Aluminum
Sodium
Calcium
Phosphorus
O
Fe
Si
Mg
S
Ni
Al
Na
Ca
P
48.86
18.84
13.96
12.42
1.39
1.39
1.31
0.64
0.46
0.14
Hydrogen
Chromium
Carbon
Potassium
Manganese
Cobalt
Chlorine
Titanium
All Others
H
Cr
C
K
Mn
Co
Cl
Ti
0.12
0.11
0.10
0.05
0.05
0.05
0.03
0.03
0.04
Compare with Chart, Cosmic Abundances of the Elements
FORMATION AND EARLY HISTORY OF EARTH
• As the Earth grew larger, its increasing gravity
caused incoming particles to collide at increasingly
higher speeds, and to produce greater amounts of
heating due to their energies of impact.
• In addition, heat generation by radioactive elements
in the early Earth helped raise its temperature, and is
still an important heat source at present.
• When Earth’s interior heated to the melting point,
heavy materials such as iron concentrated in a
central core.
• Outgassing of the molten early Earth released water
vapor and other gases which later formed the
oceans and atmosphere.
RELATIVE ABUNDANCES OF THE ELEMENTS
IN THE EARTH
(By Weight)
• WHOLE EARTH
Fe 0.35
O 0.30
Si 0.15
Mg 0.13
Ni 0.024
S
0.019
Ca 0.011
Al 0.011
Other <0.01
• EARTH’S CRUST
O 0.46
Si 0.28
Al 0.08
Fe 0.06
Mg 0.04
Ca 0.024
K
0.023
Na 0.021
Other <0.01
Origin and Early History of Earth
Three mechanisms that would cause the early Earth to heat up: (a) In accretion, impacting bodies bombard
the Earth and their energy of motion is converted to heat. (b) Gravitational compression of the Earth into a
smaller volume causes its interior to heat up. (c) Disintegration of radioactive elements releases particles
and radiation, which are absorbed by the surrounding rock, heating it.
From Earth (3rd Edition), Press & Sevier, W. H. Freeman & Co., Publisher (1982)
GEOLOGIC TIME AND THE ROCK RECORD
• Earth has existed for about 4.6 billion years. However,
our knowledge of the details of conditions and events,
is much less for earlier time periods than for more
recent ones.
• Methods for determining the ages of rock samples
are:
o Determining the relative concentrations of radioactive
elements (such as uranium and thorium) and their
daughter products (such as lead)
o Noting the relative vertical positions of layers of rock,
where such access is available (e.g., in the Grand
Canyon)
o Noting the presence of various types of fossils, where
the time periods of existence of the corresponding life
forms are known.
FORMATION AND EARLY HISTORY OF EARTH
• Earth’s original atmosphere was quite different from the present
atmosphere; it probably consisted mostly of nitrogen (N2) and
carbon dioxide (CO2) with lesser amounts of reduced gases
such as carbon monoxide (CO), methane (CH4), ammonia (NH3)
and hydrogen sulfide (H2S).
• Water (H2O), currently mostly liquid in Earth’s oceans, has about
300 times the mass of Earth’s current atmosphere. If
temperatures on the early Earth were sufficiently high, H2O
would have been the primary constituent of the atmosphere
(about 300 bars), followed by CO2 (70 to 90 bars).
• Most of the original CO2 is now locked up in carbonate rocks,
such as calcium carbonate (CaCO3), as a result of water erosion
(as carbonic acid, H2CO3) of the original silicate rocks.
• Both fossil and mineral evidence indicate that oxygen (O2) was
only a minor constituent of the atmosphere for the first half
of Earth’s existence (the Archean eon), and did not reach
near-current levels until the Phanerozoic eon (beginning 600
million years ago). (The intervening time period is known as the
Proterozoic eon.)
Evolution of Oxygen Content of Earth’s Atmosphere
From Earth (3rd Edition), Press & Sevier, W. H. Freeman & Co., Publisher (1982)
Evolution of Life and the Earth’s Atmosphere