The Origin of the Earth
Characteristics of the Solar System.
- •It is a Part of a Nebula
- •Most of the mass of the Solar System is concentrated in the sun.
- •98 percent of the angular momentum is found in the planets m*v*d
- •heavy noble gases (xenon, neon, Krypton) are rare on earth as compared
to space and the sun.
- •The Earth is layered with a thin crust, overlying a heavier mantle,
and centered with a nickel-iron core, the outer part of which is molten,
and the inner part of which is solid
- •The planets and sun each have a somewhat different density suggesting
different time and/or temperatures of origin.
Theories of Earth's Origin
The planets were spun off of the sun this theory suggests that the
planets were spun off of the sun, and are thus essentially daughters of
the sun. This requires a near collision with a large body.
Does it fit the facts? If this were the way that the solar system formed,
be less massive than it is.
- •the sun would have most of the angular momentum
- •The sun would
The nebular cloud hypothesis suggests that the solar system started
out as a nebular cloud an that the planets and sun were concentrated from
the dust and gasses in the cloud by gravitational attraction.
- •cloud would have been 30-40 light years accross
- •mass of cloud would have been 2 -10 times the present Solar System
- •originally extremely thin
- •coalesced by gravity and magnetic attraction
- •Collision heated sun until it started spontaneous fusion
- •initial solar wind drove off Earth's light elements, primitive Earth
atmosphere--what is left is only 1/1200 of original mass.
The Heating, Cooling and Coalescing of the Earth
- •Initial accretion of the Earth was cool
- •heat originated from collision, radioactive decay.
- •heavy elements (nickel and Iron) migrated to center to form core
by gravity as material became molten; lighter material floated to the
top to form crust, and material of intermediate density formed the mantle.
- •Earth began to cool, but the inside continues to be heated by radioactive
The idea that the oceans and early second atmosphere are derived
from water and gasses originally bound up in earth's rocks
Supporting evidence includes the following facts
- •meteorites contain about 0.5% by weight of water, which, extrapolated
to the earth, would fill the ocean 20 times.
- •Enough water comes out of Volcanoes to have filled the ocean basins
20 time during the history of the Earth
Water from Space
The idea is that the oceans (and early, second atmosphere) are derived
from comets, which are mostly water, which have bombarded the earth especially
in during the early formation of the earth.
Supporting evidence includes some negative evidence:
- •most magma represents part of the recycling of materials that have
already been on the surface, including water and atmospheric gases such
- •those magmas that do seem to originate directly from the interior
of the earth are much lower in water and gases than are the other magmas.
- •Comets could account for the origins of the same gases.
The Atmosphere and Life
The Composition of the Proto Atmosphere
- •Principle components would have been H2O, N2, CO2, with small amounts
of H2, and CO it is also probably that there were simple counpounds
such as CH4 and NH3.
- •This atmosphere lacked free oxygen, which would have made any organic
molecules very stable
Theories of Abiotic Synthesis
- Aleksandr Oparin (1938) Atmospheric gases and outside energy
sources could have formed the earliest organic molecules
- The Stanley Miller Experiment (1953) made an aparatus to duplicate
the early atmosphere with simulated lightening and a boiling water ocean.
and thereby created organic molecules including nucleic acids. Such
molecules might have accumulated without being either oxydized or eaten
in the early atmosphere. the next step was polymerization of the the
- the RNA first hypothesis states that only RNA was needed to
begin life and order the polymerization of organic compounds. Some viruses
have only RNA for genetic material.
- the Protein-first hypothesis was supported by the work of
Sidney Fox who demonstrated that proteins can polymerize abiotically
when exposed to dry heat. When the proteins are returned to water they
form microspheres which resemble cells in many ways.
- the Clay-substrate hypothesis was proposed by Graham Cairns-Smith.
This theory suggests that both protein and RNA are were formed simulataneously
in association with clay particles.
From Chemicals to Life -- the first Cells
Life is organized into cells wherein the processes of life occur separated
from the outside world by a cell membrane. Thus, the trick is getting the
Fox's microspheres are similar to cell membranes. They show that molecules
can self-order into cell-membrane like structures. The next step is to combine
the material to a self replicating unit.
Heterotrophs and Autotrophs
- •The first organisms were probably heterotrophs, consuming
materials around them, and they were anaerobic because there
was no free oxygen available.
- •Fermentation may be the earliest way of obtaining energy by heterotrophs.
- •Photoassimilation was possibly the next step. this is also
an anaerobic process
- •Photosynthesis developed next, photosynthesis produces free
The Formation of the Modern Atmosphere
Photosynthesis is a by product of photosynthesis. The first oxygen produced
became bound in the reduced materials in the environment--especially iron.