Download Affects an Atmosphere

How Planet Formation
Affects an Atmosphere
There are a few factors that determine whether a planet will
have an atmosphere or not. These include:
1.
2.
3.
How a planet forms.
The planet’s mass and how long it takes to "cool off”
The planet’s composition.
Let’s briefly review this process PLANET FORMATION so
that you have a basic idea of how it all works:
In the development of a planet, there are three
different processes that occur. Each planet varies in the
amount of time it takes to go through each phase and the
extent of change the planet undergoes within each stage.
Although all four phases are important, its the first phase
which determine most what the atmosphere will be like.
(NOTE: This doesn’t mean that the atmosphere will not
change over time; it just means that it is the most important
phase for what the atmosphere of a planet will be like in the
beginning.)
Stage I: Differentiation
In this first phase, the material making up the
planet separates according to density. The more dense
material will clump together in the center of the planet, and
the least dense material will "float" to the top of the planet,
forming the crust or surface. The amount of differentiation is
greater if the planet is molten (made of melted material)
when it formed. There are two heat sources which can
generate the heat required to melt the metals which make up
the inner planets: in-falling material (like meteors) will
transfer their energy in the form of heat when they hit the
planet; in addition, the decay of radioactive elements will
generate heat. It is this decay of radioactive elements that
are concentrated in the Earth’s crust that warm and soften
the rock layers, allowing for movement and change over
time.
The creation of a planetary atmosphere from the planet’s
interior is called outgassing. What is outgassing? Well,
gases from the solar nebula get trapped in the interior of a
planet. These gases were mostly hydrogen and helium.
These gases were light, and the heat from the planet
basically "blew off" these gases. As the rock material baked
and cooled, new gases were released from the rocks,
forming an atmosphere. Scientists think that planets formed
relatively rapidly, and the heat generated from the forming
planet could not escape into space. This heat melted the
planets as they formed, and in their molten state (liquid rock)
gases were allowed to escape. The gases that did not get
"blown off" into space were heavier, and that’s why the inner
planets have atmospheres with ‘heavier" gases – gravity
could hold onto those gas particles. The lighter ones
escaped.
Scientists are pretty sure that atmospheres are not formed
entirely by outgassing. Some of the gas components of an
atmosphere are thought to have been "added" later on by
comet
Carbon Dioxide (CO2) in the Atmosphere impacts and
late
Venus
96.5%
accretion of
Earth
.035%
matter in the
formation of
Mars
95.32%
planets.
You may be wondering why Earth has so little carbon
dioxide in the atmosphere when Venus and Mars have so
much. Well, it’s like this: Outgassing from a forming planet
such as Earth would release mostly carbon dioxide and
water vapor. Carbon dioxide is very soluble in water, and the
earth’s surface temperature is just right for it to be covered
with water. Much of the carbon dioxide released in
outgassing dissolves in oceans and combines with minerals
in seawater to form deposits of silicon dioxide, limestone,
etc. So the carbon dioxide has been removed from Earth’s
atmosphere and buried in the Earth’s crust (rocks).
Stage II: Cratering
This is the most violent phase of
planet formation. The forming planets are
bombarded by material from the solar nebula,
and they are hit by meteors of all shapes and
sizes. As the meteors impact the forming planet, they form
craters. Erosional processes have erased the evidence of
many craters on Earth, but if you look at the surface of
Earth’s moon on a clear night, you can see the craters easily
from Earth.
Stage III: Flooding
Flooding actually begins before the cratering
phase is complete. The broken crust and the radioactive
elements that decay will melt the rock material that forms the
planet. This molten rock will gradually seep up through
cracks in the crust, and the lava that reaches the surface of
the planet will flood the low-lying areas of the planet.
The Earth's Atmosphere
Early Atmosphere
Early Earth would have been very different and
inhospitable compared to the Earth today.
The Early Atmosphere was Hot

Why was it so hot? - Primordial heat, collisions and
compression during accretion of cosmic materials, and
the decay of short-lived radioactive elements

What were the results of it being so hot?
There was constant volcanism, the surface
temperature was too high for liquid water or life as we
know it to exist, and the surface of the earth was mostly
molten surface or thin unstable basaltic crust.

What was this early Atmosphere made of?
The early atmosphere was completely different in
composition and had a lot of Hydrogen and Helium
present (H2, He)
Cooling of the Early Atmosphere

What happened to all of the heat?
 Primordial heat dissipated to space

What were the results of the Earth cooling?
 As the Earth cooled the water vapor
condensed to form liquid water (rain), and it
accumulated on the surface of the earth
 Volcanoes formed on the Earths surface and
a new atmosphere began to form due to
volcanic out gassing
 Conditions became appropriate for the
evolution of life
First Atmosphere
The very first atmosphere was made of Hydrogen and
Helium. These gases are relatively rare on Earth compared
to other places in the universe and were probably lost to
space early in Earth's history because Earth's gravity is not
strong enough to hold lighter gases in.
Earth still did not have a differentiated core (solid inner/liquid
outer core) which creates Earth's magnetic field
(magnetosphere = Van Allen Belt) which deflects solar
winds. Once the core differentiated the heavier gases could
be retained.
Second Atmosphere
The next stage of the atmosphere was produced by volcanic
out gassing. Gases produced were similar to those created
by modern volcanoes (H2O, CO2, SO2, CO, S2, Cl2, N2, H2)
and NH3 (ammonia) and CH4 (methane) . There was,
however, no free oxygen (O2) at this time (because it is not
found in volcanic gases).
Ocean Formation - As the Earth cooled, H2O produced by
out gassing could exist as liquid water in the Early Archean,
allowing oceans to form.
Addition of O2 to the Atmosphere
Atmospheric oxygen built up in the early history of the Earth
as the waste product of photosynthetic organisms and by
burial of organic matter away from surficial decay. This
history is documented by the geologic preservation of
oxygen-sensitive minerals, deposition banded iron
formations, and development of continental "red beds" or
BIFs.
Figure from the University of Michigan's Introduction to
Global Change web site.
Today, the atmosphere is about 21% free oxygen. But how
did oxygen reach these levels in the atmosphere?
Evidence from the Fossil Record:
Oxygen is produced by plants as part of the
photosynthesis process. During the Proterozoic the amount
of free O2 in the atmosphere rose from 1 - 10 %. Most of this
was released by cyanobacteria or blue-green algae, which
also increased in abundance in the fossil record during this
time. Present levels of O2 were not achieved until about 400
MYA.
Evidence from the Rock Record:
Iron (Fe) is extremely reactive with oxygen. If we look at the
oxidation state of Fe in the rock record, we can infer a great
deal about atmospheric evolution.
Banded Iron Formation (BIF):
Deep water deposits in which layers of iron-rich minerals
alternate with iron-poor layers, primarily chert. Iron minerals
include iron oxide, iron carbonate, iron silicate, iron sulfide.
BIF's are a major source of iron ore, b/c they contain
magnetite (Fe3O4) which has a higher iron-to-oxygen ratio
than hematite. These are common in rocks 2.0 - 2.8 B.y. old,
but do not form today.
Red beds:
Continental siliciclastic deposits are never found in rocks
older than 2.3 B. y., but are common during Phanerozoic
time. Red beds are red because of the highly oxidized
mineral hematite (Fe2O3), that forms secondarily by oxidation
of other Fe minerals that have accumulated in the sediment.
Biological Evidence:
Chemical building blocks of life could not have formed in the
presence of atmospheric oxygen. Chemical reactions that
yield amino acids are inhibited by presence of very small
amounts of oxygen.
Oxygen prevents growth of the most primitive living bacteria
such as photosynthetic bacteria, methane-producing
bacteria and bacteria that derive energy from fermentation.
Since today's most primitive life forms are anaerobic, the first
forms of cellular life probably had similar metabolisms.
Today these anaerobic life forms are restricted to anoxic
(low oxygen) habitats such as swamps, ponds, and lagoons .
Our Atmosphere Today
Our atmosphere is an envelope of gases that surrounds the
Earth. It is important for life on earth because it protects the
Earth from harmful radiation and from the bombardment of
cosmic materials (like meteors). It is also used by life as a
reservoir of chemical compounds used in living systems (like
oxygen). The Atmosphere has no outer boundary, it just
fades into space. The most dense part of atmosphere (97%
of the mass) lies within 30 km of the Earth (so it has about
the same thickness as continental crust).
Chemical Composition Today

Nitrogen (N2)- 78%

Oxygen (O2)- 21%

Carbon Dioxide - .03%

Plus other misc.
gases (H2O, Ar)