Atmospheric evolution on rocky planets

The atmospheres of the rocky planets of Earth, Venus and Mars
have undergone significant changes since these planets were formed more than
four billion years ago. It is important to understand the planetary history,
geology and atmospheric composition of these planets to understand how these
conditions helped life originate, survive and develop on earth. In addition, it
is important to understand how life has and continues to affect the atmosphere to
predict changes in the future.

The evolution of atmosphere on Earth, Venus and Mars and
their current differences

The current atmosphere of Venus is mainly composed of 96.5%
carbon dioxide and 3.5% nitrogen with trace amounts of Argon, Sulphur dioxide,
water vapour, carbon monoxide and helium among others. The planet’s atmosphere
exhibits extremely high winds of up to 360 kilometres per hour   which
can be more than 60 times the rotation of the planet, comparatively the earth’s
highest winds only ever reach ten to twenty percent of the earth’ rotational
speed. Scientists have predicted the early atmosphere of Venus to be similar to
the earth’s current atmosphere, consisting of liquid water. At some point in
the evolution of Venus’s atmosphere a process between the planet’s surface
temperature and the atmospheric opacity caused a positive feedback loop called
the runaway greenhouse effect leading to the current composition of mostly
greenhouse gasses such as carbon dioxide. This effect is predicted to have been
caused by the evaporation of surface water and the rise of greenhouse gasses that
came after.

The Martian atmosphere is similar to that of Venus with 96%
carbon dioxide 1.9% argon and 1.9% argon with trace amounts of oxygen, carbon
monoxide, water and methane. The atmosphere of mars contains high amounts of
suspended particles, which when observed from the Martian surface gives it the
brown and orange coloured sky. Mars is thought to have had liquid water in
large oceans a few billion years ago with more favourable conditions in
retaining water on its surface. The Martian atmosphere may have lost its previously
thicker atmosphere due to either, erosion by solar wind in when gas was trapped
in bubbles inside the planets magnetosphere and later stripped away by the
solar winds, a collision by a large body which stripped away a large proportion
of the atmosphere, or by the planets low gravity allowing the particles in the
atmosphere to escape through molecular kinetic energy.

The atmosphere of earth mainly consists of 78.09 % nitrogen,
20.96% oxygen, 0.93% argon and 0.04% carbon dioxide. The lower portion of the atmosphere
called the troposphere of earth allows the use of photosynthesis in plants and
breathing by animals, in addition the atmosphere allows for liquid water on the
earth’s surface, absorbs ultraviolet radiation, warms the planet by the
greenhouse effect and moderate’s temperature variation in the day and night. The
atmosphere of earth has undergone numerous changes since it was first formed. The
earths earliest atmosphere consisted of primarily hydrogen, water vapour,
methane and ammonia. These gasses would be partially lost due to solar wind. The
second period of earth’s atmosphere is characterised by heavy volcanic activity
and collisions with large asteroids which produced an atmosphere of largely nitrogen,
carbon dioxide and inert gasses. In this period hints of early life forms
dating back to 3.5 billion years ago can be found. These life forms would begin
the carbon cycle and oxygenate the atmosphere by photosynthesis. The earth’s
third atmosphere was created when free oxygen began to exist in the atmosphere
after the availably of materials for oxidation began to decrease, most notably iron.
During this period oxygen levels began to grow and fluctuate reaching a peak of
30% of the earth’s atmosphere 280 million years ago. The two main mechanisms
which govern changes in oxygen in the earth’s atmosphere are plants using
carbon dioxide to produce oxygen through photosynthesis and volcanic eruptions
which release sulphur which then oxidises and reduces the content of oxygen. The
current atmosphere of earth contains 21% oxygen which supports the existence of
life.

how has the Earth’s atmosphere helped life to originate,
survive and develop

The earth’s current atmospheric composition allows for life
to survive and develop. The earth’s atmosphere can be divided up into five
distinct layers: the troposphere, stratosphere, mesosphere, thermosphere and
the exosphere some of which play a part in keeping the earth habitable. The troposphere
is the lowest portion of the atmosphere to the earth. This layer is characterised
by warm temperatures created by the solar energy emanating from the sun being
absorbed by the earth’s surface and its oceans. The atmospheric pressure also
allows for a favourable concentration of oxygen molecules and moderation of
temperature. The layer above the troposphere is the stratosphere which begins
at approximately eleven kilometres above the earth’s surface. The composition
of the astrosphere is similar to that of the troposphere with and important
difference. The stratosphere is the layer in which the ozone layer exists, the
ozone layer blocks out many forms of radiation which would be dangerous to life
forms such as ultraviolet light as well as a form of insulation for the troposphere
by holding in heat. The mesosphere lies 50 kilometres above the earth’s surface
and its upper area is the coldest part of the atmosphere, here the atmospheric
pressure does not allow for aircraft to fly through and is too low for orbital
space craft due to high atmospheric drag. It is for these reasons that the mesosphere
is the least understood of the parts of earth’s atmosphere, nonetheless it is
understood that meteors burn up in this layer due to air friction.  The thermosphere at about 85 to 100 kilometres
above the earth’s surface is the layer in which x-ray radiation is absorbed and
does not allow to reach the lower layers of the atmosphere because of this, temperatures
can reach 1500 degrees Celsius near the border between the thermosphere and the
exosphere.

In addition to the earth’s atmospheric layers there are
three cycles which support the survival and development of life on earth: the
carbon cycle, the nitrogen cycle and the water cycle. In the carbon cycle the
carbon dioxide in the atmosphere is absorbed through photosynthesis and releases
oxygen which sustains life. The nitrogen cycle is created when organisms remove
nitrogen gas from the earth’s atmosphere and absorb the molecules to for other
chemicals which plants and animals consume. The water cycle which moves the
different forms of water between the earth’s surface and the atmosphere and
distributes the water which helps sustain life.

How has life affected the atmosphere over time

The most significant impact that life has had on the earth’s
atmosphere is build-up of oxygen, called the great oxidation event. It is
estimated that this event happened about 2.45 billion years ago and was caused
by cyanobacteria undergoing the photosynthesis process. Prior to this period,
oxygen was absorbed by iron which kept the level of oxygen in check. During the
great oxidation event the low availability of these oxygen absorbing materials
meant that excess oxygen began to accumulate in the atmosphere. This caused anaerobic
organisms, meaning organisms which cannot survive in an environment of 20.95%
oxygen, to become extinct and is a significant mass extinction event in the
earth’s history. The increase in oxygen levels also reduced the levels of
methane, a greenhouse gas, and in turn triggered lowered the surface temperature
and triggered a long period of glaciation called the snowball earth. The evolution
of aerobic organisms, which absorbed oxygen, would then lower the level of
oxygen in the atmosphere and establish stable level of oxygen.

In more recent times the atmosphere of the earth has been
affected by the release of greenhouse gasses such as carbon dioxide, methane
and nitrous oxide by human activity. The increase of these greenhouse gasses
may cause the rise of temperatures on the surface of the earth by between 0.3
to 5.8 degrees centigrade. Other effects include rising sea levels, and altered
precipitation, more extreme weather, species extinction and ocean acidification