Atmospheric evolution on rocky planets The atmospheres of the rocky planets of Earth, Venus and Marshave undergone significant changes since these planets were formed more thanfour billion years ago. It is important to understand the planetary history,geology and atmospheric composition of these planets to understand how theseconditions helped life originate, survive and develop on earth. In addition, itis important to understand how life has and continues to affect the atmosphere topredict changes in the future. The evolution of atmosphere on Earth, Venus and Mars andtheir 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 atmosphereexhibits extremely high winds of up to 360 kilometres per hour whichcan be more than 60 times the rotation of the planet, comparatively the earth’shighest winds only ever reach ten to twenty percent of the earth’ rotationalspeed. Scientists have predicted the early atmosphere of Venus to be similar tothe earth’s current atmosphere, consisting of liquid water. At some point inthe evolution of Venus’s atmosphere a process between the planet’s surfacetemperature and the atmospheric opacity caused a positive feedback loop calledthe runaway greenhouse effect leading to the current composition of mostlygreenhouse gasses such as carbon dioxide. This effect is predicted to have beencaused by the evaporation of surface water and the rise of greenhouse gasses thatcame 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, carbonmonoxide, water and methane. The atmosphere of mars contains high amounts ofsuspended particles, which when observed from the Martian surface gives it thebrown and orange coloured sky.
Mars is thought to have had liquid water inlarge oceans a few billion years ago with more favourable conditions inretaining water on its surface. The Martian atmosphere may have lost its previouslythicker atmosphere due to either, erosion by solar wind in when gas was trappedin bubbles inside the planets magnetosphere and later stripped away by thesolar winds, a collision by a large body which stripped away a large proportionof the atmosphere, or by the planets low gravity allowing the particles in theatmosphere 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 atmospherecalled the troposphere of earth allows the use of photosynthesis in plants andbreathing by animals, in addition the atmosphere allows for liquid water on theearth’s surface, absorbs ultraviolet radiation, warms the planet by thegreenhouse effect and moderate’s temperature variation in the day and night. Theatmosphere of earth has undergone numerous changes since it was first formed. Theearths earliest atmosphere consisted of primarily hydrogen, water vapour,methane and ammonia.
These gasses would be partially lost due to solar wind. Thesecond period of earth’s atmosphere is characterised by heavy volcanic activityand collisions with large asteroids which produced an atmosphere of largely nitrogen,carbon dioxide and inert gasses. In this period hints of early life formsdating back to 3.5 billion years ago can be found. These life forms would beginthe carbon cycle and oxygenate the atmosphere by photosynthesis. The earth’sthird atmosphere was created when free oxygen began to exist in the atmosphereafter 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 of30% of the earth’s atmosphere 280 million years ago. The two main mechanismswhich govern changes in oxygen in the earth’s atmosphere are plants usingcarbon dioxide to produce oxygen through photosynthesis and volcanic eruptionswhich release sulphur which then oxidises and reduces the content of oxygen. Thecurrent atmosphere of earth contains 21% oxygen which supports the existence oflife. how has the Earth’s atmosphere helped life to originate,survive and developThe earth’s current atmospheric composition allows for lifeto survive and develop. The earth’s atmosphere can be divided up into fivedistinct layers: the troposphere, stratosphere, mesosphere, thermosphere andthe exosphere some of which play a part in keeping the earth habitable. The troposphereis the lowest portion of the atmosphere to the earth. This layer is characterisedby warm temperatures created by the solar energy emanating from the sun beingabsorbed by the earth’s surface and its oceans.
The atmospheric pressure alsoallows for a favourable concentration of oxygen molecules and moderation oftemperature. The layer above the troposphere is the stratosphere which beginsat approximately eleven kilometres above the earth’s surface. The compositionof the astrosphere is similar to that of the troposphere with and importantdifference. The stratosphere is the layer in which the ozone layer exists, theozone layer blocks out many forms of radiation which would be dangerous to lifeforms such as ultraviolet light as well as a form of insulation for the troposphereby holding in heat. The mesosphere lies 50 kilometres above the earth’s surfaceand its upper area is the coldest part of the atmosphere, here the atmosphericpressure does not allow for aircraft to fly through and is too low for orbitalspace craft due to high atmospheric drag.
It is for these reasons that the mesosphereis the least understood of the parts of earth’s atmosphere, nonetheless it isunderstood that meteors burn up in this layer due to air friction. The thermosphere at about 85 to 100 kilometresabove the earth’s surface is the layer in which x-ray radiation is absorbed anddoes not allow to reach the lower layers of the atmosphere because of this, temperaturescan reach 1500 degrees Celsius near the border between the thermosphere and theexosphere.In addition to the earth’s atmospheric layers there arethree cycles which support the survival and development of life on earth: thecarbon cycle, the nitrogen cycle and the water cycle. In the carbon cycle thecarbon dioxide in the atmosphere is absorbed through photosynthesis and releasesoxygen which sustains life. The nitrogen cycle is created when organisms removenitrogen gas from the earth’s atmosphere and absorb the molecules to for otherchemicals which plants and animals consume. The water cycle which moves thedifferent forms of water between the earth’s surface and the atmosphere anddistributes the water which helps sustain life. How has life affected the atmosphere over timeThe most significant impact that life has had on the earth’satmosphere is build-up of oxygen, called the great oxidation event. It isestimated that this event happened about 2.
45 billion years ago and was causedby cyanobacteria undergoing the photosynthesis process. Prior to this period,oxygen was absorbed by iron which kept the level of oxygen in check. During thegreat oxidation event the low availability of these oxygen absorbing materialsmeant that excess oxygen began to accumulate in the atmosphere.
This caused anaerobicorganisms, meaning organisms which cannot survive in an environment of 20.95%oxygen, to become extinct and is a significant mass extinction event in theearth’s history. The increase in oxygen levels also reduced the levels ofmethane, a greenhouse gas, and in turn triggered lowered the surface temperatureand triggered a long period of glaciation called the snowball earth.
The evolutionof aerobic organisms, which absorbed oxygen, would then lower the level ofoxygen in the atmosphere and establish stable level of oxygen.In more recent times the atmosphere of the earth has beenaffected by the release of greenhouse gasses such as carbon dioxide, methaneand nitrous oxide by human activity. The increase of these greenhouse gassesmay cause the rise of temperatures on the surface of the earth by between 0.3to 5.
8 degrees centigrade. Other effects include rising sea levels, and alteredprecipitation, more extreme weather, species extinction and ocean acidification