Atmosphere Evolution on Rocky Planets
Atmosphere can be defined as a gaseous compound layer surrounding a large body mass suspended by means of gravity and centrifugal force caused by rotation . Atmospheres of planets have not always been the same, its evolution comprised of complex development across million years of geologic time affected by various changes of variables inside and outside its planet . Of the interest of this essay is the atmospheric evolution of a rocky planet. Rocky planet is a terrestrial body consists mainly of silicate or metal .
Some known rocky planets with substantial atmosphere such as Venus, Mars and Earth has a very different composition of atmospheric gas . These planets experienced very different condition that causes the formation of current atmosphere. This essay will focus on the evolution of atmosphere on Earth, Venus, and Mars and compare the current atmospheric conditions of these planets. Furthermore, how the Earth’s atmosphere can sustain life and how life on earth affect Earth’s atmosphere over different period of time will also be discussed. First of all, there are some changes happenned in atmospheric condition of Venus over time. Venus has a very thick and dense atmosphere which mostly consists of carbon dioxide .
This carbon dioxide rich atmosphere is approximately 100 times greater than in Earth’s atmosphere . The carbon dioxide trap infrared radiation originated from the Sun and bounced back from the planet surface, and causes large-scale greenhouse gas effect. This condition makes the Venus’ surface to be very hot (above 450C). Experts believe that significant volume of water was covering the surface of the planet at the early stage . However, combinations of Venus location relative to the sun as well as the global warming condition evaporate the vast ocean on the Venus’s surface in a very short geological timescale . Some of this water vapor then combined with the sulfur dioxide released by volcanic to become H2SO3 (sulfuric acid) droplets while the rest is broken down by photons and swept by solar wind.
Complete removal of water due to this extreme condition stops geological and biological evolution . On the contrary to Venus, Mars is relatively far from the Sun and thus it has colder temperature compared to both Venus and Earth . Mars also has smaller mass and size than Venus and Earth, which result in weaker gravitational force necessary to hold atmospheric gas in place. This planet also does not have a magnetic field . The combination of weak gravitational force as well as no magnetic field contributes to the instability of atmosphere from the solar winds and thus result in a very low atmospheric pressure in Mars. Low temperature in Mars make the water frozes, and the low atmospheric pressure ensures that the escaped water molecule turns into vapour instead of liquid. In other words, the water in Mars is either frozen or evaporated .
This condition leads to thin, cold, and dry atmosphere, which only contains trace of water vapor . Lastly, Earth’s atmospheric evolution starts from around 4.5 billion years ago where dimmer Sun, greenhouse gas-rich, and oxygen-poor condition affecting earth’s atmosphere and early ocean. The dimmer Sun supplied less solar radiation, which is important to provide heat to the Earth surface. Since the intensity of solar radiation from Sun was less to warm the early Earth, the presence of much higher atmospheric ‘greenhouse’ helped to keep the surface warm enough to maintain water in liquid form. The temperature increases was about 30C above freezing point of water, which makes the out-gassed water remains liquid.
The earth also has an active core, which presents a strong magnetic field to counter the Solar wind and maintain the Earth atmosphere pressure . Table below shows the comparison of various variables affecting atmospheric conditions on the Venus, Earth and Mars. Table 1. Basic Atmospheric Comparisons of Venus, Earth, and Mars ,
96% CO2, 3.5% N2, 0.5% SO2
77% N2, 21% O2, 1% Argon, H2O (variable), 0.038% CO2
95% CO2, 2.7% N2, 1.6% argon, 0.7% O2
Average Temperature (C)
Rotation Period (days)
Mass (1024 kg)
Sulfuric acid clouds
H2O clouds, pollution
H2O clouds and CO2 clouds, dust
The atmospheric temperature of the planets mainly affected by its relative distance to the Sun and by its atmospheric gaseous composition. The closer the planets to the Sun, the higher heat radiation it receives from the Sun . This will affect the initial possible temperature on the planet surface. On the other hand, atmospheric pressure and composition affect the greenhouse gas effect as well as deflection of ultraviolet and heat from the Sun . Combination of these effects results in different surface temperature as shown on the table above. The atmospheric conditions of planets are related to gravity strength and temperature of the atmosphere. Atmosphere can be retained for a longer duration when the gravity is high and the temperature of atmosphere is low.  Table shows that the gravity
of Earth is the highest, followed by Venus, and Mars. The gravity of Earth and Venus are similar. Therefore, the atmosphere of both Earth and Venus can be retained for a longer period of time, while Mars’ atmosphere can only be retained in a shorter period of time. In other words, Earth and Venus has a more stable atmosphere compared to Mars.
Figure 1. Phase diagram of water in Venus, Mars, and Earth . Compare to the other two planets, Venus does not have water in any forms due to the greenhouse gas effects as well as its relative distance to the Sun, which evaporates all the available water in any form on its surface. This is shown on the Figure 1 above, that water can only form as a gaseous compound on Venus surface, this water vapor then combined with the SO2 from volcanic out-gassing to create sulfuric acid clouds. For Earth and Mars, water exists as a different form in their surfaces. Figure 1 shows the different phases of water as a function of temperature and pressure of the planet. Earth is shown to have high pressure and temperature that allows the presence of water in solid, liquid, and vapor phases. Mars has low pressure and low temperature, thus allowing the water to be stable only in solid and vapor phase.
This planet is shown to be located close to the triple point, thus the little increase of temperature and pressure for short period of time allows ice to form liquid water. Venus is shown to be present in vapor phase due to the high atmospheric temperature and pressure . Life was developed in the Earth due to the presence of liquid water. The existence of plants and some photosynthetic bacteria utilize CO2 and yield O2. Furthermore, water washes and deposits CO2 as minerals into the oceans. It leaves small amount of carbon dioxide at the atmosphere (0.038%). Therefore, the composition of CO2 decreased and O2 increased . The utilization of some natural resources and respiration by the living creatures release CO2 to the atmosphere.
This affects the composition of Earth, which initially high in CO2, become low CO2 and high O2, and become high in CO2 again. As stated above, Earth’s atmosphere helped the origination, survival, and development of life. The atmosphere of Earth contained high amount of CO2. It facilitated the Earth in maintaining warm temperature while the Sun was still dimed and less warm . This atmosphere supports the liquid water to be stable under this condition. The presence of this liquid allows the primitive bacterium to originate . Soon after the emergence of life for the first time, it begins to influence composition of Earth’s atmosphere. Free oxygen (O2) was produced as the product of photosynthesis, and dominated the chemistry of atmosphere. This oxygen is used by animals and other living creatures to breath and creates energy necessary to sustain life.
The oxygen is also responsible for the formation of protective ozone (O3) layer in the atmosphere. Ozone was formed by recombination of oxygen by solar radiation. The Ozone screens ultraviolet (UV) from sunlight which and prevent most of UV sunlight to reach Earth’s surface . Therefore, the presence of Ozone allowed the colonization of more advanced life from the seas to land after the accumulation of oxygen from photosynthesis . To conclude, the stable atmosphere of Earth sustains the temperature and Earth’s condition, and is suitable for living creatures. Life has affected the Earth’s atmosphere over different period of time. Over millions of years the level of carbon dioxide in Earth has become decreased to a very low level, and oxygen began to increase significantly. The carbon dioxide in atmosphere become lower is due to the fact that CO2 might be dissolved in the oceans, absorbed by plants for photosynthesis, or locked up in sedimentary rocks. The increase proportion of oxygen is because of oxygen production from photosynthesis by plants .
The existence of human marked another change in Earth’s atmosphere. The development of industrialized country begins as human develop technology and exploit fossil fuels to provide energy. The exploitation of natural resources that entrap CO2, result in the release of this greenhouse gas (CO2) and polluted the atmosphere. The most common greenhouse gas which responsible for Global warming is CO2. Other gases that contribute to Global warming are methane from digestive systems of grazing animals, nitrous oxide from fertilizers, fluorinated gases, and gases used in industrial process and refrigeration , . Every greenhouse gases possess different abilities to trap heat. Methane could produce more than 20 times of the CO2 effect. Nitrous oxide has 300 times higher ability compared to CO2 . As more greenhouse gases released to the air, the Earth’s atmosphere is again affected. Greenhouse gases could absorb heat and prevent the heat from escaping into the space. The production of greenhouse gases is faster than the ability to remove it and thus leads to global warming.
Global warming is where the Earth’s average temperature rises quickly . Although the presence of CO2 could has no effect on ozone layer, the presence of other gases, such as Chlorofluorocarbons (CFC) has thousands times heat-trapping potential higher than CO2. This could lead to the depletion and thus disappearance of ozone layer . In Conclusion, the atmosphere evolution on Venus, Mars, and Earth are different, and thus results in the different current atmospheric conditions. The evolution of atmospheric condition is affected by some factors such as the distance from the Sun and the size of the planet, its rotation speed, gravity and most importantly its magnetic fields. Also affecting are the initial materials available, which later vaporized and suspended by the gravity and centrifugal forces of the planet. Earth atmosphere can sustain life by the presence of high amount of CO2 in the atmosphere to maintain warm temperature of the Earth while the Sun was still dimmed and less warm. Earth’s atmosphere also allow the formation of liquid water in stable condition, thus the presence of water allow the origination of life.
Life has a great role in affecting Earth’s atmosphere over different period of time. While initial life on earth help transform the CO2 rich atmosphere into N2 and O2 rich atmosphere capable to support complex life system, global warming and ozone depletion occur as the result of both natural and human-made industrial activity. These examples of atmospheric changes outline the fact that life on the planet can significantly alter atmospheric condition of the planet. Human as the most complex life form to live on earth has the capability and responsibility to maintain earth’s atmosphere conditions. Reference
 Science Dictionary. 2013. ‘Atmosphere Science Definition’. Your Dictionary. Online. Internet Explorer. 18 May, 2013. Available : http://science.yourdictionary.com/atmosphere  Encyclopedia Britannica. 2013. ‘Evolution f the Atmosphere’. Britannica. Online. Internet Explorer. 18 May, 2013. Available : http://www.britannica.com/EBchecked/topic/1703862/evolution-of-the-atmosphere  Princeton. 2013. ‘Terrestrial Planet’. Princeton. Online. Internet Explorer. 18 May, 2013. Available : http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Terrestrial_planet.html  Montana Space Grant Consortium (MSGC). 2013. ‘Discovering Rocky & Gas