The Evolution and Current Composition of the Atmospheres of Earth and Mars

The Evolution and Current Composition of the Atmospheres of

Earth and Mars

The essay begins by looking at the birth of our solar system and formation of Mars and Earth. The origins of the atmospheres of the two planets are considered and attention then turns to the early atmospheres themselves. Regarding Mars, predominance is given to the mechanisms responsible for the removal of the planets atmosphere. This is mainly because its early removal precluded further evolution. Consideration is then given to the evolution of Earth’s atmosphere from the Hadean, through the Archean and Proterozoic. Due emphasis is placed on the appearance of life in the Archean and its effect on atmospheric chemistry. The essay ends by looking at the two planets present day atmospheres.

Mars is a terrestrial or ‘Earthlike’ planet composed of rocks and metals and was formed in much the same way as the Earth, by the accretion of planetesimals by gravitational attraction during the birth of our solar system 4.5 billion years ago. Two processes could have formed the early atmospheres of Earth and Mars. During the “Heavy Bombardment” period Earth and Mars were frequently struck by large amounts of residual material which had escaped earlier accretion. Some of this material would have taken the form of comets, which are composed mainly of water, carbon dioxide and other gases locked up in their frozen form. Comets could have carried in the water and gasses that formed the early atmospheres.

Both Earth and Mars probably began as undifferentiated mixtures of planetisemals and other solar nebular materials. Their recent accretion from planetesimals, whose immense kinetic energy transformed to heat upon impact, would have caused global melting. Through the process known as ‘differentation’ the heavier materials sank to the planets interiors forming iron cores, whilst the lightest materials rose to the surface forming the crust. Differentation would have been synonymous with intense volcanism and plate tectonic activity, consequently the atmospheres could have formed from the water and gasses emitted by ‘outgassing’. Evidence of plate tectonic activity on Mars is sparse, but the planet was certainly volcanically active. The Martian volcano “Olympus Mons” is probably the biggest volcano in the solar system at 27 km high and 600 km wide.

Because of the intense heat, the early atmosphere of Mars (Noachian < 3.5 billion years ago) would most likely to have been steam, which condensed out onto the surface as the planet cooled. For a short time the very early Noachian atmosphere contained a vast amount of hydrogen produced by the reaction of water with the molten iron core. The hydrogen was subsequently vented via volcanoes to the surface and quickly lost to space due to its lightness. Volcanoes also released carbon dioxide, nitrogen, sulphur dioxide and methane. Carbon dioxide and nitrogen were the dominant gasses of the early Martian atmosphere, which may have been denser than Earth’s present atmosphere.

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The strong evidence of liquid water on Mars 3.6 to 4 billion years ago suggests a thick greenhouse. A CO² atmosphere becomes unstable with liquid water around. CO² was the major constituent of Mars’s early atmosphere, and still is (Table 1). CO² is also a greenhouse gas. As Mars cooled and volcanic activity ceased, the flux of CO² from the interior of the planet came to an end, whilst rock weathering and the subsequent burial of carbonate minerals continued a one-way flux of CO² into the planets interior. This process continued until nearly all the CO² had been removed from the atmosphere thereby removing Mars’s greenhouse. The water today may be stored as ice below the Martian surface or in the polar icecaps. A similar process functions on Earth with one critical difference, CO² is pumped back into the atmosphere by volcanoes. As Earth is much bigger than Mars its internal heat engine is still running strong enabling this outgassing, balancing carbon in Earth’s reservoirs and maintaining the atmospheric greenhouse.

Table 1. Planetary atmospheres: Their present composition and an estimate of Earth’s atmosphere without life. (Lovelock, J., 1995).

During the Hadean (4.6-3.7 Billion Years ago) Earth was highly radioactive resulting in intense volcanism, which pumped huge quantities of CO² into the early atmosphere as well as water vapour, nitrogen, carbon monoxide and hydrogen. This was a reducing atmosphere with no free oxygen. Oceans would have formed as Earth cooled and atmospheric water condensed out. Huge amounts of CO² were removed from the atmosphere by rock weathering.

Lovelock J. (1991) postulates that in the reducing atmosphere on Earth during the Archean (3.7 –2.5 billion years ago) ‘There would have been a continuous production of hydrogen gas from the reaction of oxides in basalt rock with CO² and water. Water would have been split releasing hydrogen to the atmosphere and locking the oxygen into the various carbonates of sodium, potassium, calcium, magnesium and iron’. Lovelock believes that if it hadn’t been for the evolution and intervention of life, this process would have continued until all the water had been removed from Earth. The first simple bacteria are thought to have evolved around 3.6 billion years ago. Micro-organisms on the seafloor metabolised hydrogen into hydrogen sulphide, retaining the hydrogen produced by the reaction of water with the seafloor rocks. Additionally, photosynthetic algae produced oxygen as a by-product of photosynthesis and in the reducing atmosphere this oxygen combined with hydrogen to form water.

There is no compelling reason to suspect that Mars’s atmosphere was any different to Earth in terms of reduction properties and also offers another plausible explanation for the disappearance of water from the surface of Mars. By the same processes that occurred on Earth, the hydrogen and oxygen fractions of water would have been split, the hydrogen being lost to space and the oxygen locked up in the surface rocks. The heavily oxidised surface rocks give Mars its characteristic red colour. Another possibility is that due to Mars’s small size and weaker internal heat engine than Earth’s, there was no system of plate tectonics cooling the mantle relative to the core creating convection currents to keep its dynamo turning. The solar wind is powerful enough to strip away atmospheres. It is prevented from doing so on Earth by the magnetic field, which deflects the solar wind around and behind the planet. Scientists have predicted that the solar wind stripped away Mars’s atmosphere during the planets first two billion years.

Archean photosynthetic organisms on Earth started removing atmospheric CO² and producing oxygen, but oxygen levels did not build up for a considerable time (Fig 2). This was due to reaction of oxygen with reducing compounds in rocks, methane and organic matter. Any oxygen produced by photosynthesis quickly combined with the plentiful reducing substances at that time. The reservoirs of reducing substances eventually became saturated and free oxygen could remain in the air although still at low levels. The atmosphere was dominated by nitrogen with CO² and methane levels around 0.1 and 1 per cent (Table 2.1).

Fig 2. The variations of oxygen, methane, and carbon dioxide during the history of life on Earth. The abundance of gasses shown on the vertical scale is logarithmic, i.e. in powers of ten: 1 means 10 ppm, and 5 means 100, 000 ppm. The horizontal axis shows time expressed as eons before present. O²-Oxygen, CO²- Carbon Dioxide, CH4- Methane. (Lovelock, J., 1991).

Table 2.1. Estimate of the Archean atmospheric composition before and after life appeared. (Lovelock, J., 1995).

On Earth during the Proterozoic (2.5-0.7 billion years ago) the addition of oxygen to the atmosphere was slow but steady. Once sufficient oxygen had built up it reacted with sunlight in the stratosphere to form ozone. The ozone protected organisms as it does today from harmful UV radiation and enabled the colonisation of the land by vascular plants about 400 million years ago. The increased carbon burial caused oxygen levels to rise rapidly to around present day levels of 21 per cent. Simultaneously there was a reduction in methane as oxygen levels rose and a switch from reducing methane dominated atmosphere to an oxygen dominated oxidising atmosphere (Fig 2).

Throughout Earth’s history an atmospheric greenhouse has protected it from freezing or burning up despite significant changes in solar luminosity. The sun was burning 25 per cent less brightly back in the Hadean than it is now. The Earth had more internal heat at this time due to its recent accretion from planetesimals and higher radioactivity. As a result volcanoes and plate tectonics emitted copious quantities of CO² providing a thick atmospheric blanket keeping Earth warm despite a 25 per cent less luminous sun. As atmospheric CO² levels have fallen the greenhouse effect has been reduced compensating for the increase in solar luminosity over the same time frame and keeping Earth’s surface temperature relatively constant.

Since the Industrial Revolution the burning of fossil fuels has been pumping vast quantities of CO² back into the atmosphere far quicker than rock weathering can remove it resulting in a net increase. This may be responsible for the phenomenon known as ‘global warming’. Vegetation loss through agriculture and logging has also increased CO² levels by removing organisms that would have photosynthesised. Other greenhouse gasses are also being added to the atmosphere. Chloroflurocarbons have received the most publicity due to their destructive action on the ozone layer.

It can be seen from Table 1 that Earth and Mars differ wildly in the composition of their present day atmospheres. Atmospheric pressure on Mars today is less than 1/100th that of Earth. The Martian atmosphere contains very little water vapour because the low temperature causes it to condense. The water is locked up either in the polar ice caps or as ice below the surface. CO² forms 95.3% of the atmosphere, nitrogen 2.7% and the rest a mixture of trace gasses. The thin atmosphere is very inefficient at transporting heat causing the CO² to freeze as dry ice at the poles during winter.

Despite both Mars and Earth starting out with the same raw materials and early on in their history having very similar atmospheres, they have ended up with drastically different atmospheric compositions. The appearance of life on Earth and its subsequent effect on atmospheric chemistry had profound implications for life in terms of its own continuance on the planet. In short, life made further life possible. The Earth’s size (relative to Mars) and its system of plate tectonics and volcanism have also played key roles in forming and then enabling Earth to retain its priceless atmosphere, the pre-requisite for life. (1642 words).

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