Earth, Venus and Mars

There have been various times in Earth history when the extent of ice has been considerably greater than it is now. There were five such periods in which great ice ages occurred, as listed in Table 1.3. All were characterized by extensive polar ice sheets and cooler global temperatures than at present. 

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Hydrogen isotopes have also been fractionated during planetary geologic processes. Compared to the Earth, Venus and Mars have significantly elevated D/H ratios (5D = 125 000 permil and 4000 permil, respectively). These fractionations are thought to result from preferential loss of H relative to D from the atmospheres of these planets (Robert el al., 2000) atmospheric escape of hydrogen from the Earth was apparently not important. 

In the study of atmospheric temperatures and composition one is often interested in the emission from a particular atmospheric constituent. For the analysis of the vertical temperature profile on Earth, Venus, and Mars, thermal emission from the CO2 molecule can be used. If the same gas is contained in the absorption cell, the radiation of interest is being filtered out. How does one measure the radiation that has just been removed from the beam This can be accomplished in several ways. For example, consider an absorption cell with two windows on opposing ends exposed to a beam of radiation. Wavenumbers outside the gas absorption band and in the transparent gaps between lines will penetrate the cell without a noticeable effect. Radiation within the width of strong lines will be absorbed and will cause a temperature rise in the gas. The corresponding pressure increase may be registered by a sensitive pressure transducer. The resulting infrared detector is sensitive only to radiation specifically tuned to the gas in the cell. Such detectors have been produced (the Patterson-Moos cell), but have, as far as we know, never been applied to planetary work. 

Table 1.2. A comparison of the atmospheric compositions of Earth, Venus and Mars (based on data in Wayne, 2003).

The special position of the Earth among the terrestrial planets is also shown by the availability of free water. On Venus and Mars, it has not until now been possible to detect any free water there is, however, geological and atmospheric evidence that both planets were either partially or completely covered with water during their formation phase. This can be deduced from certain characteristics of their surfaces and from the composition of their atmospheres. The ratio of deuterium to hydrogen (D/H) is particularly important here both Mars and Venus have a higher D/H ratio than that of the Earth. For Mars, the enrichment factor is around 5, and in the case of Venus, 100 (deBergh, 1993). 

Capitalize Earth, Sun, and Moon only when used in an astronomical sense. Venus and Mars are the closest planets to Earth. 

While considerations of the origin of planetary noble gases have been predominantly focused on those presently found in the atmosphere, noble gases still within the Earth provide further constraints about volatile trapping during planet formation. A wide range of noble-gas information for the Earth s mantle has been obtained from mantle-derived materials, and indicates that there are separate reservoirs within the Earth that have distinctive characteristics that were established early in Earth history. These must be included in comprehensive models of Earth volatile history. Also, data are now available for the atmospheres of both Venus and Mars, as well as from the interior of Mars, so that the evolution of Earth volatiles can be considered within the context of terrestrial-planet formation across the solar system. 

The most important single physical event took place roughly 4.5 Ga ago, 25 -30 Myr after the birth of the solar system. At this stage, Earth was probably a substantial fraction of its present mass, with a segregated core. Sunwards of Earth, Venus and Mercury had formed outwards, were Mars-like planets. Then, the Earth suffered its largest collision a defining moment in habitability. 

Water, in liquid oceans, is what makes Earth s tectonic history different from Venus and Mars. On Earth oceanic crust and hence plate is cooled quickly by water, because the surface is close to 0 C, not 500°C as on Venus. This cooled plate thickens and becomes dense more quickly than it would on Venus. Plates fall into the astheno-sphere as a steady regular process. Andesite volcanism is fluxed by water given off by subducted oceanic crust. Mars is so cold that reintroduction of water to the interior does not occur. The only volcanism in the past billion years on Mars appears to have been from deep-rooted plumes. 

Further support for solar nebular fractionation comes from the compilation of Brown and Mussett (1993) who showed that the terrestrial planets have densities, which when corrected for their different internal pressure, vary significantly from that of chondritic meteorites. In fact Mars with an uncompressed density of ca. 3.7 Mg.m-3 is the only planet which lies in the chondritic range of 3.4—3.9 Mg.m-3. Earth, Venus, and Mercury are denser than chondrites and the Earth s Moon is less dense. These compositional differences are thought to reflect differences in the Fe/Si ratio between the different planets, which in turn reflects the fractionation of Fe from Si in relation to proximity to the Sun, during the condensation of the solar nebula, further supporting the view that the silica-depletion relative to chondrite took place in the solar nebula. 

The dynamic nature of planet Earth is important here for the periodic resurfacing of the planet means that fresh supplies of lifesupporting elements are provided at the surface, preventing the premature exhaustion of the process of life. There is probably a close connection between the presence of water on the Earth and its dynamic nature, for planets that "dry out" cease to be active. This is perhaps the principal reason for the differences between Earth and its near neighbors, Venus and Mars. The presence of water and the dynamic nature... 

The outer planets also tend to have a number of satellites with (at last count) 56 orbiting Saturn, 63 around Jupiter, 27 around Uranus, and 13 around Neptune, compared to the virtual absence of satellites in the inner planets Mercury with 0 Venus, 0 Earth, 1 and Mars 2. 

Current evidence indicates that all major bodies in our solar system originated about the same time, approximately 4.6 Ga ago. The oldest rocks taken from the surface of the Moon and meteorites found on Earth are about 4.5 Ga old. The Moon, Mercury, Venus, and Mars have cratered surfaces that appear to be the result of the same type of meteoric activity that produced craters on Earth. The early atmosphere on Earth was probably similar to those found today on nearby planets on which life did not evolve. Hence, we can use information about those planets to help us infer the nature of the conditions on Earth under which life presumably evolved. 

It follows from this discussion that our atmosphere has many peculiar characteristics, on the one hand relative to the Venus and Mars or, on the other hand, considering it separately in the Earth-atmosphere system. The question therefore arises how did this anomalous gas cover of the Earth form and what... 

It is generally believed that the solar system condensed out of an interstellar cloud of gas and dust, referred to as the primordial solar nebula, about 4.6 billion years ago. The atmospheres of the Earth and the other terrestrial planets, Venus and Mars, are thought to have formed as a result of the release of trapped volatile compounds from the planet itself. The early atmosphere of the Earth is believed to have been a mixture of carbon dioxide (C02), nitrogen (N2), and water vapor (H20), with trace amounts of hydrogen (H2), a mixture similar to that emitted by present-day volcanoes. 

The auroral phenomena are not unique to Earth. The aurora is found on magnetized planets such as Jupiter, Uranus, and Neptune. On the other hand, nonmagnetized planets such as Venus and Mars have no aurora. 

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