Planets, composition atmospheres

Sasaki S. (1990) Heating of an accreting protoplanet by blanketing effect of a primary solar-composition atmosphere. Lunar Planet. Sci., abstract 1067. 

The most recent mission to Venus is the European Space Agency s Venus Express spacecraft, launched on November 5, 2005. The spacecraft reached the planet in April 2006 and settled into orbit on May 6. It has now transmitted some of the best images of and data about the planet s atmosphere ever obtained, including the first images ever of its south pole. Among the new data transmitted by Venus Express are the chemical composition of the lower atmosphere, temperature variations at different levels of the atmosphere, temperature measurements of the planet s surface, and reactions between oxygen and nitrogen oxides in the middle and upper atmosphere. 

The availability of space-based observatories, beginning in the 1960s, provided a new and promising way of collecting further data about the composition of the Martian atmosphere. A number of the early U.S. and USSR flights confirmed earlier Earth-based discoveries and provided new values for previously calculated variables. For example, both USSR Mars and U.S. Mariner spacecraft confirmed the concentrations of carbon dioxide and water vapor in the planet s atmosphere, and Mariner confirmed the general distribution of water vapor at various locations above the planet s surface and at various seasons. 

Voyager 1 flew by Saturn on November 12, 1980, at a distance of three planet radii from the planet s atmosphere, while Voyager 2 flew past the planet on August 26, 1981, at a distance of 2.67 planet radii. The two spacecraft carried a variety of instruments that allowed them to measure the chemical composition of the planet s atmosphere, the presence and strength of its magnetic field, any electrical properties associated with the planet, and other physical characteristics of the planet, its satellites, and ring system. 

Habitability and the HZ are first order functions of the stellar flux at the planet s location as well as the planet s atmospheric composition. The latter determines the albedo and the greenhouse effect in the atmosphere. The inner and outer boundaries of the HZ differ for clear and cloudy conditions because the overall planetary albedo A, is a function of the chemical composition of the clear atmosphere as well as the fraction of clouds, A = Adear + Adoud-... 

The main differences among studies of the HZ are the imposed chemical composition and cloud fraction of the planet s atmosphere. Examples of atmospheres with different chemical compositions include the original CO2/H2O/N2 model with a water reservoir (e.g., Earth s), or model atmospheres with high H2/He concentrations [33] or limited water supply [34]. Two concepts are commonly used throughout the literature for cloud free [35] and cloudy atmospheres [36], assuming Earth-like planets. According to these models, the HZ is an annulus around a star where a rocky planet with a CO2/H2O/N2 atmosphere and sufficiently large water content (such as in Earth) can host liquid water permanently on a solid surface. 

There appears to be a correlation between the mass of the planets and the mass and composition of their atmospheres. Generally, only those planets of high mass were able to retain much of their atmospheres. Nitrogen, hydrogen, and helium are probably abundant, though not yet detected, on the heavier planets. Table 25-V also reveals a considerable range in the surface temperatures of the planets. The higher temperatures on the terrestrial planets also contributed to the loss of their atmospheres. 

The Earth is a highly unusual planet because life did evolve on it and it thrived to the extent that the surface and atmosphere of the planet were greatly modified. The Earth is unique in this respect relative to all known astronomical bodies (Taylor, 1999). The Earth s location, composition, and evolutionary history are all significant factors in the planet s success in nurturing life. Critical factors include its temperature, its atmosphere, its oceans, its long-term stability and its "just right" abundance of water and other light element compounds. 

However, we need information on the atmospheric composition in order to plan and carry out simulation experiments. Although the four terrestrial planets originated from the same solar matter, their atmospheres are completely different. This is due to ... 

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). 

The planets nearest the Sun have a high-temperature surface while those further away have a low temperature. The temperature depends on the closeness to the Sun, but it also depends on the chemical composition and zone structures of the individual planets and their sizes. In this respect Earth is a somewhat peculiar planet, we do not know whether it is unique or not in that its core has remained very hot, mainly due to gravitic compression and radioactive decay of some unstable isotopes, and loss of core heat has been restricted by a poorly conducting mainly oxide mantle. This heat still contributes very considerably to the overall temperature of the Earth s surface. The hot core, some of it solid, is composed of metals, mainly iron, while the mantle is largely of molten oxidic rocks until the thin surface of solid rocks of many different compositions, such as silicates, sulfides and carbonates, occurs. This is usually called the crust, below the oceans, and forms the continents of today. Water and the atmosphere are reached in further outward succession. We shall describe the relevant chemistry in more detail later here, we are concerned first with the temperature gradient from the interior to the surface (Figure 1.2). The Earth s surface, i.e. the crust, the sea and the atmosphere, is of... 

Observe the chemical composition of the atmospheres and surfaces of comets, planets and satellites. 

The volatile materials would have vaporised from the surface of the planetesimals once the temperature reached 160 K below this temperature water sticks to silicate surfaces and condenses, ultimately freezing into ice. The new gaseous material is swept away from the planetesimals by the solar wind of particles, leaving bare planetesimals too small to acquire and maintain an atmosphere. The temperature gradient and location within the solar nebula are then important to the ultimate nature and composition of the planets themselves and interplanetary debris. 

The period of emergence of life on Earth is constrained to be between the period 4.0-3.7 Gyr ago, for which there is no fossil record. Urey postulated that all of the planets formed from the same solar nebula and so the early Earth should have an atmosphere with a composition the same as that of Jupiter (known at the time),... 

The trace gases in the earth s atmosphere are only a few percent of its composition but they make the planet livable. They absorb radiant energy at infrared wavelengths much more efficiently than they absorb radiant energy at solar wavelengths, thus trapping most of the radiant heat emitted from the earth s surface before it escapes. 

Fig. 14.4 Oxygen isotopic composition of atmospheric species measured to date (After Thiemens, M., Ann. Rev. Earth Planet. Sci. 34, 217 (2006)). For these data, m, Equation 14.31 is approximately (0.7 < m < 0.9) indicating for these materials the fractionation is better described as anomalous rather than mass independent ...

Bottinga Y, Craig H (1969) Oxygen isotope fractionation between CO2 and water and the isotopic composition of marine atmospheric CO2. Earth Planet Sci Lett 5 285-295 Bottinga Y, Javoy M (1973) Comments on oxygen isotope geothermometry. Earth Planet Sd Lett 20 250-265... 

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