Atmospheric escape

Many of the processes responsible for isotope fractionations in the Earth s atmosphere may also occur in the atmospheres of other planetary systems, such as the atmospheric escape of atoms and molecules to outer space. Likely unique to Earth are isotope fractionations related to biological processes or to interactions with the ocean. One aspect of atmospheric research which has great potential for the application of stable isotope investigations is the study of anthropogenic pollution. 

The existence of hydrogen has been known since the 16th century. The gas, however, was reported in different ways, such as inflammable air. Henry Cavendish was the first to elucidate the chemical nature of hydrogen in 1776 and prepared it by several methods. Lavoisier in 1783 named this element hydrogen, which means, water former. Practically all gaseous hydrogen from earth s atmosphere escaped into outer space during formation of the earth. 

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. 

The equation shows 24 carbon, 28 hydrogen, 20 nitrogen, and 102 oxygen atoms before and after the reaction. The difference is in how these atoms are grouped together. The products formed in this case are all gaseous materials that quickly mix into the atmosphere, escaping our notice. 

Formation by precipitation from the hot gaseous silicate atmosphere escaped from the Earth, as proposed by Ringwood 30), may be considered a variant of the common fission hypothesis (Model a). Fission can be excluded because of the arguments presented in Chap. VII. There are a number of severe constraints that are difficult to overcome in both the precipitation hypothesis and the twin-planet model. 

We postulate, in view of these constraints, that rare gases were delivered to the Earth and fractionated in the atmosphere mainly within the first 100 Ma, possibly within 50 Ma, when most of the terrestrial accretion took place. During this period, atmosphere and proto-mantle rare gases were actively exchanged and rare gas fractionation took place following a combination of impact degassing, atmospheric escape and... 

Note that equations containing t involve only time differences, not their absolute values (e.g., Eqns. 7 and 11). Consequently the fractionations of initial isotopic and elemental ratios generated by hydrodynamic escape do not depend on any specific choice for to, the time in solar evolutionary history when atmospheric escape begins. Permitted values of to are constrained, however, by the solar EUV flux needed to drive an escape episode. For example, as noted in the following section, EUV-driven Xe loss from Earth requires a flux that exceeds the present solar level by a factor of 450, and thus a to no later than -100 Ma if the flux history follows the i = 90 Ma exponential in Figure 10. It is assumed that to, whatever its value, marks the time at which dust and gas in the nebular midplane had cleared to the extent that solar EUV radiation could penetrate to planetary distances, so that EUV-driven atmospheric loss would not have occurred prior to to (Prinn and Fegley 1989). 

Water was surely abundant in early atmospheres and throughout Earth s history with a cooled lithosphere. If there were water. Earth had an oxidizing atmosphere, while hydrogen liberated from water escaped from Earth s atmosphere just as it continues to do today. Both hydrogen gas and helium, when released into Earth s atmosphere, escape from Earth never to return, because the thermal velocity of these gases is above the escape velocity of a mass of their size. They are two of a very few substances that are not automatically recycled. We have no other choice than to recycle unless garbage is to be disposed of in outer space, a highly unlikely event. 

Lammer et al., 2008 [192] discussed the origin and evolution of Venus , Earth s, Mars and Titan s atmospheres from the time when the active young Sun arrived at the Zero-Age-Main-Sequence. Thermal and various nonthermal atmospheric escape processes influenced the evolution and isotope fractionation of the atmospheres and water inventories of the terrestrial planets efficiently. 

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