Volatiles inventory

Comets are rich in volatile elements, but they probably delivered no more than 10% of Earth s volatile inventory. There are several reasons for this. Comets have a very low impact probability with Earth over their dynamical lifetime ( 10 Levison et al., 2000), limiting the amount of cometary material that Earth could have accreted. In addition, if most of Earth s water was acquired from comets, it seems likely that Earth s noble gas abundances would be higher than observed by several orders of magnitude (Zahnle, 1998). Einally, water measured spectroscopically in comets differs isotopically from that of seawater on Earth, with the cometary D/H ratio being greater by a factor of 2 (Lunine et al., 2000). 

Many of the important characteristics of the atmosphere are related to the concentrations of particular molecular species, such as O2. Here the absolute abundances of carbon, nitrogen, and hydrogen (as H2O) as supplied from the mantle are considered in the context of models of noble gas degassing from the solid Earth. A convenient reference for comparing surface volatile inventories to mantle reservoirs is obtained by dividing the surface volatiles into the mass of the upper mantle, the minimum size of the source reservoir. However, this should not be taken to imply a particular model of degassing. 

Venus. Venus is characterized only by the immensely valuable but still incomplete and relatively imprecise reconnaissance data from the Pioneer Venus and Venera spacecraft missions of the late 1970s. Additional in situ measurements, at precisions within the capabilities of current spacecraft instrumentation, are now necessary to refine atmospheric evolution models. Unfortunately, the possibilities of documenting the volatile inventories of the interior of the planet are more remote. A significant question that must be addressed is whether nonradiogenic xenon on Venus is compositionally closer to SW-Xe (as seen on Mars) or to the U-Xe that is seen on the Earth and so is expected to have been present within the inner solar system. Also, the extent of xenon fractionation will be an important parameter for hydrodynamic escape models if intense solar EUV radiation drove hydrodynamic escape on the Earth, it would also impact Venus, while losses from the Earth driven by a giant impact would not be recorded there. 

The sedimentary evidence implies the existence of oceans. Although the initial deep volatile inventory of the planet would have been removed by the late great impact that formed the Moon, much of the water presently in Earth s oceans would have degassed from the hot mantle or infallen as comets soon after that great impact, and the ongoing volcanism would have added more. 

The above assumption with regard to materiai at risk wiil be overly conservative in most process scenarios for the less volatile fission products by two to four orders of magnitude, but this will be only about a factor of four in terms of potential total dose consequences, as depicted later in Table 3.4-1. However, this assumption will encompass future unanticipated processing scenarios which might render the less volatile inventory more available for release. Based on this assumption, the hazard and accident analyses are not dependent on any specific isotope processing steps, since there are no process operations which could render the material more volatile. Thus, these process operations are not relied on to mitigate any release consequences and therefore do not perform any Safety Function. 

Occasionaily, abnormal processing events are anticipated which would cause the volatile inventory at a process station to be eievated. 

Sequence H Same as G except that operator does not detect venting and/or takes no corrective action. Complete volatile inventory would eventually be released to SCB. 

Sequence C This scenario results from the loss of both off-site and on-site power, if a target is in process and both the target and SCB are intact, gradual diffusion of residual quantities of contaminants (<1% of target volatile inventory) inside the SCB into Zone 2A and subsequently into Zone 2 could occur. 

Sequence D This scenario assumes that the total loss of power occum simultaneously with failure of a target during processing, at the time that the inventory is most voiatiie. In this case, the contents of the SCB at the time of loss of power are 100% of the volatile inventory of the target. 

Epoch 0 in our model is centred at about 3.9 Gyr ago. The atmosphere was most likely spectroscopically dominated by carbon dioxide that originated from volcanoes or the original volatile inventory, with nitrogen being the most abundant gas, and trace amounts of methane. Therefore, in our input model for this epoch we use 10% CO2, current amounts of CH4, and no O2, O3, or N2O in the atmosphere. 

Lebofsky, L. A., Jones, T. D., Herbert, F. (1989). Asteroid volatile inventories. In Origin and Evolution of Planetary Atmospheres, 192-229, ed. S. K. Atteya, J. B. Pollack, M. S. Matthews. Tucson University of Arizona Press. 

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