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Nitric oxide formation by meteoroids in the upper atmosphere
Discussions
864
NITRIC OXIDE FORMATION METEOROIDS IN THE UPPER ATMOSPHERE*
BY
This paper presents interesting and valuable calculations which suggest that production of nitric oxide by meteoroid impact upon the upper atmosphere is an important process. The results presented indicate that this is a much more satisfactory explanation for the observed distribution of NO than those propounded in earlier models. The assumptions made by the authors are, in general, adequately discussed by them. However, it seems worthwhile to comment further upon some of the astronomical aspects of their work. Their initial assessment of the total meteoritic material collected by the Earth in a year seems rather large. A calculation by Hughes (1975) indicates a figure of between 30,OOtland 8000 tonnes; somewhat less than the authors’ figure of 1,000,000 tonnes. It is easy to sympathize with the authors’ feeling of frustration over the diverse nature of values for physical attributes of meteors. This is not only a result of lack of basic meteor information. but also reflects the wide range of sources considered likely to be responsible for the interplanetary debris. The authors’ problems over mass and velocity distributions could by resolved, at least partially, by considering the known sources of as much meteoroid material as possible. For instance, there are a number of occasions during the year when the Earth passes through a meteor stream. At these times, the flux of meteoroidal material is noticeably increased. Since this debris has been identified with decayed comets, the physical parameters of the material should be more easily established than that of the sporadic flux of debris which may originate from a number of diverse sources. This known fluctuation through the year may also bear upon the authors’ assumptions concerning the annual influx of material and the global production rate of nitric oxide. Menees and Park also assume that the vapour from the meteoroid is of identical composition to that of the air (which is taken to consist only of Nz and 0,); noting incidentally that this assumption is not crucial since the vapour-to-air molar fraction is r 0.01. I mention this assumption only to bring me to my final comment. It would be particularly interesting to know what effect carbon compounds and especially CO* had upon the chemical reactions in the wake of the meteoroid. This comment stems not only from the obvious fact that the majority of meteoroids, and thus their vapour trails, will contain substantial proportions of carbon-based material, but also from a desire to know whether similar reactions would * Menees G. P. and Park C. (1976) Atmospheric Enuironmerit 10, 534-545. t Present address: Mathematics Department, University of Salford, Manchester, England.
be expected m an atmosphere dominated by CO, whilst containing small but significant mixing ratios of Nz and OZ. The far-reaching astronomical interest in this type of theory, if it may be applied to a CO, atmosphere, is obvious. It is now generally believed that the atmospheres of the terrestrial planets were formed by a combination of degassing and vaporization of infalling debris early in the history of the Solar Svstem (Walker. 1977). The exact leneth of time over which impacting of debris continued is st;l not established. During this period of almost continuous bombardment of the evolving planetary atmospheres, the type of reactions described by Menees and Park would, presumably, have been a dominant force in the evolution of the upper atmosphere and ionosphere and possibly also affected the evolution of the troposphere. As far as I am aware, no calculations have ever been made to assess the effect upon an evolving terrestrial atmosphere of such meteoroidal impacts. The planet Mars would probably have suffered these effects to a greater extent and over a larger time period than the other terrestrial planets due to its proximity to the asteroid belt and its probable slowed accretion caused by the perturbing gravitational influence of Jupiter. It is possible that a similar reaction to those discussed by the authors for the Earth could be a contributing factor to the anomalously small ratio of N,:CO, in the atmosphere of Mars compared with the total degassed volatiles on Earth. The widely held belief that a considerably larger amount of nitrogen was originally degassed by Mars than is now present in its atmosphere seems‘to have been confirmed by the atmospheric entry experiments performed by the Viking spacecraft. McElroy (1972) has suggested that the escape of nitrogen from the evolving Martian atmosphere was aided by non-thermal mechanisms in the upper atmosphere. The number density profiles he discusses would presumably be modified by the type of meteoroid-forced chemistry developed by Menees and Park. I urge the authors to consider extending their calculations to the other terrestrial planetary atmospheres, since impact by interplanetary debris is an effect suffered by all the planets.
REFERENCES
W. (1975) Cosmic dust influx to the earth. Space Res. 15, 531-539. McElroy M. B. (1972) Mars: an evolving atmosphere. Science 175, 443-445. Walker J. C. G. (1977) Euotufion of’ fhe Atmospiicrt,. Hafner Press (to be published). Hughes
D.
864
NITRIC OXIDE FORMATION METEOROIDS IN THE UPPER ATMOSPHERE*
BY
This paper presents interesting and valuable calculations which suggest that production of nitric oxide by meteoroid impact upon the upper atmosphere is an important process. The results presented indicate that this is a much more satisfactory explanation for the observed distribution of NO than those propounded in earlier models. The assumptions made by the authors are, in general, adequately discussed by them. However, it seems worthwhile to comment further upon some of the astronomical aspects of their work. Their initial assessment of the total meteoritic material collected by the Earth in a year seems rather large. A calculation by Hughes (1975) indicates a figure of between 30,OOtland 8000 tonnes; somewhat less than the authors’ figure of 1,000,000 tonnes. It is easy to sympathize with the authors’ feeling of frustration over the diverse nature of values for physical attributes of meteors. This is not only a result of lack of basic meteor information. but also reflects the wide range of sources considered likely to be responsible for the interplanetary debris. The authors’ problems over mass and velocity distributions could by resolved, at least partially, by considering the known sources of as much meteoroid material as possible. For instance, there are a number of occasions during the year when the Earth passes through a meteor stream. At these times, the flux of meteoroidal material is noticeably increased. Since this debris has been identified with decayed comets, the physical parameters of the material should be more easily established than that of the sporadic flux of debris which may originate from a number of diverse sources. This known fluctuation through the year may also bear upon the authors’ assumptions concerning the annual influx of material and the global production rate of nitric oxide. Menees and Park also assume that the vapour from the meteoroid is of identical composition to that of the air (which is taken to consist only of Nz and 0,); noting incidentally that this assumption is not crucial since the vapour-to-air molar fraction is r 0.01. I mention this assumption only to bring me to my final comment. It would be particularly interesting to know what effect carbon compounds and especially CO* had upon the chemical reactions in the wake of the meteoroid. This comment stems not only from the obvious fact that the majority of meteoroids, and thus their vapour trails, will contain substantial proportions of carbon-based material, but also from a desire to know whether similar reactions would * Menees G. P. and Park C. (1976) Atmospheric Enuironmerit 10, 534-545. t Present address: Mathematics Department, University of Salford, Manchester, England.
be expected m an atmosphere dominated by CO, whilst containing small but significant mixing ratios of Nz and OZ. The far-reaching astronomical interest in this type of theory, if it may be applied to a CO, atmosphere, is obvious. It is now generally believed that the atmospheres of the terrestrial planets were formed by a combination of degassing and vaporization of infalling debris early in the history of the Solar Svstem (Walker. 1977). The exact leneth of time over which impacting of debris continued is st;l not established. During this period of almost continuous bombardment of the evolving planetary atmospheres, the type of reactions described by Menees and Park would, presumably, have been a dominant force in the evolution of the upper atmosphere and ionosphere and possibly also affected the evolution of the troposphere. As far as I am aware, no calculations have ever been made to assess the effect upon an evolving terrestrial atmosphere of such meteoroidal impacts. The planet Mars would probably have suffered these effects to a greater extent and over a larger time period than the other terrestrial planets due to its proximity to the asteroid belt and its probable slowed accretion caused by the perturbing gravitational influence of Jupiter. It is possible that a similar reaction to those discussed by the authors for the Earth could be a contributing factor to the anomalously small ratio of N,:CO, in the atmosphere of Mars compared with the total degassed volatiles on Earth. The widely held belief that a considerably larger amount of nitrogen was originally degassed by Mars than is now present in its atmosphere seems‘to have been confirmed by the atmospheric entry experiments performed by the Viking spacecraft. McElroy (1972) has suggested that the escape of nitrogen from the evolving Martian atmosphere was aided by non-thermal mechanisms in the upper atmosphere. The number density profiles he discusses would presumably be modified by the type of meteoroid-forced chemistry developed by Menees and Park. I urge the authors to consider extending their calculations to the other terrestrial planetary atmospheres, since impact by interplanetary debris is an effect suffered by all the planets.
REFERENCES
W. (1975) Cosmic dust influx to the earth. Space Res. 15, 531-539. McElroy M. B. (1972) Mars: an evolving atmosphere. Science 175, 443-445. Walker J. C. G. (1977) Euotufion of’ fhe Atmospiicrt,. Hafner Press (to be published). Hughes
D.