Gravitational separation, composition and structural parameters of the night atmosphere at altitudes between 100 and 210 km

Planet.

Space Sci..

1963, Vol.

11, pp. 441 to 449.

Pergamon

Press Ltd.

Printed

in Northern

Ireland

GRAVITATIONAL SEPARATION, COMPOSITION AND STRUCTURAL PARAMETERS OF THE NIGHT ATMOSPHERE AT ALTITUDES BETWEEN 100 AND 210 KM A. A. POKHUNKOV Translated by 0. M.

BLUNN

from Zskusstvennye Sputniki Zendi, No. 13, p. 110 (1962)

The neutral gas composition of the upper atmosphere was measured by a 5-stage frequency mass-spectrometer which, as in previous experimentG2) was mounted in a recoverable geophysical container jettisoned from the rocket. The launch was made at night on 23 September 1960 at 00.56 hr local time at middle latitudes in the European part of the U.S.S.R. The container was lifted vertically by a large geophysical rocket to an altitude of 210 km. Measurements were taken with the container a few score to a few hundred metres away from the rocket after jettisoning. This experiment employed the improved radio frequency mass-spectrometer which had been used for experiments in 1959. The device has been described elsewherec3). Its general appearance, method of mounting and technical characteristics have already been described in this journalt4). At altitudes between 100 and 210 km during ascent and descent 51 and 50 mass spectra of the atmospheric gases were obtained respectively. The mass spectra were recorded by a galvanometric oscillograph which was accommodated in the sealed instrument compartment of the container. The minimum detectable ion current to the collector of the analyser was 4 x lo-l4 A (for an input resistance R = 1O1lQ). A characteristic feature of the spectra relating to the ambient atmospheric gases was the periodic modulation of the ion currents due to rotation of the container and the ram effect due to its velocity. This enables the contribution to the spectra due to gas emitted from the container to be taken into account in subsequent analysis of the data. Ion peaks were recorded with the following mass numbers: 1, 2, 12, 14, 16, 17, 18, 28, 29, 30, 32, 34, 36,40 and 44, which were identified respectively with H1, H,, C, N,, 01, OH, HzO, Nz, N14 N15, NO, 02, 016018, A?, Ar40, CO, and N,O. In addition, gases with mass numbers 9, 10 and 42 were recorded. Their identification is considered latter. RESULTS AND DISCUSSION

Gravitational separation and temperature of the atmosphere A study of the relationship between altitude and the ratio of the ion currents Ar and N, shows that this ratio decays almost exponentially with increasing altitude during the ascent and descent (Fig. 1). This indicates that gravitational separation of Ar and N, occurs. Comparison of the laboratory-measured ratio, corresponding to the gas content at ground level, with the value obtained during flight (corrected by the mass discrimination factor*), shows that gravitational separation begins at altitudes of 105-I 10 km. * In this particular case the factor is not X’M(N,)/M(Ar) = 0.8375 (see Repnevljl) in view of the fact that in the ion source ionization of the undistorted input flow (discrimination factor = I) is just as probable as output flow modified in the diffusion ratio. In this case the correction coefficient is obviously (1 + 0.837)/2 C=Y 0.92. 441

442

A. A. POKHUNKOV

h, km FIG. I. VARIATION OF THE RATIO OF THE ION CURRENTS Ar AND WITH ALTITUDE DURING ASCENT (1) AND DESCENT (2)

N,

This conclusion regarding the presence of gravitational separation in the night atmosphere at middle latitudes U.S.S.R. agrees with the results of the first two experimentsoJJ which were carried out in the summer of 1959 during morning hours at the same latitudes, and with the data of American authors c6) obtained during rocket probes of the upper atmosphere at Fort Churchill (59” N. Lat). Yet there is disagreement with other datat7) by the same authors obtained during investigation of the upper atmosphere by rockets at White Sands (30” N. Lat). Mirtov(*) casts doubt on the conclusion regarding the absence of gravitational separation in the atmosphere above White Sands on the grounds that there were systematic errors in the experiments. There are good grounds to suppose that the atmosphere of the Earth, at least in middle and circumpolar latitudes, is in a state of gravitational separation at altitudes above 100-l 10 km. The level of initial separation cannot apparently be rigorously defined, and the small number of experiments allows no conclusion to be drawn regarding the existence of latitudinal or seasonal variations in the altitude of the level of separation. In this experiment, as previously”s2), atomic nitrogen at altitudes between 100 and 210 km was not detected in amounts exceeding l-2 per cent of the concentration of molecular nitrogen N,. Therefore, assuming that Dalton’s law is applicable to Ar and N, throughout the range of altitudes where it is possible to ignore the effect of the photo-chemical reactions for which absorption of molecular nitrogen occurs, it can be assumed that the barometric formula holds good for the variation of partial pressures: p(N2) = p0(N2) exp (--M@lRT). Here p, g and T are the pressure, acceleration of the force of gravity at the altitude h respectively. In this case the relative concentration altitude according to the formula

n(WndN2) n(Ar) n,(Ar) = -

exp

-

(MA, -

Mp: )gh

RT

1 ’

and the temperature of Ar varies with

(1)

GRAVITATIONAL

SEPARATION

where n,(Ar) and n,(N,) are the absolute concentrations separation. Putting k for [n(Ar)]/[n(N&], it is seen that

k = k, exp

c

- i

!

443

of Ar and

N, at the level of

,

where H = RT/[(M,, - M,)g] is the altitude of a homogeneous atmosphere for the relative concentration Ar. But since the ratio of the ion currents (apart from the proportionality factor) corresponds to the ratio of the concentrations, by applying formula (2) to the experimental curve for the variation of the ratio of the ion currents Ar and N, with altitude, values of H can be calculated and consequently the temperature T of the atmosphere at different altitudes can be calculated. It was possible to calculate T up to 185 km by this method. Above this level, for the analysis of the pressure of N, according to the barometric formula, the temperature T was determined by linear extrapolation of the temperature with altitude up fo 210 km. The results are given in the following Table. The error in determining T was about 10 per cent of the measured magnitude. TABLE

h, km

T, “K

1010/cm3

1010/cm3

m,), 101°/cm3

I’> a.e.m.

P* 1O-6 mm Hg

100 110 120 130 140 150 160 170 180 190 200 210

215 265 325 395 490 600 715 785 825 860 895 925

740 170 48 17 I.6 3.6 1.9 1.1 0.13 0.50 0.33 0.23

180 40 II 3.8 1.63 0.73 0.36 0.21 0.13 0.080 0.050 0.032

68 20.7 7.9 3.48 I.86 1.1 0.65 0.44 0.32 0.25 0.19 0.15

27.9 27.6 27.2 26.9 26.5 26.1 25.8 25.5 25.1 24.9 24.4 24.1

250 64 23 10 6.7 4.3 2.7 I.8 1.2 0.73 0.53 0.38

Wd,

n(W

1

10 &,:’ 490

110 31 II 5.0 2.3 1.2 0.75 0.49 0.34 0.24 0.16

Main composition, pressure and density of the atmosphere The main components of the atmosphere from the experiment between 100 and 210 km are Nz, 0, and 0,. In the range between 100 and 150 km isotopes N,(Nr4N15) with the mass number 29 a.e.m. were also detected, whilst between 100 and 126 km isotopes 0,(016018) with mass number 34 a.e.m. were detected. The relative concentrations of these isotopes at all observed altitudes were constant and equal respectively to (7.6 & 0.6) x 1O-3 for N, and (4.1 5 0.6) x 10e3 for 02, which agrees well with the relative spread of the isotope N14Nls (76 x 10e3) and of 01601s (4.0 Y 1O-3)(Q). study was made of experiments carried out by In a previous paperC4) a comparative mass-spectrometers with constructionally different analysers and on the basis of this comparison a correction coefficient was obtained which took into account the various reactions which change the composition of the analysed gases. This coefficient was used to produce ratios of the concentrations of 0, and 0, to Nz in the atmosphere at different altitudes. The resulting ratio values can serve as a basis for determining the absolute concentration of atmospheric gases if appropriate laboratory calibration data are to hand regarding the dependence of the ion currents on the partial pressures. In view of the absence of data concerning the orientation of the container, it was impossible to re-calculate the measured 8

A. A. POKHUNKOV

FIG. 2. VARIATION OF THE ION CURRENT N, WITH ALTITUDE DURING ASCENT (1); VARIATION OF THE PRESSURE OF N2 IN THE ATMOSPHERE (2). AS CALCULATED BY THE BAROMETRIC

FORMULA

partial pressures of the gases in the analyser in terms of the pressure of these gases in the atmosphere and the velocity of the oncoming flow. Figure 2 shows the variation of the ion current N, with altitude during ascent, modulated by the rotation of the container. The curve for descent is similar in shape. At the summit of the trajectory where the speed of the container is close to zero, the pressure pn in the atmosphere can be found from the thermal effusion formulaoO):

where T, is the temperature of the device, taken to be 300”K, T,, is the temperature of the atmosphere and pd is the pressure of N, in the device (analyser) from the laboratory calibration curve. Taking the value of T, at 210 km from the Table, it is found that the pressure at this altitude less the background due to container gas contamination is (2.2 & 0.4) x lo-’ mm Hg. Having applied the barometric formula and using this value of the pressure at 210 km as p. in this formula, the distribution of the pressure N, can be obtained at all altitudes where the temperature is known right down to 100 km. The resulting distribution of the pressure N, in the atmosphere is shown in Fig. 2 (curve 2). The necessary temperature values at different altitudes are taken from the Table. Since altitude intervals Ah = 2.5 km were used, it is assumed that in the interval Ah atmospheric temperature and the acceleration of gravity are constant quantities. Further calculations using previous data(l) concerning the relative concentrations of O,, O2 and N, enabled the distributions of the absolute concentrations of the main atmospheric gases, pressure and density of the atmosphere at

GRAVITATIONAL

SEPARATlON

445

t, set FIG. 3. VARIATION WITH

OF THE ION CURRENTS H1O, OH, H, AND ALTITUDE AND FLYING TIME DURING ASCENT

altitudes between 100 and 210 km to be obtained. cluded in the Table. SMALL

ATMOSPHERIC

H,

The results of these calculations

are in-

ADMIXTURES

Group H,O, OH, H,, H, the range of In this experiment all the stated gases except H, were detected throughout altitudes investigated (Fig. 3). Unlike the experiments in 1959°.z’, the registered amount of H, was much less and the behaviour of the corresponding ion current was quite different. The ion current H,O decreases with altitude and its variation tends to be symmetrical It will be seen from Fig. 3 that the modulation of the about the summit of the trajectory. ion current of Hz0 is hardly pronounced compared with that of N, (Fig. 2) due to alteration in the position of the inlet aperture in relation to the inflowing stream. This indicates that a mass-spectrometer in the upper layers of the atmosphere mainly registers the molecules of H,O adsorbed on the surface of the container. Only that part of the ion current which undergoes modulation in conformity with the change in orientation of the inlet aperture However, the possibility cannot be can be caused by H,O from the upper atmosphere. excluded that modulation is explained by the incidence in the analyser of H,O molecules

446

A. A. POKHUNKOV

which appeared during gas-separation and underwent reflection in interaction with the molecules of the counter flow. The maximal value of the pressure of H,O in the upper atmosphere (I 15 km), estimated from the percentage modulation of the ion current, does not exceed 3 ‘/ IO--; mm Hg, which is approximately 0% per cent of the total pressure of the atmosphere at this altitude. Bearing the foregoing in mind, this value is to be regarded as the upper bound of the H,O content of the night atmosphere at altitudes over 100 km. On dissociation of H,O in the ion source of a mass-spectrometer. hydroxyl HO and atomic hydrogen H, arc formed which subsequently are subjected to ionization and form corresponding ion currents on the collector. In view of this for each device in the laboratory the ratio of the ion currents H,O and products of dissociation (OH and H,) can bz obtained. The presence of the OH registered by the mass-spectrometer, as compared with the ratio of the ion currents I(OH)/I(H,O) obtained in flight and laboratory conditions can be fully explained (allowing for measurement error) by the dissociation of H,O in the ion source of the mass-spectrometer. Taking the magnitude of the measurement error as the upper bound of the OH content of the atmosphere, it is found that the relative OH content of the night atmosphere at altitudes higher than IO0 km does not exceed 6 ,. 10 -3 per cent of the total amount of all gases. The behaviour of the ion current H, resembles that of H,O and OH (Fig. 3) as regards lariation with altitude, which is indicative of the dissociative relationship of H, and H%O. However, from 145--l 50 km upwards. the ion current HI decreases more slowly than is necessary to satisfy the laboratory ion current ratio I( H,O) : f(OH): /(H, 1. If the ion current H, is represented as the sum of two currents (an equivalent current Mhich satisfies the ratio and an “excess” current), then at altitudes upwards of I50 km the excess current is greater than the equivalent current by a factor of I.5 to 2. The concentration of H, corresponding to the excess ion current is no less than IO”/cm”. The possibility is not excluded that the presence of this excess quantity of H, is explained by atmospheric hydrogen, but for a final conclusion it is necessary to carry out more experiments. The variation of the ion current H, detected at altitudes up to 130 km on the ascending branch is correlated with the variation of the ion current H,. It may therefore be concluded that the registerable molecular hydrogen H, is associated by its continuance with the re-combination of atomic hydrogen H, inside the analyser of the mass-spectrometer. The absence of Hz from the spectra at altitudes over 130 km may serve as proof that the concentration of H, in the atmosphere above the stated level does not exceed 2 ,.’ IO’/cm’ (the threshold of registration for H, in this particular device). From the ratio of the ion currents H, and I-l,. with due regard to mass discrimination. the degree of re-combination of atomic hydrogen in the analyser can be determined. For the mass-spectrometer in question, not more than 3-4 per cent of atomic hydrogen H, This circumstance, in contrast to the statement undergoes re-combination in the analyser. confirms the possibility of taking quantitative measurements of the conby IstominC1l). centration of atomic hydrogen in the atmosphere by mass-spectrometers of existin_e design. He/i/l/l? (tie)

Within the limits of sensitivity of the device after opening (unsealing?) the analyser, no atmospheric helium was detected in any recording. This gives grounds to say that at altitudes upwards of 100 km the concentration of He does not exceed 6 x lO’/cm”. Comof the atmosparison w,ith the data of American authors w shows that the composition phere is better studied by a container which is separated from the rocket than by one which

GRAVITATIONAL

SEPARATION

447

is mounted on it. In the latter case the rocket-emitted gases inevitably exert a strong influence. Thus, in particular, the helium concentratioI1 at altitudes 130-l 70 km (over 10s~c~~) reported by Meadows and Townsend w and the character of its variation with altitude must on the whole be attributed to the gas emission of the rocket where He was used as a working gas. To form refined conclusions regarding the light gas content of the upper atmosphere. using a jettisoned container, further experiments are necessary with massspectrometers which arc more sensitive to hydrogen and helium. CO, nttn N,O

of

Little can be said about the content gases with the mass number 44 in view of the fact that in this experiment the CO, gas contamination by the container was high. The variation of the ion current CO, 1 N,O decreases regularly with increasing altitude and is weakly modulated by the counter flow (like the ion current H,O). In the same way as for H,O, the upper bound of the CO,) - N’,O content of the atmosphere was determined from the percentage modulation of the ion current corresponding to 44 a.e.m. The maximum value of the concentration of the mixture of these two gases is I per cent at the altitude of 120 km, which roughly coincides with the figure obtained in the 1959 experimentri*2i. Some part of this mixture of gases can consist of the CO, formed in the reactions of atomic and molecular atmospheric oxygen on the oxide-tllorilIm cathode of the ion source. The gas which was detected at alI altitudes with the mass number I2 a.e,m. (carbon C) must be regarded as having been formed during the dissociation of CO, in the ion source of the mass-spectrometer. This is indicated by the fact that the ratio of the ion currents of gases with mass numbers 12 and 44 is independent of altitude [/(C):/(CO, ‘b N,O) O.OlS]. NO At altitudes between 130 and 180 atm in ascent and descent a small amount of gas was detected with the mass number 30. This was identified as NO. Apparently a considerable part of the registcrcd NO was formed inside the analyser as a result of ion exchange rebetween the actions of the type 0,; j- N, = NO-- :- N. as indicated by the correlation variation with altitude of the ratios of the concentrations NO/N, and O,Nf. In view of this the stated value can be regarded only as the upper limit of the concentration of NO in the atmosphere at altitudes 130-I 80 km.

At altitudes between 100 and I25 km in ascent and descent gases with the indicated mass numbers were detected which are not characteristic of the atmosphere near ground level. The ratios of the ion currents of two of these gases with mass numbers 9 and IO to the ion current N, are roughly the same at all altitudes between 100 and 125 km and equal of respectively to (5 .- 2) ‘d 10e5 and (4 11: 1.5) ’ IO- >. The ratios of the concentration in these gases to the concentration of N,, having regard to the light mass discrimination the analyser. are approximately eight times greater than the reduced ratios of the ion curPreliminary identification with rents. These gases have not as yet been finally identified. beryllium Be and the isotope of Boron B in still leaves a number of questions open such as: are these elements of the upper atmosphere of cosmic origin?, why is the principal isotope of Boron B” absent? Discovery of gas with the mass number 42 is of considcrablc interest. This gas was

448

A..4.POKHUNKOV

FIG. 4. VARIATION OF THE RELATIVE CONCENTKATION OF Mg*"O WITH ALTITUDE DURING ASCENT (1) AND DESCENI (2)

detected during ascent and descent at altitudes between 103 and 126 km. It was identified as the oxide of one of the isotopes of magnesium Mz6. It is common knowledge that there are three Mg isotopes with mass numbers 24, 25 and 26, the relative extent of which is in the ratio 78, 60: 10, 1 I : 11, 29. The relative concentration of the oxides of these isotopes, the mass numbers of which are respectively 40,41 and 42, are in the same ratio. The stated identification is not at variance with the existence of the three Mg oxides, since oxides with the mass numbers 40 and 41 could not be isolated on the mass-spectrograms for the following reasons. The ion peak of the oxide of the principal isotope Mg24 cannot be isolated because it coincides with the ion peak of Ar, the ampIitude of which is greater than the peak of Mg2*0 by one order of magnitude. Owing to the slight over-correction of the d.c. amplifier at high frequency (in order to reduce the effect of the input RC), the ion peak of the oxide Mg”“O was unresolved since it coincided in time with the negative over-correction pulse from the ion peak of Ar. From Fig. 4 which shows the variation of the ratio of the ion currents corresponding to Mgz60 and Nz, it is apparent that within the limits of experimental error the absolute concentration of all the Mg oxides in the altitude range 103126 km is approximately 10s/cti. The presence of such a comparatively large amount of MgO in the upper atmosphere indicates that it is of extra-terrestrial origin. It indeed seems that meteorites are the source of MgO in the upper atmosphere, the elements Mg and0 being the principal elements in their composition (PJ). The mean percentage of these elements by weight is respectively 13.9 and 34.6 per cent of the total mass of matter. In meteorite matter Mg is mainly in the form of the oxide MgO, the content of which in stone meteorites reaches 40.2 per cent of their gross weighto3J, whilst the weight ratio between stone and iron meteorites is stated to be 6: 1(r2). Thus MgO represents a considerable proportion of meteorite matter. In the metallic phase the Mg-content of meteorite matter is only O-03 per cento4). Such a low metallic Mg fraction, side by side with the considerable amounts of atomic oxygen at altitudes greater than 100 km which can easily combine with Mg, may explain the absence from the mass-spectrograms of ion peaks corresponding to metallic Mg. The inter-action of micro-meteorites, which form the bulk of meteorite matter, with the atmosphere leads to heat and the gasification of a considerable part of meteorite matter. In this respect it is obvious that the rate of vaporization depends on the temperature, which

GRAVITATlONAL

SEPARATION

449

in turn increases with increasing atmospheric density. Thus, the absollutc concentration of the gaseous matter due to the vaporization of meteors must increase with increasing atmospheric density, which is observed if the relative concentrations of MgO given in Fig. 4 are re-stated in absolute values. As regards changes in the relative concentrations of MgO and N?, it will be seen from Fig. 4 that there is a maximum at the altitude 117-I I8 km which was detected during descent as well as ascent. The occurrence of this maximum can be understood from consideration of the existence of gravitational separation at altitudes greater than 100 km and the presence of an extended transitional layer between an atmosphere with turbulent cross-mixing and an atmosphere where cross-mixing of the gases is diffusive in character in accordance with the laws of molecular physics. In this transitional layer gravitational separation may be reduced by eddies from the lower-lying turbulent atmosphere. In consequence the MgO concentration of the transitional layer decreases owing to partial crossmixing with the layers of atmosphere which lie below the level of meteorite vaporization and which therefore have no MgO content. From consideration of the curve for the variation of the relative concentration of MgzGO and N, (Fig. 4) it may be concluded that although gravitational separation has already set in at altitudes of 105-I 10 km the transitional layer extends upwards to 117-I 18 km. Above this level the relative concentration of MgZ60 still decreases, but in accordance with the laws of gravitational separation of MgZ60 and N,. The total amount of MgO detected by the mass-spectrometer at altitudes between 103 and 126 km (assuming that it is uniformly distributed throughout Earth’s atmosphere) is approximately 7 x loll g. The mean daily amount of matter incident on Earth from of these sporadic meteorites is, according to Mitrov u5), 5 x IO” g. From comparison figures, it is estimated that approximately one year is required to form the stated amount of MgO. In conclusion the author wishes to thank B. A. Mirtov for his constant interest in the paper and his valuable advice, and V. G. lstomin and A. D. Danilov for their critical observations and their discussion of the results. The author also wishes to thank A. A. Perno, R. F. Starostinoi and G. I. Podsoblayevoi for the active part which they took in the experiments and their help in handling the material. REFERENCES i. 2. 3. 4. 5. 6. 7. 8. 9. 10.

1I,

12. 13. 14. 15.

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