Venus EUV measurements of hydrogen and helium from Venera 11 and Venera 12

Adv. Space Res. Vol.5, No.9, pp.119—124, 1985

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VENUS EUV MEASUREMENTS OF HYDROGEN AND HELIUM FROM VENERA 11 AND VENERA 12 J. L. Bertaux,* E. Chassefiere* and V. G. Kurt** *Service d’Aéronomie du C.N.R.S., B-P. no. 3, 91370 Verrièresle-Buisson, France * Space Research Institute, 84/32 Profsoyuznaya, Moscow 117810, U.S.S.R.

ABSTRACT Lyman a and 58.4 nm Hel radiations resonantly scattered were observed with EUV spectrophotometers flown on Venera 11 and Venera 12. The altitude distribution of hydrogen was derived by limb observations from 250 km (exobase level) to 50,000 km. In the inner exosphere (up to = 2,000 lan of altitude) the distribution can be described+b~ a classical exospheric distribution with T 0 275 ± 25 K and n 4 — 2, a factor 3 250 to to 6 110 lower than 1eve~ that l0~ atom. cm~ at 250 km. The integrated number density of from km (~he predicted by aeronomical This number of CO2 absorption) is models. 2.1 x 1012 atom. cm density decreases from the morning side to the afternoon side, or alternately from equatorial to polar regions. Above 2,000 km a ‘hot’ hydrogen population dominates, which can be simulated by T IO3 K and n l0~atom. cm3 at the exobase level. The optical

thickness

of helium above

141 km

(the

level

of

CO

2 absorption for 58.4 nm 3. determined This is about 3 ti~ies less corresponding than what was obtained with the radiation) waa to be t 3, to a density at Bus 150 Neutral km of Mass of Pioneer Venus, and about twice less than ONMS measurements, but is 1.6 Spectrometer a 106 cm in agreement with earlier EVil measurement by Mariner 10 (2 ± 1 a 106 cm3). GEOMETRICAL PAR.AMETERS OF OBSERVATION The multichannel EVil spectrophotometers were placed on board Venera 11 and Venera 12, flying by Venus in December 1978. They recorded the EIJV intensity in 10 discrete wavelength channels across the illuminated disc of Venus (see figure 1). A complete description of the instrument is contained in Bertaux et al., 1981/1/. Two channels were devoted respectively to the resonance emissions of H (121.6 nm, Lyman a) and He (58.4 ma) atoms. Projected on the planet, the full size of the field of view was 770 x 220 krn, from the pericenter distance of 40 x LO~ km. The optical axis of the instrument was fixed in an inertial reference system.

-

~VINUS ~DAYSROIMION

P4 Fig.

I

The track of the line of sight is represented in projection on the disc of Venus, with the full size of the field of view (0.33 X 1.1°). The South Pole (S) has been placed on top. The time is running from left to right, as in the following figures. Bright limb observations are sounding the polar exosphere region. The lirection of the cloud motion defining the 4 days rotation is iud icut el 119

120

J.L. Bertaux, E. Chassefiere and V.G. Kurt

LYMAN a EMISSION AND DISTRIBUTION OF HYDROGEN

After subtraction of the interplanetary Lyman a signal, three different zones can be distinguished in the Venus La emission recorded with Venera 11 and 12 : the disc, the inner exosphere and the outer exosphere (the exosphere is seen on both sides of the planet). From the spatial variation of La emission, the vertical distribution of H can be determined with the help of radiative transfer computations, and comparison of models to data. Because the shape only of the intensity curve is used, there is no need to know the exact value of the calibration factor of the instrument, nor the. value of the solar flux F 5. From relative measurements near the limb, an absolute determination of the density distribution can be achieved. This can be done safely only above 250 km (the altitude of the exobase), whereas below 250 kin only the total vertical column density Nt can be determined down to the level of CO2 absorption at 110 kIn, with no details between 110 and 250 kin. In the inner exosphere, the La distribution at the bright limb data is well represented by a classical exospheric distribution (with no satellite atoms) defined by the following parameters n~(atom density at exobase) and T0 (exospheric temperature)

Latitude (°S) Tc (°K) 3) ~ (atom. cur2) cnf Nt

VENERA 11

VENERA 12

79 300 ~ 25 4 + ~ x 10” 1.7 x 1012

59 275 ±25 6 x 10’ 2.6 a 1012

When spherically symmetric models are compared to the data, a good fit cannot be obtained simultaneously for both the bright limb and the dark limb, where more hydrogen is present. The integrated number density decreases smoothly and up to a factor of 2 along the track of the line of sight on the duyside, which could be either an efiuct of local time or latitude. If it is sri effect of local time, it indicates an asymmetry around noon between the morning and the afternoon sides. Such an effect is present in PVO in situ measurements with more hydrogen in the morning (Brinton et al., 1980 /2/). In addition to the thermal exospheric component, another “hot” component of H is clearly identified, populating the outer exosphere up to = 40,000 km of altitude. Its distribution can be simulated above 4,000 km by an exospheric distribution having — 1O3 atoms. cm3 at 250 km and T 3°K, and the density is higher than the Mariner 10 density by a factor of 2 (T~kacs 7 — 10at al., 1980 /3/), probably as a result of increased solar activity. Von Zahn at al. (1983 /4/) concluded that observed non—thermal H atoms may be pr~,duced by a combination of a ionospheric source (charge exchange of 0+ and 112 , and H with H) with a contribution from solar wind charge exchange. There is also some Indication in the Venera data of a source of hot atoms at a high altitude. All the above conclusions concerning hydrogen were published in Bertaux at al., 1982 /5/. BELUJM

Introduction Two different

techniques can be used and have been used to measure helium density in the upper atmosphere of Venus, and five instruments using these techniques have Indeed yielded results: the Bus Neutral Mass Spectrometer (BNMS /6/) and the Orbiter Neutral Mass Spectrometer (ONNS /7/8/), both on Pioneer Venus mission, and three EUV epeccrophotometers on Mariner 10 /9/, Venera 11 (V 11) arid Venera 12 /1/. The Venus EUV radiation at 58.4 rim is the result of resonance scattering of solar photons by helium atoms in the upper atmosphere of Venus, lying above the altitude where CO 9 absorbs completely the solar radiation (about 140 Icin). Of course, observations of th!s resonance radiation is restricted to the dayside of Venus. However, there is a particular circumstance which allows the derivation of an absolute determination of helium column density N (z ~ 140 kin) from photometric observations of S8.4 rim radiation along the illuminated disc of Venus, even if neither the absolute sensitivity of the instrument nor the value of the exciting solar flux are known. This Is because the helium optical thickness t at line center corresponding to N (z ~ 140 kin) is ~oderate (between I and 10), and thi shape of the intensity variation across the planetary disc

Venus EUV Measurements

of Hydrogen and Helium

121

sensitive to the exact value of -t in this range I to 10. The only assumption required for the analysis is that t s%ould be approximately constant across the disc. This assumption is fortunately valid~, as demonstrated by ONMS results between 6 and 18 hr of local time where resonance radiation can be observed (Von Zahn et al., 1983 /4/). is

Since the final result of the analysis indicates, for both V 11 and V 12 measurements, a helium density which is a factor of = 3 below the currently admitted results of the BNMS (Von Zahn at al., /6/), it was felt necessary to present a careful and convincing statistical analysis of Venera 11 arid 12 data. The Radiative Transfer Computation A radiative tran8fer computer code has been developed to predict the 58.4 rim intensity I emerging from the upper atmosphere of Venus through resonance scattering of solar photons. Spherical symmetry of He and CO 2 distributions was assumed, which is justified from ONMS observations in the range of local time observations. The lower boundary of the helium 58.4 nm airglow, usually simulated by a totally absorbing medium at a fixed level, is represented more accurately in the present model the CO2 actual vertical distribution of Von Zahri et al. /6/ is taken into account to compute the progressive absorption of He I 58.4 rim radiation when photons are flowing downward, both for the direct and the multiple scattering. When the source function S is found, then the emerging intensity can be computed, for the VENERA geometrical conditions of observations, taking also into account the progressive absorption by CO2. As in the case of H Ly a, the emerging intensity is proportional to the exciting solar flux at line center F5, but depends on the helium content (measured by ‘t ) in a non—linear fashion when ~t ~ 1. The counting rate of the instrument C, when oI,serving the intensity I, is proportional to the intensity I with the calibration factor a (in Counts! Rayleigh for the counting time of I s) Ca

I—a F5 f(’t)—Af(t) 0 0

where f (‘t ) is an Increasing function of conditions) %hich is accessed to only through the problem.

(for the relevant geometrical solution of the radiative transfer

t0

It can be remarked that a and F5 appear as their product and cannot be distinguished from a comparison of measurements C to computations f ( t0) only the parameter A — a 2 S A)~ at Venus F5 will be derived from this comparison. However, for practical purposes the solar flux d~stance, intensities in —Rayleigh unitsphotons from the(cm computer coda. F at lineallowing center to was predict fixed at a value Fs 8.6 a 1O~ Adjustaent of )~delto ~ta The vertical distribution of helium as measured by the BNMS (Von Zahn et al. /6/) was taken as a reference. The vertical column density N (z 140 1cm) — 3.3 a 1013 cm2, and yields -a value of t — 7.56, when the temperature is 275°K. Since it was rapidly obvious that the modgl intensity curve did not fit the data well, other vertical profiles were also used, obtained by scaling down the reference BNMS profile, providing the following values of t : 7.56, 5.04, 4.03, 3.02, 2.02, 1.01. For these six vertical distributions, the fntensities I~were computed across the disc and computed to data C 1. When necessary, for intermediate values of i~ , a linear interpolation was made to predict Ij. The adjustment is made with a least-square method, trying to minimize the mean deviation between computed intensities and measurements difference between the C 1n1easur~ment~the a1 + C, counting raterepresents, and the predicted a and where c~ for theone, itha l~. Two parameters are to be found

actual

The estimated variance V which is to be minimized over the n measurements is V

—

(n—I)

E (C 1

and, by derivation, the optimal value each value of ~r

a Ii)2

a0~~of the parameter a is found immediately for E —

Ciii E

122

J.L. Bertaux, E. Chassefiere and V.G. Kurt

The intensity measurements of Vll(figures 2 and 3) and V 12 (figures 4 and 5) are compared with the model predictions fo~ r 0 7.56 (reference value) and for the optimal value r~ found from the least—square exercise, 2.4 and 2.63 for V 11 and V 12, respectively. Computed inteuaities in Rayleigh intensities units come from the model from assuming F9 =rates 8.6 xwith iO~photons— 1,whereasmeasured are converted counting the value of a (cm2.s.~)’ 0 ~ found for the particular choice of ta,. As expected, intensities are larger for the 1a~ger value of soQ_

•

C —

—

V~1TMi.1.5~

,00

—

I’

: ::,!

*11 TAU.2,iO

,00

:: : : (~

,,,1~111~1ci1,

~.

-

5A~

Fig. 2,3 He I 58.4 ma of Venera 11 are plotted as a function of time, during the crossing of the illuminated disc. Each error bar is computed from the dispersion of 30 individual measurements. Measurements are compared with a model with a constant helium optical thickness ~t — 7.56 above 140 km corresponding to BNNS data and t~ = 2.40, which°providesthe best fit to the data. The intensity scale in Rayleigh is adjusted to fit the model. 919 TAti.i.M

noc~

~

512 TeJ.n.u) -

~/1 ~::( Fig. 4,5 Same as figures

A vieual inspection the data much better a substantial “limb necessary to produce determination of the

2, 3, but for Venez-a 12 measurements.

indicates clearly that the model with the optimal value t,, fits than the model with the reference value ~t 7.56, which pFesents darkrning effect which is not observed i~the data. Still, it is a quantitative criterion of the fit quality, which will allow the range of acceptable values for to.

The estimated variance V is plotted on figures 6 and 7 as a function of function provides the continuous solid line betweeri the discrete values of value is found at the minimum of this function.

c~. A spline ~r, and the

It is clear from figures 3 and 5 that, even with the optimal values t,,, the model deviates significantly from the measurements, since many measurements dif’ler from the model by much more than the individual error bar. This is probably due to small horizontal fluctuations of the actual column density of helium. In order to estimate a bracket around the central value of ‘t , these fluctuations can be treated as random and independent errors around the mean ~alue of t~ following a normal law with a zero mean value and a variance o2 for which we have one estimated value V. In such a caae, 2 law. It is a ft is well known that the random variable (n —o2 1) V follows a x

Venus EVil Measurements of Hydrogen and Helium

law with n

1 degree of freedom, with a mean value n

—

Therefore the standard deviation the variance uncertainty

V bar

are represented

—

—

V

I)

on

E (C

1

~

—

(n o2 Ii)l

at

V

is

—

1 and a variance 2 (n

/2 (n—I).

and

7,

was computed (figures

1).

t.

t,

6 and 7), and an Only three of them ç (at the the cor~esponding

which corresponds to the optimal value

minimum V0~ of V) and two bracketing values, error bar is just excluding the error bar for i~o _________________________________________________________

190

which are

such that

__________________________________________________________

liii DI$O&t.Lutae

-~

—

For each value of

was assigned to V for all values of

on figures 6

123

‘

100.

100

~01

00

U12 OISJS.115E

Fig. 6,7 The variance V measuring the deviation of Venera 11 and Venera 12 He I 58.4 nm data from a model with a constant vertical optical thickness t is shown as a 6racketlng ‘r.~ is atshown (see discussion function of ‘t. The Optimal value ‘t1, of t is found the ~tinimumof V. in The text). range of acceptable values of t The results are as follows +0.6

+1.2

V 11

~

—

2.4

—

0.4

~12

-t~

-

2.6

—

0.5

These intervals include not only real random errors but also possible fluctuations of the real optical thickness along the track of the line of sight.

‘tv

It should be remarked that both results are yielding the same value for = 2.5, considerably lower than the reference value t 7.56. If 2 o intervals are considered instead of 1 a, the maximum ac~eptable values (t~ — 3.6 for VII, — 5.0 for V12) are still substantially lower than 7.56. Let

us assume however that

the average value of

would be

7.56.

In such a case,

systematic departures between observed and predicted °intensities could be attributed

to

be transformed into a density departure a linear a non—constant value of ‘t 2n across the disc (a diurnal variation of He with distribution). interpolation the tablesc of intensities for the six selected values of t , in order The intensity indeparture to take into account the non—linear dependence of intensities on the dei~sities. The result of this exercise is displayed on figure 8, for V 11 and ‘t — 7.56. Measured intensities near the terminator, between 6 and 8 hr indicate a gradual decrease by a factor of 2 of the density, whereas actual in—situ data from ONMS show a much sharper decrease of helium density at the terminator (06 hr). This major discrepancy is also an indication that the average value -t = 7.56 does not represent the reality well. On the contrary, the same exercise for ~-t — 2.40 and V1 shows a better agreement to a constant density (figure 9). There is no reason to be~ieve that, at two days interval separating V11 and V~, the helium content would have changed. Therefore, we adopt a common value from both Sets of data with a con~e1jv~tive uncertainty margin 9responds t’o a helium concentration at 150 km 2.5 — O’6 (adopting BNMS relative This vertical optical profile) thickness cot a (150)

—

1.6 ~

~ 106 cm3

which is not compatible, even at 2 a , with the BNMS value of 5.33 a 106 cm~3 (Von Zahn at al., 1980 /6/). On the contrary, the actual measurement of OHMS indicates a constant value during the day of approx. 2.3 x 106 cm~ at 157 kin, which can be extrapolated downward to 2.9 x 106 cm3 at 150 kin, in better agreement with Venera value. ONMS densities are usually multiplied by a factor 1.6 to fit other measurements, in JASR 5:9—i

124

J.L. Bertsux, E. Chassefiere and V.G. Kurt

10-

,

,

,

10.

,

VII 1*0.1.55

VII 1*0.3.00

5

5

II

~

1.0

‘

0. I

I

.

.

$

1

~

$

$

10

II

19

L~M4 Ui 19

~

1.0

‘ ‘ ‘

I

‘

~,

10

—

•

I

1

I

$

15

11

t~ 13

~0LL 13

114 10

Fig. 8,9 Deviations of a measured from computed intensities have been transformed into relative He density diurnal variations respectively for ¶ — 7.56 and r — 2.40. For the value r — 7.56 corresponding to BNMS measurements, ° the diurnal variation between 6 an~ 7.5 hr does not fit at all ONMS measurements,

particular the drag measurements analysis. It is not obvious why this corrective factor would be the same for all chemical species, and should also be applicable to helium.Analyzing the Mariner 10 data of He I 58.4 nm with a method similar to the one 3, presented here, Kumar and Broadfoot /9/ found a density at 145 Ian of 2 ±1 a 106 cm’ fully compatible with Venera results. Therefore, we estimate that the helium absolute densities in the upper atmosphere of Venus are about a factor of 3 lower than indicated by BNMS results, and this will apply to the whole content of helium in the atmosphere of Venus. This has implications on the outgassing rate of the Venus crust, perhaps partially balanced by non—thermal escape mechanisms of helium atoms (thermal escape is negligible at the exospheric temperature of 300 K). ACKNOWLEDGMENTS

The analysis presented in this paper was sponaored under contracts n

84 CNES 201.

REFERENCES N.N. ROMANOVA and A.S. SMIRNOV Venera 11 and Venera 12 observations of E.U.V. emissions from the upper atmosphere of Venus — Planet. Space Sci, 29, 149—166 (1981).

1.

J.L. BERTAUX, J.E. BLANONT, V.M. LEPINE, V.G. KURT,

2.

H.C. BRINTON, H.A. TAYLOR, H.B. NIEMANN, H.G. MAYR, A.F. NAGY, T.E. CRAVENS and D.F. STROBEL, Venus nightime hydrogen bulge — Geophys. Res. Letters, 7, 865—868 (1980).

3.

P.Z. TAKACS, A.L. BROADFOOT, G.R. SMITH and S. KUMAR, Mariner 10 observations of hydrogen Lyman—alpha emission from the Venus exosphere Evidence of complex structure — Planet. Space Sci., 28, 687—701 (1980).

4.

U. VON ZAHN, S. KUMAR, H. NIEMANN, R. PRINN, Venus, p. 299—430, edited by D.M. HUNTEN, L. COLIN, The University of Arizona Press, Tucson (1983).

T.M. DONAHUE, and V.1. MOROZ,

5. the

J.L. BERTAUX, V.M. LEPINE, V.G. KURT and A.S. SMIRNOV, Altitude profile of H in atmosphere of Venus from Lyman a observations of Venera 11 and Venera 12 and Origin of the hot exospheric component. — Icarus, 52, 221—244 (1982).

6.

U. VON ZAHN, K.H. FRICKE, D.M. HUNTEN, 0. KRANKOWSKY, K. MAUERSBERGER and A.0• NIER The upper atmosphere of Venus during morning conditions — J. Geophys. Res., 85, 7829—7840 (1980).

7.

11.8. NIEMANN, W.T. KASPRZAK, A.E. HEDIN, n.M. HUNTEN and N.W. SPENCER Mass spectrometric measurements of the neutral gas composition of the thermosphere and exosphere of Venus — J. Geophya. Res., 85, A 13, 7817—7827 (1980).

8.

A.E. HEDIN, thermosphere

9.

—

H.B. NIEMANN and W.T. KASPRZAK, Global empirical model of 3. Geophys. Res., 88, Al, 7383 (1983).

the Venus

S. KUMAR, A.L. BROADFOOT, He 584 A emission from Venus : Mariner 10 observations Geophys. Res. Let., 2, 8, 357—360 (1975).

—