Explorer XVII data on neutral particle concentrations in upper atmospher

Reber, C. A.

English

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stb THE REFLECTIVITY OF JUPITER IN THE
o k)', ULTRAVIOLET*
_ P. STECHER
THEODORE
__ Four spectral scans of J lpiter were ubtained by means of objective grating rocket spectrome-
ters. The geometrical reflectivity is interpreted in terms of Rayleigh scattering by obtaining a
/ numerical relationship between reflectivity and optical depth. This gives an upper limit to the
amount of molecular hydrogen above the cloud layer of 10.2 ± 1 km. atm. The effect of Raman
scattering by molecular hydrogen increases this to a 10.6_+1 km. atm. An absorption feature is
found to be consistent with its identification being photodissociation of H2.
I. THE OBSERVATIONS I GEOMETRICALEFLECTIV,TYU; 'LIPITER
0,8 I' 0pt_collyTeuckottoThe spectrum of Jupiter in the ultraviolet was o7
obtained from an Aerobee rocket on July 23, 1963 I_
06
at 06h02=U.T. Three additional spectra were
obtained in a similar manner on November 14, _ 05
1964 at 10h28_U.T. The instruments used were
objective grating photoelectric stellar spectrome- 03
ters similar to those described by Stecher and 02 ] "_'__-_ ,o_ .... .,Milligan (1962). Spectral scanning is the conse- Ol _- _ .6_.... .,
quence of the spin of the rocket. The observa- , , , ,
tions were telemetered to the ground. The exit %. 2, 2z _3 2'4 25 2_ 2'7 2'8 2'9 3'o
slit widths were between 502_ and 90_. The total
spectral range covered was from 1050_ to 4000_. Fmva_ 1.--Circles: Observed average geometrical reflec-tivity of Jupiter as a functior of wavelength with mean
The instruments were calibrated in both a relative errors as determined from four spectra. Solid curves:
and an absolute sense prior to flight in the labors- Computed geometrical refiectivities for various amounts
tory. The accuracy of the absolute calibration of molecular hydrogen.
is believed to be considerably better than the limit
set by noise in signal for these observations. A Ephemeris values. The phase angle of Jupiter
complete description of the instrumental tech-
was 11.72 degrees on July 23, 1963 and almost
niques will be included in a forthcoming paper on
stellar energy distributions, zero on November 14, 1964.
The discussion of planetary photometry by II. INTERPRETATION
Harris (1961) is now followed. In Figure 1 the
geometrical reflectivity, io, as a function of wave- The literature on Jupiter, in particular recent
length is given between X2000 and X3000. The polarization measurements and molecular hydro-
mean error for each value of p was determined in gen quadrupole line measurements, indicates the
the standard manner from the observations, no possible presence of a Rayleigh atmosphere at
measurable signal was detected belowX2000. The ultraviolet wavelengths above the cloud layer.
solar flux values used in obtaining p are those Upper limits for the number of atoms or molecules
given by Tousey (1963) and were obtained by above the cloud layer may be derived immediately
photographic spectrophotometry. The necessary for each species if the reflectivity is known in
geometry was obtained using the appropriate terms of optical depth. Tables have been tabu-
lated for the total intensity emerging from the top
*Published as Goddard Space Fliokt Center Document X--813-6@-391,
ne_b,_ 10e4. of a Ray:eigh atmosphere by Coulson, Dave and
1158
1966028656-1214
https://ntrs.nasa.gov/search.jsp?R=19660028677 2020-03-11T17:51:33+00:00Z
-:" PLANETARY A£MOSPHERES 1159
:_ Sekera (1960). This tabulation is for optical but would require more accurate observational
i_ depths, r, up to unity as a function of zenith dis- material at the shorter wavelengths.
_: tance of the sun, _0; zenith distance of the emer- In Figure 1 the observed geometric reflectivities
gent rays, #, and the azimuthal angle, _. From as a function of wavelength (circles) are compared
these values p may be obtained as a function of r. with ones calculated for a given atmosphere. The
-_ For our purposes, Jupiter is assumed to be at best fit to the data under the assumption of zero
opposition even through one observation was reflectivity in the clouds is a 10.2 km. atm. of H2
;! made at quadrature, since the pha _e integral is or a 200 km. arm. of He or any combination of the
< almost constant for small phase angles. This two resulting in the same total reflectivity. If
approximation makes less than three percent dif- the clouds are assumed to form a Lambert surface
_ ference for that observation (Talley and Horak, with a reflectivity of 0.25 the total number of
1956). The plane parallel atmosphere is made scattering atoms or molecules is reduced by a fac-
tangent to a unit sphere at each point on the tor of ten and the form of the reflectivity curve
,_: illuminated hemisphere by the restraint _0=_. becomes incorrect for Rayleigh scattering. The
;_ The value of the emergent intensity is then pro- possibility that the clouds have a small reflectivity
jected onto a unit disk. Integration over the sur- can not be excluded. The derived atmosphere is
_: face then gives the geometric reflectivity for a therefore an upper limit.
_ given value of optical depth. The aximuthal The above analysis has been carried out under
angle is 180°. Curves in the p- r plane were con- the assumption that the scattering is coherent.
structed in this manner in the range r= 0 to r-1 In the case of most molecular gases including
i: for ground reflectivities of 0, 0.25, and 0.8. The molecular hydrogen this is known not to be the
'_ values of p for the optical thick case are given for case. Raman scattering from H., in the lowest
Rayleigh scattering and for isotropic scattering by vibrational state is five percent of the Rayleigh
_ Harris (1961). The extrapolation from r= 1 to scattering at X1216 (Dalgarno and Williams,<
r = ¢o was made with the aid of geometric reflec- 1962). Isotropic scattering may be used to ap-
tivities calculated in a similar malaner from the proximate the effect by setting the particle albedo
-_' available X-Y functions for isotropic scattering equal to 0.95 and comparing it with the result for
_:_ given by Mayers (1962) and by Sobouti (1963). a particle albedo of unity. In the case of an
_ Over the range of r to be used in this paper the optically thick atmosphere one concludes that 43
i' extrapolation error is probably negligible, percent of the incident photons emerge in the
.o_! The total number of atoms or molecules in a Raman lines. Brant (1963) found 34 percent of
_ column of unit area perpendicular to and above the incident Lyman-a photons in the Raman lines
the cloud layer _snow obtained from r = na under by means of an approximation to the Rayleigh
the assumption of only one constituent. Here n phase function.
": is the number of atoms or molecules and a is the There is a half electron volt difference between
: Rayleigh scattering cross section per atom or the incident photon and the Raman scattered
_ molecule. The Rayleigh scattering cross section photon. This energy difference means that the
_:, for molecular hydrogen is given by Dalgarno and Raman photon has a wavelength several hundred
Williams (1962)L The Rayleigh scattering cross angstroms longer than the incident photon at the
section for helium is given by Dalgarno (1962). wavelengths under consideration here. This
The molecular scattering cross section for these would appear as a second order effect except on
gases does not follow the Rayleigh law in the ultra- the longer wavelength side of an absorption fea-
violet. This is physically due to the increased ture in the planet or when there is a strong wave_
probability of inducing a dipole moment as a length variation in the source function. The ab-
:i resonance is approached. In the range of intel_st sorpticm bands of ammonia begin at 2100A and
' here the deviation of molecular hydrogen is result in an almost continuous absorption at
greater than that of helium. This in principle shorter wavelengths reaching an optical depth of
: would allow an abundance r_,,tio determination more than 10_at k1216 (Kuiper 1952)(Watanabe,
et al. 1953). This is apparently indicated by the
The ordinate in Fig. 1, Ap. J. 136, 691, 1962 should read ),_¢(_)
XlO-c_,. data although the solar continuum becomes so
1966028656-1216
1160 PUBLICATIONS OF GSFC. 1964: I. SPACE SCIENCES
weak here that the determination is unreliable, both of these determinations are much too small
The effect here is that only the Rayleigh compo- and it should be on the order of a 270 kin. atm.
nent and not the Rayleigh plus Raman appears up The latter value would gi_e an optical depth of
to )`2200. Similarly, the absorption feature be- nine at ),3000 and correspond to a conservative
tween )`2400 and )`2700 reduces the Raman scat- atmosphere throughout the ultraviolet. This is
tered component between )`2700 and )`3000. Us- not observed. Casual inspection of photographs
ing isotropic scattering as an approximation and of Jupiter taken on blue plates also indicate this
assuming that the ratio of the Raman scattering not to be the case since the surface markings
cross section to the Rayleigh cross section is a would be obscured near the edge of the disk. This
constant independent of wavelength the total large optical depth would also require U and B
scatteringatmospher(, would be considerably more. color intensities several times what is observed
Da]garno and Williams (1964) have kindly (Harris, 1961). 0pik (1962) derives an optical
communicated to me in advance of publication depth by inspection of published photographs
the Raman scattering cross section for H_ between which is in remarkably good agreement with that
)`2000 and )`3000. They give Q=2.75×10-46/)` '4 obtained in this paper. 0pik then proceeds to
where h' is the wavelength of the scattered radia- derive an atmosphere that is consistent with the
tion in centimeters. This is a smaller cross sec- molecular weight he obtained from a rediscussion
tion than was anticipated and leads to only 0.4 of the occultation measurement of Baum and
kin. atm. increase of H2. Thus, we have an upper Code (1953). His resulting atmosphere consists
limit of a ]0.6 kin. atm. of molecular hydrogen, mostly of helium giving a pressure of 11 atmo-
The total mean error is estimated to be ___one kin. spheres at the top of the cloud layer. Spinrad
atm. and Trafton (1963) find from the widths of meth-
There are several possible causes of the absorp- ane lines a pressure equal to or less than 2.8 atmo-
: tion feature between )`2700 and )`2400 which ap- spheres at the top of the clouds. If we adopt 2.8
pears to be real. If for the sake of conjecture, atmospheres at the top of the clouds. If we adopt
this absorption is assumed to be the forbidden 2.8 atmospheres as the upper limit then we are
photodi,_soci,_ion of molecular hydrogen, an em- limited to less than 50 kin. atm. of helium. The
pi:ical absorption coefficient may be derived, cross section for Rayleigh scattering in helium is
This may be done by following the path of the five percent that of molecular hydrogen. A 50
photon by using isotropic scattering for which kin. atm. of helium is insufficient to give the
X-Y functions are available (Sobouti, 1963). A observed reflectivity. The amount of molecular
particle albedo of unity is equated numerically to hydrogen then becomes at least 8 km. atm. for
the Rayleigh scattering cross section and the par- zero reflectivity clouds.
ticle albedo giving the observed mean absorption The conclusion of this paper is that Jupiter has
depth is determined. If the Rayleigh scattering less than 10.6_+ 1 km. atm. of molecular hydrogen
is considered to be due entirely to H_ a cross sec- above the clouds. If the reflectivity of the clouds
tion of 1X 10-_ cm2is obtained for photodissocia- is zero in this part of the ultraviolet and the upper
tion of//2. Dalgarno (1964) considers 10 -2_ to limit to the pressure is 2.8 atmospheres at the
be a good estimate. Herzbe_'g (1964) points out cloud tops then Jupiter has at least a 8 km. atm. of
that H2S would also absorb in this wavelength molecular hydrogen. Inherent throughout this
region so the identification is by no means paper is the assumption that the atmosphere has
certain, the same physical depth at all wavelengths longer
than 2000 Angstroms.SUMMARY AND DISCUSSION
Zabriskie (1962) determined at 4.6 km. atm. of REFERENCES
//t from their quadrupole bands. Spinrad and BAv_, W. A. and CoD_.,A. D., 1953,A. J., 58, 108.
Trafton (1963) empirically derived a 27 km. arm. BaANT,J. C., 196,_,Planetand Space Sci., 11, 725.CovLaoN,K. L., DAv_.,J. V. and S_K_.aA,Z., 1960.Tables
from these bands. Foltz and Rank (1963) con- Related to Radiation Emerging from a Planetary Atmo-
cluded from laboratory measurements and theo- sphere with Rayleigh Scattering, (Berkeley and Los
retical calculations on the quadrupole bands that Angeles, University of California Press).
J
L
1966028656-1217
PLANETARY ATMOSPHERES 1161
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DALGARNO,A. and WH_LIAMS,D. A., 1964, private corn- SPINRAD, H. and FRAFTON, L. M., 1963, Icarus, 9, 19.
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FOLTZ, J. V. and RANK, D. H. 1963, Ap. J., 138, 1319. TALL_Y, R. L. and HOIIAK, II. G., 1956, Ap. J., 123, 176.
HxaalS, D. L., 1961, Planets and Satellites; The Solar TousEY, R., 1963, Spat.e Sci. Rev., 2, 3.
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1966028656-1218


Data from Explorer 17 on composition of the upper atmosphere

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