Observations of the short period variations in ultraviolet polarization and carbon dioxide line intensity for the atmosphere of the planet Venus are discussed. Theoretical polarization-phase curves for the integrated disk of Venus are developed. The amplitude of the short-term variation in the polarization as a function of wavelength is analyzed. Curves are developed to show the relationship between percent polarization and phase angle for various wavelengths

Bowell, E.

English

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(NASA-CR-142115) POLARIZATION OF VENUS: N75-172594
OBSERVATIONS OF SHORT-TERM QUASI-PERIODIC
VARIATIONS (Lowell Observatory) 13 p HC
$3.25 CSCL 03A Unclas
G3/89 17563
POLARIZATION OF VENUS: OBSERVATIONS OF SHORT-TERM
QUASI-PERIODIC VARIATIONS*
Edward Bowell
Planetary Research Center
Lowell Observatory
Flagstaff, Arizona 86001
Transcript of a paper presented at a conference on the
atmosphere of Venus, Goddard Institute for Space Studies,
New York, 15-17 October 1974.
*To be published as a NASA or GISS report.
https://ntrs.nasa.gov/search.jsp?R=19750009187 2020-03-22T22:44:53+00:00Z
DR. BOWELL: Short-term variations have been noted in CO2 line
strengths, and of course morphological variations in the cloud deck
have been well documented. But up to now, no one has, to my knowledge,
discovered any short-term variations in polarization. There are sea-
sonal variations which have been discussed by Coffeen.
Figure 1 is a composite, as it were, showing both variations in
polarization and in CO2 line strengths. The CO2 measurements are by
Barker, who will talk about them later. The interval concerned is
August to September 1973. The polarization observations were made in
September, and the CO2 measurements in August.
The sine curve should simply be interpreted as a reference curve.
It doesn't purport to fit fe observations. What it does purport to do
is to show that both the CO2 and polarization variations are in phase
and have a common period of something like 5.5 to 6 days.
Let me explain the CO2 and polarization scales. The peak-to-peak
variation in CO2 line strengths, in August 1973, is about 20%. The
semi-amplitude of the variation in the polarization is something like
2-1/2 thousandths (0.25'percent). This is a fairly small number in
polarimetric parlance. Observations of the whole disk, as these are,
can be made to an accuracy of something like half a thousandth. There-
fore, looking for a 1 or 2 thousandths variation is quite a difficult
job.
What's causing the variation in polarization? Is it variation in
particle size? Is it variation in the variance of the particle size
-2-
size distribution? Is it variation in the thickness of an overlying
Rayleigh atmosphere?
Figure 2 is an attempt to show that the variation in the ultra-
violet polarization is not coming from changes in refractive index
of the particles. The phase angle at the time of the observations
was in the range 60 to 80 degrees, and this diagram, from a paper by
Hansen and Hovenier, indicates that quite radical changes in refrac-
tive index would not change polarization very greatly. One should
note that this doesn't preclude very large changes in refractive index
causing a variation in polarization. Yet, in all other previous obser-
vations no great variation in polarization has been observed at small
phase angles where there would be enormous differences in the observed
polarization if the refractive index were to change on a short time
scale.
Here is another polarization-phase curve (Figure 3) which this
time is intended to show that the parameter known as fR, the contri-
bution of Rayleigh scattering to the phase matrix, could indeed change
the polarization drastically over the relevant range of phase angles
(60 to 80 degrees). In fact, a change of 0.001 in fR corresponds to
a change of about 0.001 in the polarization at phase angle 70 degrees.
I would suggest that the observed variations in polarization are
indicative of changes in the height of the absorbing layer in the cloud
and that this is reflected in changes in thickness of the Rayleigh
scattering layer above the cloud. To give you some figures: assuming
-3-
that the top of the absorbing cloud is at the 50-millibar level, which
everyone else has done, then a change of 2-1/2 thousandths in the
polarization would result from a change of pressure at the level of
the cloud top of something like 3 millibars. So this is a change of
3 millibars in 50 millibars occurring planetwide on a time scale of
days.
Figure 4 shows the amplitude of variation in polarization (units
are thousandths) versus wavelength. It bears a resemblance to a figure
by Coffeen (see Figure 5) in an article about contrast of ultraviolet
features on Venus. The drop from the ultraviolet down into the blue
here is fairly steep. In green I can't readily attach any variation
in polarization; the spot marks the analytical position.
Figure 5 is Coffeen's diagram of contrast versus wavelength. There
is a steeper slope, but it can't readily be compared with the previous
slide of polarization variation as a function of wavelength because
Coffeen's contrast data were derived from a series of scans taken
instantaneously, as it were, and refer to different regions over the
planet, whereas the variation in polarization is a temporal phenomenon
which is integrated over the whole planetary disk.
There seems, therefore, to be a tie-up between variation in polar-
ization, between CO2 line strengths, and if this bad comparison between
contrast and polarization variation is to be believed, between ultraviolet
features and variation in polarization.
- 4 -
•I think this goes back to what AndyYoung was saying earlier,
that all these different parameters we're measuring, probably referring
to the same 50-millibar level, just have to correlate. The next step
will be to refine all the measurements: to link in the polarization and
the CO2 observations with greater certainty, to link those in turn with
the visual aspect of clouds in the ultraviolet, and eventually to move
from.a global to a localized appraisal of these parameters. This is
certainly technically feasible with regard to polarimetry. One can
make pretty accurate local polarization measurements and, with luck,
observe changes in weather on Venus over a time scale of hours (this
has already been done to a limited extent).
DR. KURIYAN: Could you tell us how you measure the polarization
to this accuracy?
DR. BOWELL: Dr. Dollfiis could best answer that. It was his
polarimeter. A half-wave plate rotates in front of an analyzer, usually.
One measures the modulation of the output signal, and this is a measure
of the polarization. You must have a fairly instantaneous method of
measuring the change of the polarized signal.
DR. KURIYAN: And you could get it to an accuracy of .01 percent?
DR. BOWELL: I reckon it's on the order of .05 percent. This is
a common accuracy astronomically. However, Venus is a different object
because it has to be observed during the day. The sky is equal in
intensity to Venus. and is usually many times more polarized. So that's
the real problem.
-5-
DR. SAGAN: You are proposing a pressure modulation of a few
millibars to explain the polarization variations?
DR. BOWELL: Yes.
DR. SAGAN: What variation in ultraviolet contrast would result
from the same pressure modulation?
DR. BOWELL: I haven't calculated that.
DR. SAGAN: I am wondering if these are compatible numbers. Jim
Hansen thinks the answer is no.
DR. HANSEN: The magnitude of the changes in the Rayleigh optical
thickness that you're talking about would have a negligible effect on
the contrast, on the brightness.
DR. BOWELL: Therefore, you don't think the polarimetry would
necessarily be correlated with the ultraviolet markings?
DR. HANSEN: That's right, not necessarily. I suspect that it may
be correlated, but I think that explanation for the contrast variation
is wrong. I think your data are very impressive. The magnitude of the
effect that you see is clearly much larger than the non-systematic
errors. But it would be easy to construct half a dozen different models
which could give you that type of variation, and I don't think you can
choose between those models until you have local polarization measure-
ments of bright- and dark areas. But you need to have these as a function
of phase angle, and since things are changing, on a short time scale, you
can't wait for the phase angle of Venus to change as seen from the earth.
So far as I can see, the only way to solve the problem is with measure-
ments from an orbiting spacecraft.
-6-
DR. BOWELL: Oh, yes, indeed.
DR. YOUNG: I would point out that essentially we have this kind
of observation from the ground. It's easy to see the 002 variations.
We made a special effort to try to match up the CO2 variations with
ultraviolet features. We just don't see any variation at all between
bright and dark ultraviolet features on the same day. I mean what we
see is quite like what Ted Bowell sees, namely something like 5 or
10 percent variation in the apparent amount of gas. He sees it in
Rayleigh scattering and we see it in C02 absorption, but it's the
apparent amount of gas in the line of sight. But when you look at a ,
bright area and a dark area in the ultraviolet photographs and ask
if there is any difference between them, my answer is that the average
difference is 1 + 3 percent. It's very mysterious as to why the dif-
ference is so small. I surely expected I would see some difference
and nothing came out.
FIGURE CAPTIONS
Figure 1. Short-period variations in ultraviolet polarization (3540
and 3790 X) and CO2 line intensity. Ordinate units represent
about- 0.002 change in polarization and 10% change in CO2
line intensity.
Figure 2. Observations and theoretical polarization-phase curves for the
integrated disk of Venus. The effect of varying particle
refractive index is shown. [J. E. Hansen and J. W. Hovenier,
J. Atmos. Sci 31, 1137, 1974]
Figure 3. Observations and theoretical polarization-phase curves for
the integrated disk of Venus, showing the effect of varying
fR, the Rayleigh contribution to the phase matrix. [J. E.
Hansen and J. W. Hovenier, J. Atmos. Sci. 31, 1137, 1974]
Figure 4. Amplitude of the short-term variation in the polarization
of Venus as a function of wavelength. Observations of
September 1973.
Figure 5. Venus cloud contrasts as a function of wavelength [from
an article by D. L. Coffeen, in "Planetary Atmospheres,"
ed C. Sagan et al., Reidel, 1971].
Normalized relative CO2 line intensity [o] and u.v. polarization [.] of Venus
5 10 15 20 25 30 5 10 15
August September Date [UT] 1973
X =.365 ,.
14 I ....... h r -1.33 (a- 8 1) 8=.07
r " "b = .07
n r - 1. 40 (a - I ) fR = .045
12 r -- -  1n(.50 (a I 1.2P-)
I I
I
8 -
o 6 i 1:-
o 6
I 
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o .
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0 20 40 60 80 100 120 140 160 180
Phase Angle
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0 20 40 60 80 100 120 140 160 180
Phase Angle
C RU J1e 3
Amplitude of the short-term variation in
polarization of Venus as a function of wavelength.
Observations of August, September 1973
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3500 4000 4500 5000 5500
Wavelength, A
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Venus
17 Jan 1969
2300 UT
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Polarization of Venus: Observations of short-term quasi-periodic variations

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