Strong 10 micrometer line emission from (c-12)(o-16)2 in the upper atmosphere of Venus was detected by heterodyne techniques. Observations of the absolute Doppler shift of the emission features indicate mean zonal wind velocities less than 10 m/sec in the upper atmosphere near the equator. No evidence was found of the 100 m/sec wind velocity implied by the apparent 4-day rotation period of ultraviolet cloud features

Betz, A. L.

Johnson, M. A.

Mclaren, R. A.

Sutton, E. C.

English

NASA Technical Reports Server

General Disclaimer 
One or more of the Following Statements may affect this Document 
 
 This document has been reproduced from the best copy furnished by the 
organizational source. It is being released in the interest of making available as 
much information as possible. 
 
 This document may contain data, which exceeds the sheet parameters. It was 
furnished in this condition by the organizational source and is the best copy 
available. 
 
 This document may contain tone-on-tone or color graphs, charts and/or pictures, 
which have been reproduced in black and white. 
 
 This document is paginated as submitted by the original source. 
 
 Portions of this document are not fully legible due to the historical nature of some 
of the material. However, it is the best reproduction available from the original 
submission. 
 
 
 
 
 
 
 
Produced by the NASA Center for Aerospace Information (CASI) 
https://ntrs.nasa.gov/search.jsp?R=19760025002 2020-03-22T13:00:36+00:00Z
HETERODYNE DETECTION OF CO 2 EMISSION LINES
AND WIND VELOCITIES IN THE ATMOSPHERE OF VENUS*
A.L. Betz, M.A. Johnson, R.A. McLaren + , E.C. Sutton#
Department of Physics, University of California, Berkeley
Berkeley, California 	 94720
.4
Received
0
o ABSTRACT
N U1
M
I
ro r
^ m
12 16Strong 10 pm line emission from
	 C	 0 2 in the upper
r a Ln
Z o
atmccohere of Venus has been detected by heterodyne techniques.
Observations of the absolute Doppler shift of the emission
features indicate mean zonal wind velocities less than
m
.-.0 10 m/sec in the upper atmosphere near the equator.
	
No evidence
^ z0H
_1U
a
was found of the 100 m/sec wind velocity implied by the apparent.Z
O
Vn
W A
to
U ,
H H ro 4-day rotation period of ultraviolet cloud features.
U h•.1
W U C
H O W
W .4 o
raW4'
w ^
i
(a
#National Science Foundation Fellow.A H
O 3C
W
W
E-+
A
z
V)
Z
z
+NATO Postdoctoral Fellow 1974-75
W -W W Present Address:
	
Department of Astronomy University of
^cn
>
Toronto, Toronto, Canada	 M5S lA7
ww
'. z o
^' H
wr a WXo
-t^O
o
xw`" *This work was supported in part by NASA grants NGR 05-003-452
F-+ a4 rn
i
a^no
U) Ln t04
and NGL 05-003-272 and National Science Foundation grant
U H E Vi
V)
V
w
H
a
x
a AST 75-20353.
.2 N
z0xJt
W ^\	 v
^^
`^ U F,
R^CElVED
tMSA S;1 FACILITY
lili UT r1 l.A"
.j y
2.
I. Introduction
In the infrared spectral region, heterodyne spectroscopy
has been shown to be a useful technique for achieving
resolving powers exceeding 10 6
 (Peterson et al, 1974). The
ability to convert optical frequencies down into the radio
frequency domain permits the spectral analysis to be performed
electronically, and frees the spectrometer from mechanical
constraints as a limit on achievable resolution. In this
Letter, we wish to report the heterodyne detection of intense
emission lines from the 950 cm -1 vibrational-rotational
transitions of 12C1602 in the atmosphere of Venus. The observed
intensities are much larger than those expected from a gas in
LTE and indicate a nel-thermal excitation mechanism. The
linewidths are typically only 1.4 x 10 -3 cm-1 and yet the
features are well resolved, allowing the line center frequencies
to be measured to an accuracy exceeding 3 x 10 -5 cm-1 (1 MHz).
The observed frequencies have been combined with a
knowledge of the transition rest frequencies and the center of
mass velocity of the planet to determine the line of sight
wind velocities of selected areas on the planet. These
Doppler velocities are of interest in measuring the magnitude
of any systematic rotational motion of the atmosphere.
Apparently, some uncertainty still exists whether the 4-day
(LOO m/sec) rotational period of ultraviolet cloud features
on Venus represents a true motion of the atmosphere, or perhaps
only the phase velocity of a wave phenomenon (Young, 1975).
Although certain spectroscopic evidence appears to confirm
the existence of 100 n/sec retrograde wind motions in the j
3.
upper atmosphere (Traub and Carlton, 1975), a conclusive
theoretical explanation of the phenomenon has not as yet
emerged. Since the technique employed in this experiment
measures the line of sight velocity in a absolute sense,
it can distinguish between a horizontal (tangetial) wind
velocity on the limb and any vertical velocity component which
may exist at the center of the planet. A wave phenomenon
in the atmosphere might be expected to produce periodic
vertical motions.
(II) Instrument Description
The general principles of heterodyne detection at
infrared frequencies have been reviewed by Blaney (1975), and
only the details of this spectrometer are described here.
The instrument is installed at the McMath Solar Telescope of
Kitt Peak National Observatory and utilizes one of the two
--------------------------------------------------------------
*Operated by the Association of Universities for Research in
Astronomy, Inc., under contract with the National Science
Foundation.
--------------------------------------------------------------
81 cm auxiliary solar telescopes. As illustrates'. in Figure 1,
the source radiation collected by the telescope falls upon a
dichroic mirror which transmit, the visible image to an eye-
piece and reflects the infrared beam to a focus at a dual
beam focal plane sky chopper. The infrared signal is then
directed through a 958 transmitting NaCl beamsplitter, where
it is combined with a few milliwatts of the output power
of a stabilized 12C1602 laser. The laser oscillates on any
	 I
4.
one of a number of vibrational-rotational transitions of
the 00°1 -(10°0, 02°0] 1 950 cm-1
 band of 12C1602' The combined
beams are focussed onto a cooled77
	 -(	 K) mercury cadmium
telluride (HgCdTe) photodiode mixer, which generates a difference
frequency current over the radio frequency bandwidth of 0 to
1500 MHz. After amplification, a 200 MHz segment of this
intermediate frequency spectrum is converted into the band
of 50 to 250 MHz by a single-sideband mixer and directed into
a multichannel RF filter bank. The local oscillator frequency
for this SSB mixer is calculated to place the Dopplar-shifted
line center at the mid-frequency point of this filter bank,
and is tracked in frequency to correct for changes in Doppler
shift caused both by the earth's rotation and by changes in
the interplanetary velocity during the course of the measurement.
The filter bank then analyzes the 50 to 250 MHz spectrum into
40 independent channels of 5 MHz (1.7 x 10 -4 cm-1 ) each.
The 40 detected power outputs are synchronously demodulated
at the sky chopping frequency (150 Hz) and multiplexed into a
computer for on-line integration and analysis. After each
integration cycle, a blackbody radiation source is placed in
the infrared signal beam to calibrate the system. The
sensitivity is measured to be 2.7 x 10 -16 watts for a signal-
to noise ratio of 1 after a 1 second integration in each 5 ;411z
channel. With an input infrared frequency of about 3 x 10 i3
 Hz,
this channel bandwidth implies a spectrometer resolving power
6of 6 x 10.
5•
(III) Observations
The observations were conducted in 1975, November and
December, when Venus was at an average phase of 0.65 and
approximately 20 aresec in diameter, about 5 times the 4 aresec
beam size. An observation cycle consisted of 4 minutes
integration in each of the two signal be&-s, followed by a
2 minute blackbody calibration and zero check, yielding a net
8 minutes integration on the source.
A typical narrow emission line observed in the center
of a broader CO 2 absorption feature is shown in Figure 2. This
line is the same moleculer transition, 00°1 -(10°0, 02°0110
as that of the CO 2 laser oscillator. As shown in the figure,
the strength of the emission appears to be proportional to
the solar intensity at the observed area; no emission is seen
from the dark half of the planet. The line shape is Gaussian,
with a halfwidth consistent with a kinetic gas temperature of
about 200K near the subsolar region, and 185K near the termina-
tor. However, the central intensity of the strongest line is
15 times greater than that expected from an optically thick
region at a kinetic temperature of 200K. A more detailed
analysis of the non-thermal emission mechanism is treated in
an accompanying paper by Johnson et al. (1976), along with a
discussion of similar emission features observed in the
atmosphere of Mars.
From November through December, 1975, the Doppler shift
of the receding planet varied from 1160 to 1080 MHz. The
tracking oscillator frequency was calculated from the planetary
ephemeris for Venus provided in the American Ephemeris and
a
Y
j
6.
Nautical Almanac so that an emission line with zero velocity
with respect to the planetary center of mass should appear
centered in the filter bank. Any deviation of the line
frequency from the center of the filterbank could be caused
by a combination of the following principal effects:
(1) an offset of the CO 2 laser frequency from the natural
frequency of the transition,
(2) an offset of the RF tracking oscillator frequency from
the correct value,
(3) a velocity component of the atmospheric emission region
'	 with respect to the planetary center of mass velocity, projected
along the line of sight towards the earth.
The last effect is the desired quantity of the experiment,
while the first two produce systematic errors in an absolute
velocity measurement. Errors related to the first effect
can in principle be accounted for. Although the laser is
frequency stabilized to the center of its gain profile to
a precision better than t 0.2 MHz (Skolnick, 1970), corres-
ponding to a velocity uncertainty of t 2 m/sec, it does not
oscillate at the true center frequency of the transition.
For our laser operating on the P(16) transition at a total
pressure of 14 Torr, the shift was measured to be 0.7 t 0.1 MHz
to lower frequency, based on a calibration with a low pressure
CO 2 absorption cell. Errors of the second type are contributed
by the uncertainties in the planetary ephemeris or a lack of
precision in setting the tracking frequency, but are estimated	 1
to be less than f 0.2 Mhz (t 2 m/sec) during these observations.
The remaining significant source of error lies in the frequency
signal
The la
larger
Janet.
3 to 5
as a function
tter effect is
velocity
Typically the
times less than
9
T
7.
4.
calibration of the filter bank, but the accuracy here is
better than t 0.1 MHz (t lm/sec). After correcting for the laser
frequency shift, the systematic error is less than 6 m/sec in
determining the average velocity of any 4 aresec region on Venus.
(IV) Results and Conclusions
The Doppler velocities measured on 7 days are listed
in Table 1 for three different positions along the equator
of Venus. Each velocity value is determined from the
deviation of the obser , ied line center from the frequency
expected for the center of mass velocity. The line center
frequency is derived from the center of gravity of the line
profile and is consistent with an independent calculation
using a least squares fit to a Gaussian profile. The
quoted uncertainties in %elocity are one standard deviation
estimates from the formal statistical error and do not include
the effects of any systematic contribution. This statistical
error is governed completely by the signal to noise ratio
of the line detection, which depends upon the length of
integration time, the weather conditions at the time of the
observation, and the strength of the
of the solar illumination on Venus.
the dominant one contributing to the
uncertainties at the center of the p
line emission near the terminator is
that of the subsolar limb.
Because the listed velocities correspond only to the
component seen along the line of sight, the limb values
are tangential or horizontal velocities, while those at
f
i
8.
the center are, of course, vertical components. A few observa-
tions for which the velocity uncertainties exceeded t 20 m/sec
have been omitted from the Table. Generally, these values are
from observations attempted under poor weather conditions for
which the infrared signal dropped to less than half of its
normal value; however, the mean values of all these omitted
measurements lie within the bo , inds of the std"ed velocities.
The principal conclusion of :his investigation is that
at no tiric. was a Doppler velocity measured which could be
interpreted as a 100 m/sec (4 day) retrograde rotation of the
atmosphere. Both the absolute velocity data from the limb
and the differentia]. values between the limb and center of
the planet are more consistent with a mean rotational velocity
near zero. Some of the day-to-day variations, however, do
appear significant enough to indicate localized inhomogeneities
as large as 20 m/sec. The variations in velocity at the
center of the planet also imply that the vertical motions
there are of the same scale as the horizontal variation on
the limb, although the vertical values are not as well deter-
mined due to the larger statistical uncertainties. In
comparing these results with previous spectroscopic investi-
gations, one should bear in mind that these emission lines
are formed at an altitude near 120 km (Johnson et al. 1976).
s
This is almost twice as high as the altitude of the broader
CO 2 absorption lines (Traub and Carleton, 1975) and ultraviolet
j
cloud features, for which large 100 m/sec retrograde velocities
have been reported.
i
9.
Finally it should be emphasized that these results are
only a first attempt to utilize heterodyne techniques to
determine wind velocities on Venus, and that a number of
significant improvements are possible. By stabilizing the
laser frequency to the Lamb dip in a low pressure CO2
absorption cell, a long term absolute frequency accuracy of
a few parts in 1010 can be achieved (Freed and Javan, 1970).
This, when combined with a more accurate planetary ephemeris
such as one derived from planetary radar measurements
(Shapiro, 1976), should reduce the total systematic error to
less than 1 m/sec for absolute Doppler velocity measurements.
The uncertainity for the line of sight velocity would then
be limited only by the detection statistics to less than
t 5 m/sec for a 16 minute integration.
The success of infrared heterodyne techniques depends
strongly on the latest advances in HgCdTe photodiode
technology, and we would especially like to thank D. Spears
(Lincoln Laboratory) for developing the devices used in this
experiment. The CO 2 laser was constructed from the design
of C. Freed (Lincoln Laboratory); the CO 2 absorption cell was
provided by R. Jacobs ( Lawrence Livermore Laboratory), and
the multichannel filterbank was on generous loan from the
National Radio Astronomy Observatory, Greenbank, W. Va.
We are grateful to the staff of KPNO for their assistance in
the use of Observatory facilities, and we also acknowledge
informative conversations with 1. I. Shapiro and C.H. Townes.
ITRODL`C1RIIXry OF TH"
ANAL P,^(; r_ IS POOR
rL
11
10.
FIGURE CAPTIONS
Figure 1: Diagram of Experimental Apparatus
Figure 2: 1 X 160 2 emission lines observed at three locations,
4 aresec in diameter, on the 20 aresec disc of
Venus. Integration times are 16 minutes for posi-
tions 1 and 2, and 32 minutes for position 3. The
frequency scale is centered on the expected Doppler
shift for the center of mass velocity of the planet.
Ths intensities are calibrated as a Rayleigh-Jeans
temperature, TRJ , defined as received power density,
W•Hz-1 , divided by Boltzman's constant, k.
ti»
	ssr	 i	 _
	
•	 11.
REFERENCES
Blaney, T.G. 1975, Space Science Reviews, 17, 5, 691.
Freed, C. and Javan, A. 1970, Appl. Phys. Lett. 17, 2, 53.
Johnson, M.A., Betz, A.L., McLaren, R.A., Sutton, E.C. and
Townes, C.H. 1976, (to be published).
Peterson, D.W., Johnson, M.A., Betz, A.L. 1974, Nature 250,
128.
Shapiro, I.I. (1976) private communication.
Skolnick, M. 1970, IEEE, Journ. Quant. Elect., QE-6, 2, 139.
Traub, W.A., Carleton, N.P. 1975, J. Atmos. Sci., 32, 6, 1045.
Young, A. 1975, Icarus 24, 1-10.
I
TABLE 1
LINE OF SIGHT VELOCITIES (m/nec)*
1.'
DATE L'.MB (1) MID-RADIUS	 (2) CENTER (3`
22 Nov 75 -3 *	 8 -------- -7 ± 12
24 Nov 75 -9 t	 6 -13 t	 6 -8 t 9
25 Nov 75 -2 t	 3 -31 ±	 6 -8 ± 13
11 Dec 75 -8 ±	 4 12 ±	 6 2 t 17
12 Dec 75 -15 !	 3 -------- 14 '- 8
17 Dec 75 -5 *-	 3 -7 *-	 6 17 t 10
24 Dec 75 7 '-	 9 -12 '-	 7 -7 = 8
*Absolute line of sight wind velocities with respect to the
center of mass of Venus for three positions along the equator.
The integration times of individual observations vary from 32 to
88 minutes. Negative velocities imply notion towards the observer.
M
A.L. Betz, Department of Physics, University of California,
Berkeley, California 94720
M.A. Johnson, Department of Physics, University of California,
Berkeley, California 94720
R.A. McLaren, Department of Astronomy, University of Toronto,
Toronto, Canada M5S lA7
E.C. Sutton, Department of Physics, University of California,
Btrkeley, California 94720
ii
40 FILTERS & RF DETECTORS
50 250 MHz
SYNCHRONOUS DETECTORS
AND MULTIPLEXER
COMPUTER
150 Hz
Ref.
1 LINE PROFILE
I	 -
	BLACK BODY	 '
1
150 Hz CHOPPER
BEAM / ^ iSPUTTERS
COz LASER  Z^
GERMANIUM LENS
I PHOTOMIIEI GUIDING
EYEPIECE
AMPLIFIER
0.1-1500 MHz
TRACKINGSSB MIXEROSCILLATOR
LOWER	 UPPER
VENUS
12-17-75
12C 1602 P(16)
25
a
30
20
2
i N
= 15
v
d'	
3
2c-I
10
5
0
-100 -50	 0	 +50
MF%r.-  AHERTZ
+100


Heterodyne detection of CO2 emission lines and wind velocities in the atmosphere of Venus

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server