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Space Administration
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Charles Redmond
Headquarters, Washington, B.C.
(Phone: 202/755-3680)
Peter Waller
Ames Research Center, Mountain View, Calif,
(Phone: 415/965-5091)
RELEASE NO: 81-33
VENUS CLOUD STUDIES SHOW CHANGES IN LONG-TERM WIND PATTERNS
Significant discoveries concerning the pattern of atmos-
pheric changes on Venus have been made based on two years' worth
of Pioneer orbiter observations. The discoveries indicate a long
term period of change for both the planet's wind patterns and for
the existence of a haze layer above the cloud tops.
The Pioneer orbiter has taken about 1,000 pictures of Venus
clouds and extensive measurements of the particles composing
those clouds.
The most noteworthy of the discoveries is that Venus'
plane.t-wide wind patterns change dramatically over a period of
several years. Two patterns have been discerned — a mid-lati-
tude jet stream pattern and a cloud and wind pattern which acts
like a solid body.
March 9, 1981
(HASa-Sews-Eelease-81-33) VENOS CLOUD
STUDIES SHOW CHANGES IN 10NG-TEB8 HIND
PATTERNS (National Aeronautics and Space
Administration) 7 p
N81-J374U
Unclas
00/91 - 19834 .
https://ntrs.nasa.gov/search.jsp?R=19810069321 2020-03-21T11:47:19+00:00Z
-2-
A second discovery shows that the high-altitude haze layer
which completely envelopes Venus' clouds appears and disappears
over several-year periods. This haze is a "smog layer" extending
above the main cloud region by about 30 kilometers (18 miles).
This altitude on the Earth would be well into our stratosphere.
The Pioneer Venus orbiter is expected to return pictures and
other data until 1985. The orbiter reached Venus in December 1978,
and the four Pioneer probe craft entered the atmosphere at the
same time. Cloud pictures and polarimetry data are provided by
the cloud photopolarimeter. Dr. Larry Travis, NASA Goddard Insti-
tute for Space Studies, New York City, is principal investigator.
The two new discoveries of a multi-year change in the pattern
of global winds, and similar changes in the planet-wrapping haze
layer could help explain the major remaining mystery of Venus'
atmosphere: Why, on a planet which has almost no axial rotation,
do the upper level winds circle the planet at tremendous speeds of
360 kilometers per hour (225 miles per hour). These winds cover
the planet completely, blowing at virtually every latitude from
equator to pole. Their speeds can be determined from the speeds
at which the clouds, carried by the winds, travel around the
planet.
Wind speed measurements from top to bottom of the atmosphere
by the four Pioneer probe craft show that these high-speed, cloud-
level winds are coupled to lower altitude winds, which also have
very high speeds.
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The 360 km/hr (225 mph) cloud level winds blow around the
planet at an altitude of 65 km (40 mi.)- Wind speeds then range
down to 192 km/hr (120 mph) at 50 km (30 mi.) altitude and to a
still-very-high 80 km/hr (50 mph) at 20 km (12 mi.) altitude.
The mass of the moving atmosphere constituting these high-
speed winds is several times that of the entire atmosphere. It
represents about a quarter of Venus' atmosphere which is about
100 times denser than Earth's.
Despite the scale of these high-speed, upper level winds,
well over half of Venus' tremendously dense atmosphere, near the
planet's surface, is almost stagnant. From the surface up to
10 km (6 mi.) altitude, wind speeds are only about 3 to 18 km/hr
(2 to 11 mph).
In a general way, the high-speed winds can now be explained
as due to the transfer of momentum from Venus' slow-moving, mas-
sive lower atmosphere to higher altitudes where the atmosphere is
less massive, so that the same momentum results in a much higher
velocity.
These new discoveries of long-term changes in global wind
patterns and an enormous haze envelope which appears and dis-
appears could help scientists further define the "driver" for the
planet's high-speed winds. Any future general atmosphere circula-
tion model for Venus will have to produce these long-term changes
in wind and cloud patterns.
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Details of these and other major findings from the two-year
analysis of the Venus cloud and polarimetry data include:
• It is now clear that the high-speed movements of Venus'
clouds around the planet are not caused by wave motions in the
atmosphere, as was thought by a number of scientists, but are
real winds, though there are some wave motions as well. These
planet-circling winds, which carry along the clouds, are the same
ones which were measured by the four Pioneer Venus probe craft as
they descended to Venus' surface in December 1978. These winds
blow in an east to west direction, circling the planet once every
four days at speeds near the equator of 360 km/hr (225 mph), and
near the poles (at around 70 degrees latitude) of 160 km/hr (100
mph). The Pioneer cloud pictures show the region of Venus' main
cloud deck at altitudes between 60 and 65 km (37 and 40 mi.) above
the planet's surface.
• The global pattern of these planet-circling, cloud-level
winds appears to change periodically. For the past two years of
Pioneer observations, Venus' clouds and cloud-level winds have
been evidencing "solid body" rotation. That is, they move around
Venus as though they were made up of one solid, planet-encasing
body. This pattern of motion, of course, means wind speeds are
much higher at the equator than at the poles.
• However, in 1974 when the Mariner spacecraft flew past
Venus, the clouds did not circle the planet as a solid body,
and there were mid-latitude jet streams at around 45 degrees
latitude.
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These higher-speed winds had velocities of around 400 km/hr
(250 mph), while wind velocities at the equator were some 40 km/hr
(25 mph) lower, at 360 km/hr (225 mph).
This seems to indicate that there is an irregular cycle of
change in the pattern of these cloud-level winds — perhaps sev-
eral years in length. The duration and rate of change of this
cycle of changing wind patterns would be of fundamental interest
in understanding the high-speed flow of Venus' upper level winds
around the planet, as well as the behavior of the general atmos-
phere circulation.
• Measurements of Venus' cloud level winds show that, in
addition to circling the planet, they also blow toward the poles
at speeds of around 25 km/hr (15 mph). These equator-to-pole
winds (also seen by the four Pioneer probes at lower altitudes)
carry heat from the Sun, absorbed near the Venus equator, to the
poles.
The speeds of these equator-to-pole winds agree with the
wind measurements by the four Pioneer probes. This indicates
that the cloud-level winds are the upper limb of an equator-to-
pole Hadley cell circulation loop, which carries Venus' equa-
torial heat poleward.
• The so-called global "Y" pattern of Venus clouds with the
tail of the "Y" extending eastward around the planet, and the arms
westward, appears at times, but is not typical. The "Y" was first
seen in ground observations.
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Sometimes the "Y"f which occasionally extends two-thirds of
the way around the planet, disappears completely. At other times,
it is so changed that it forms a "C" or other shape. In general,
the planet shows a whole range of global cloud patterns in addi-
tion to the "Y".
• In addition to its well-known veil of clouds, two years of
Pioneer polarimetry measurements show that Venus is currently en-
veloped in an 18-mile-thick blanket of high-altitude haze. The
haze is present everywhere, but has about three times more par-
ticles per unit volume at the poles than at the equator. At the
poles, the-haze is so thick that it obscures the base clouds be-
neath it. This haze of tiny sulfuric acid droplets is the "sealer"
of Venus' greenhouse effect, holding additional heat beyond that
which would be trapped by the clouds and atmosphere alone. The
planet's 482 Celsius (900 Fahrenheit) surface temperature would
be somewhat lower without the haze. Furthermore, inclusion of
-*•
haze effects makes the Venus heat radiation models developed by
scientists match the cloud top atmosphere structure observed by
the Pioneer Venus instruments.
Venus' main clouds consist of sulfuric acid particles two
microns in diameter, while in the haze layer, the particles are
smaller, only a quarter of this size.
Over the next five years of the pioneer Venus orbiter's
extended mission (through 1985), Dr. Travis will be watching
the clouds for changes.
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He will be looking for changes in the global circulation
pattern, such as the one which occurred in the five years sepa-
rating the Pioneer and Mariner missions (1974-1979). Determin-
ing such factors as the rapidity of the change and the frequency
of its occurrence should lead to further understanding of the
Venusian high-speed winds.
The Pioneer Project is managed by NASA's Ames Research Cen-
ter, Mountain View, Calif. The spacecraft were built by Hughes
Aircraft Co.
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(Index: 23, 28, 33)


Venus cloud studies show changes in long-term wind patterns

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