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NATIONAL ACADEMY OF SCIENCES NATIONAL ACADEMY OF ENGINEERING INSTITUTE OF MEDICINE NATIONAL RESEARCH COUNCIL
June 18, 2004 Current Operating Status
1990 Update to Strategy for Exploration of the Inner
Planets
4
Venus
PROGRESS
This section summarizes advances in our understanding of Venus since 1978 and
outstanding problems that require resolution by future missions. The decade
following the 1978 report witnessed the acquisition of the first geophysical data
relevant to the planet's interior. The Pioneer Venus radar altimeter provided a
global topographic map, and radio tracking of the orbiter supplied gravity data over
limited regions. In contrast to Earth, Venus's gravity and topography are highly
correlated, and interpretations of the combined data may involve mantle and
lithosphere dynamical processes peculiar to Venus. The Pioneer Venus
magnetometer placed a strict upper limit on any internally generated global
magnetic field; the implications of this for the planet's internal structure and
theories of the geodynamo are profound and include the possibility that the core is
completely liquid.
The Soviet Venera 15 and 16 radar missions, as well as high-resolution Earth-
based radar observations, have revealed a complex tectonic history for the Venus
surface. There are unique features visible in these images that could have
important implications for internal processes. Although the impact crater record
provides a broad constraint on surface ages, limited resolution (1 to 2 km) has
prevented identification of the complete record with certainty. The Magellan
mission should provide resolution sufficient to characterize the cratering history
and to search for a possible relic crater population prior to climatic instability. The
mission should also test hypotheses about the timing, style, and rate of surface-
modification processes including erosion, possible fossil river beds, volcanic
resurfacing, and perhaps plate tectonics.
Experiments to determine major element composition of surface rocks were
performed by seven of the Soviet Venera landers. The elemental inventories are all
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consistent with two types of basalts: high-potassium basalt near a suspected
impact feature and tholeiitic basalts in the lowland plains, similar to ocean floor
basalts on Earth.
Noble gas abundances in the atmosphere of Venus were measured by the Pioneer
Venus and Venera probes, but the data contain serious conflicts and are subject to
profound differences of interpretation. The deuterium abundance in the
atmosphere was tentatively determined from the 1978 Pioneer Venus data to be
0.016. This elevated value has been a prime driver for the argument that the planet
has lost a massive amount of water over the age of the solar system, although an
alternative interpretation involving comet impacts is also viable. In addition, the
reported abundance of water in the atmosphere has a very peculiar altitude profile
when the Pioneer Venus and Venera measurements are combined. Since the
water and the deuterium abundances are key quantities in understanding the
evolution of the atmosphere and the planet's surface, improved abundances and
height profiles are essential.
Global changes in the composition of the Venus atmosphere have recently been
shown to occur on short time scales, on the order of months or years. Analysis
indicates that the global average abundance of SO2 in the visible-part of the
atmosphere decreased tenfold between 1978 and 1983, perhaps as result of the
recovery of the atmosphere from a massive volcanic eruption.
Pioneer Venus provided important information on ionospheric and solar wind
interactions, atmospheric thermal structure, zonal and meridional circulation, and
planetary-scale waves and tides. The Soviet VEGA balloons collected additional
information about the dynamics of the atmosphere of cloud level.
SCIENTIFIC OBJECTIVES
Measurements from Pioneer Venus, Venera; VEGA, and ground-based
investigations have provided a quantitative data base but have also raised
fundamental issues concerning the planet's interior, surface, and atmosphere that
demand further study.
Determination of the interior structure is a primary objective for future exploration
following successful completion of the Magellan mission. The existence or absence
of a metallic core needs to be established, and the properties of the core—such as
its radius, density, and physical state—must be determined. Knowledge of core
mass and size will determine average mantle density and will constrain models of
the composition of the interior. A seismometer array deployed by landers or
penetrators on the surface would directly determine the planet's deep internal
structure.
The necessity for a long-lived seismic array raises serious difficulties for instrument
operation at the high temperatures of the Venusian surface. Development of
reliable electronics to withstand operating temperatures of 500° C is required.
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Seismic probing would yield detailed radial profiles of seismic velocities and reveal
any major density discontinuities in the core and mantle. It would be possible to
determine if the planetary core were partially solidified as a test of explanations for
the lack of a planetary magnetic field and theories of the geodynamo.
The development of high-temperature electronics for a long-term seismic array
would also enable the establishment of a more general-purpose geophysical array
to measure such quantities as surface heat flow-an important constraint on the bulk
content of radiogenic elements in the interior. Long-term radio tracking of surface
geophysical stations would reveal important characteristics of the planet's rotation,
an additional source of information on internal structure and atmosphere-solid
planet angular momentum exchanges. While Magellan may provide global
topographic and gravity data, higher-resolution data on particular locations will be
necessary to understand the interior dynamics and surface tectonic processes
responsible for certain geological features. Should Magellan fail to collect the
expected global topography and extensive gravity data, the acquisition of this
information would remain a high-priority science objective.
Direct sampling of rock from a highland region has never been done and is crucial
to determining whether such rocks have a granitic composition similar to .that of
continental shields on the Earth. Elemental analyses could be improved on future
lander missions but require the development of electronics that can function at the
high surface temperatures.
A complete elemental inventory along with mineralogical analysis would be key to
interpreting the surface record and would provide important information on the
chemistry of surface-atmosphere interactions on Venus. Such interactions are a
critical component in the short- and long-term evolution of the atmosphere.
While important initial measurements of atmospheric dynamics and composition
were made by the Venera spacecraft and by the Pioneer Venus orbiter and probe,
some fundamental issues remain unresolved. New measurements of noble gas
abundances, isotope ratios, molecular species, aerosols, and atmospheric
dynamics are all needed.
In the upper atmosphere, the abundance of molecular oxygen is a key indicator of
photochemical processes, but O2 has never been detected. The abundance of free
O2 in the lower atmosphere would also be an important indicator of surface-
atmosphere chemical equilibrium. Other photochemically important species that-
have not been measured or are poorly measured include compounds of O, Cl, and
S above the cloud tops.
In the upper haze region, a number of questions remain after Pioneer Venus.
Sulfur dioxide is a major absorber in the far-UV region of the spectrum, but
additional work is needed to characterize absorbers in both the near- and far-UV
ranges. Better measurements of these absorptions are vital for understanding the
global heat balance and greenhouse effect. This requires high-resolution
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spectroscopy in the near-UV region, photometric and polarimetric studies of the
bright and dark features, and laboratory work on cross sections.
Controversy also remains as to whether Pioneer Venus electric field data detected
lightning. Whether lightning exists and whether it is generated in volcanic eruptions
or in the cloud decks (which do not appear to precipitate or have large potential
instabilities) are important open questions that should be addressed in future
missions.
The dynamics of the superrotating Venus atmosphere needs additional study as
well. What is the nature of the mean and temporally variable zonal and meridional
circulations? What maintains the dominant superretrograde rotation? How do
planetary-scale waves and tides transport heat and momentum? What is the
interaction of cloud dynamics and circulation? Direct measurements of winds have
utilized the motions of the Pioneer Venus and Venera descenders, but only at
altitudes below 65 km. Venusian winds can .be measured to very high altitudes
from future spacecraft using remote sensing,techniques, including Doppler shifts in
IR spectral lines and microwave rotational transitions of CO. LIDAR measurements
should be developed for applications on both Venus and Mars. The aeronomy of
the upper atmosphere,, involving photochemistry and the wind fields, requires
measurements, well above the clouds. Temperature sounding and wind
measurements are needed at all latitudes and heights for a comprehensive
understanding of the circulation system. Finally, new measurements of ionospheric
composition and dynamics are important to further advances in our understanding.
Approaches here include long-term UV imaging, remote temperature
measurements in the cloud levels and above by a long-term orbiter, balloons and
surface weather stations for circulation in and below the clouds, and theoretical
efforts to develop three-dimensional global circulation and dynamical models.
UPDATED RECOMMENDATIONS
Essentially all of the strategy developed for Venus in the 1978 report remains valid.
That report presumed that the Pioneer Venus and Venera investigations of the
middle and lower atmosphere would completely determine its basic structure,
dynamics, and chemical composition: The 1978 strategy therefore emphasized
complementary investigations. The Pioneer Venus and Venera results, however,
left some significant questions unanswered and raised important new issues.
Hence, science objectives pertaining to middle and lower atmospheric composition
and dynamics remain of primary importance, in addition to the objectives stated in
the 1978 report. These were
1. to obtain a global map of the topography and morphology of its surface at
sufficient resolution to allow identification of the gross processes that have shaped
the surface;
2. to determine the major chemical and mineralogical composition of the surface
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material;
3. to determine the concentrations of photochemically active gases in the 65- to
135-km altitude region; and
4. to investigate the physical and chemical interactions of the surface with the
atmosphere and the composition and formation of atmospheric aerosols.
Therefore, COMPLEX recommends that characterization of the basic structure,
composition, and dynamics of Venus's atmosphere be a primary objective.
The 1978 report also considered study of the Venusian interior using passive or
active seismic techniques as a secondary objective, while noting that it would be a
primary objective if it appeared feasible. At that time, acquisition of such data
required new technology development of instrumentation able to function for long
durations at the surface temperatures. The current committee regrets to note that
the required technology developments have not occurred in the intervening
decade.
The committee wishes to elevate studies of the planet's interior to primary status,
together with studies of the surface and the atmosphere, so that acquisition of
seismic data is now regarded as a primary objective. The committee therefore
reiterates the following recommendations of the 1978 report, restated as follows:
that acquisition of seismic data from Venus be maintained as a highly desirable
goal, that serious study of instruments operating at Venusian surface temperatures
be undertaken, and that preliminary studies be conducted to determine the
technical feasibility of sample return from Venus.
Finally, the committee notes that a more sensitive search for any intrinsic magnetic
field of Venus is of primary significance for determining the nature of the planet's
interior.
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