Non-evidence of lightning and associated volcanism at Venus

Reports of unpredicted lightning and its spatial association with mountains of possible volcanic origin are provocative features of the 1980's literature on Venus. These reports are based upon interpretation of low-frequency 100 Hz electric field noise observed from the Pioneer Venus Orbiter during 1978–1986. These speculations have been repeatedly challenged in the literature. Even though explosive volcanism, like lightning, is discounted in the literature, researchers have been prompted to believe in present-day eruptions by the suggestion that volcanic plumes might stimulate the otherwise unexpected lightning. Recent introductions of a distinct set of higher-frequency electric field noise has resulted in further claims for lightning, but these results, like those derived from the 100 Hz data are discounted be several independent studies. Commenting on the large body of 100 Hz data, Russell (1991) abandons earlier reports of the planetographic clustering of this noise, and states that active volcanoes are not the source of the Venus lightning. This welcome acknowledgement leaves unresolved problems. First, this brief comment is quite insufficient to correct the widespread and flawed perception that Venus is currently experiencing widespread lightning, stimulated by volcanic disturbances. Second, this admission leaves unexplained the origin of the voluminous 100 Hz data set. The foregoing problems, combined with negative results of recent independent studies, indicate strongly that the Pioneer Venus results provide no reliable evidence of either lightning or volcanism at Venus.     

The Habitability of Venus

Space Science Reviews - Venus today is inhospitable at the surface, its average temperature of 750 K being incompatible to the existence of life as we know it. However, the potential for...     

Current knowledge of Venus

As an introduction to the remaining papers in this issue, a summary is given of our current knowledge of Venus, with emphasis on recent progress and on the contributions to be expected from the Pioneer Venus missions. Headings are surface and interior, clouds and lower atmosphere, dynamics and thermal structure, neutral upper atmosphere, and ionosphere and solar-wind cavity.     

The atmosphere of Venus

The investigations of Venus take a special position in planetary researches. It was just the atmosphere of Venus where first measurements in situ were carried out by means of the equipment delivered by a space probe (Venera 4, 1967). Venus appeared to be the first neighbor planet whose surface had been seen by us in the direct nearness made possible by means of the phototelevision device (Venera 9 and Venera 10, 1975). The reasons for the high interest in this planet are very simple. This planet is like the Earth by its mass, size and amount of energy obtained from the Sun and at the same time it differs sharply by the character of its atmosphere and climate. We hope that the investigations of Venus will lead us to define more precisely the idea of complex physical and physical-chemical processes which rule the evolution of planetary atmospheres. We hope to learn to forecast this evolution and maybe, in the far future, to control it. The last expeditions to Venus carried out in 1978 — American (Pioneer-Venus) and Soviet (Venera 11 and 12) — brought much news and it is interesting to sum up the results just now. The contents of this review are:

1. The planet Venus — basic astronomical data.
2. Chemical composition.
3. Temperature, pressure, density (from 0 to 100 km).
4. Clouds.
5. Thermal regime and greenhouse effect.
6. Dynamics.
7. Chemical processes.
8. Upper atmosphere.
9. Origin and evolution.
10. Problems for future studies

Here we have attempted to review the data published up to 1979 and partly in 1980. The list of references is not exhaustive. Publications of special issues of magazines and collected articles concerning separate space expeditions became traditional last time. The results obtained on the Soviet space probes Venera 9, 10 (the first publications) are collected in the special issues of Kosmicheskie issledovanija (14, Nos. 5, 6, 1975), analogous material about Venera 11, 12 is given at Pis'ma Astron. Zh. (5, Nos. 1 and 5, 1978), and in Kosmicheskie issledovanija (16, No. 5, 1979). The results of Pioneer-Venus mission are represented in two Science issues (203, No. 4382; 205, No. 4401) and special issue of J. Geophys. Res. (1980). We shall mention some articles to the same topic among previous surveys: (Moroz, 1971; Sagan, 1971; Marov, 1972; Hunten et al., 1977; Hoffman et al., 1977) and also the books by Kuzmin and Marov (1974) and Kondrat'ev (1977). Some useful information in the part of ground-based observations may be found in the older sources (for example, Sharonov, 1965; Moroz, 1967). For briefness we shall use as a rule the abbreviations of space missions names: V4 instead of Venera 4, M10 instead of Mariner 10 and so on. The first artificial satellites of Venus in the world (orbiters Venera 9 and 10) we shall mark as V9-O, V10-O unlike the descent probes V9, V10. Fly-by modules of Venera 11 and Venera 12 we shall mark as V11-F and V12-F. Pioneers descent probes — Large (Sounder), Day, Night and North — will be marked as P-L, P-D, P-Ni, P-No, orbiter as P-O, and bus as P-B.     

The clouds of Venus

The current state of knowledge of the Venusian clouds is reviewed. The visible clouds of Venus are shown to be quite similar to low level terrestrial hazes of strong anthropogenic influence. Possible nucleation and particle growth mechanisms are presented. The Pioneer Venus experiments that emphasize cloud measurements are described and their expected findings are discussed in detail. The results of these experiments should define the cloud particle composition, microphysics, thermal and radiative heat budget, rough dynamical features and horizontal and vertical variations in these and other parameters. This information should be sufficient to initialize cloud models which can be used to explain the cloud formation, decay, and particle life cycle.     

Plasma waves at Venus

Many significant wave phenomena have been discovered at Venus with the plasma wave instrument flow on the Pioneer Venus Orbiter. It has been shown that whistler-mode waves in the magnetosheath of the planet may be an important source of energy for the topside ionosphere. Plasma waves are also associated with thickening of the ionopause current layer. Current-generated waves in plasma clouds may provide anomalous resistance resulting in electron acceleration, possibly producing aurora. Ion-acoustic waves are observed in the bow shock, and appear to be a feature of the magnetotail boundary. Lastly plasma waves have been cited as evidence for lightning on Venus.     

The exploration of Venus

This paper is an introduction to a special issue ofSpace Science Reviews dedicated to the exploration of Venus and the role played by the Pioneer Venus program. The Pioneer Venus program consists of a Multiprobe and Orbiter mission, both to be launched and to encounter Venus in 1978. The evolution of the program is traced from its conception in 1968 as the Goddard Space Flight Center Planetary Explorer Program through its transfer to Ames Research Center in 1971 as Pioneer Venus to the present.     

Balloon Position Determination on Venus

A position determination technique for balloons floating in the Venusian atmosphere is described. Measurements are all made using a single station of the NASA Deep Space Network. One coordinate of the fix is derived from a ranging measurement, and the other coordinate is derived from observation on earth of the polarization of a signal radiated from a linearly polarized antenna on the balloon, aligned to vertical by balloon local gravity.     

Venus Interior Structure and Dynamics

No two rocky bodies offer a better laboratory for exploring the conditions controlling interior dynamics than Venus and Earth. Their similarities in size, density, distance from the sun, and young surfaces would suggest comparable interior dynamics. Although the two planets exhibit some of the same processes, Venus lacks Earth’s dominant process for losing heat and cycling volatiles between the interior and the surface and atmosphere: plate tectonics. One commonality is the size and number of mantle plume features which are inferred to be active today and arise at the core mantle boundary. Such mantle plumes require heat loss from the core, yet Venus lacks a measurable interior dynamo. There is evidence for plume-induced subduction on Venus, but no apparent mosaic of moving plates. Absent plate tectonics, one essential question for interior dynamics is how did Venus obtain its young resurfacing age? Via catastrophic or equilibrium processes? Related questions are how does it lose heat via past periods of plate tectonics, has it always had a stagnant lid, or might it have an entirely different mode of heat loss? Although there has been no mission dedicated to surface and interior processes since the Magellan mission in 1990, near infrared surface emissivity data that provides information on the iron content of the surface mineralogy was obtained fortuitously from Venus Express. These data imply both the presence of continental-like crust, and thus formation in the presence of water, and recent volcanism at mantle hotspots. In addition, the study of interior dynamics for both Earth and exoplanets has led to new insights on the conditions required to initiate subduction and develop plate tectonics, including the possible role of high temperature lithosphere, and a renewed drive to reveal why Venus and Earth differ. Here we review current data that constrains the interior dynamics of Venus, new insights into its interior dynamics, and the data needed to resolve key questions.     

Clouds and Hazes of Venus

More than three decades have passed since the publication of the last review of the Venus clouds and hazes. The paper published in 1983 in the Venus book summarized the discoveries and findings of the US Pioneer Venus and a series of Soviet Venera spacecraft (Esposito et al. in Venus, p. 484, 1983). Due to the emphasis on in-situ investigations from descent probes, those missions established the basic features of the Venus cloud system, its vertical structure, composition and microphysical properties. Since then, significant progress in understanding of the Venus clouds has been achieved due to exploitation of new observation techniques onboard Galileo and Messenger flyby spacecraft and Venus Express and Akatsuki orbiters. They included detailed investigation of the mesospheric hazes in solar and stellar occultation geometry applied in the broad spectral range from UV to thermal IR. Imaging spectroscopy in the near-IR transparency “windows” on the night side opened a new and very effective way of sounding the deep atmosphere. This technique together with near-simultaneous UV imaging enabled comprehensive study of the cloud morphology from the cloud top to its deep layers. Venus Express operated from April 2006 until December 2014 and provided a continuous data set characterizing Venus clouds and hazes over a time span of almost 14 Venus years thus enabling a detailed study of temporal and spatial variability. The polar orbit of Venus Express allowed complete latitudinal coverage. These studies are being complemented by JAXA Akatsuki orbiter that began observations in May 2016. This paper reviews the current status of our knowledge of the Venus cloud system focusing mainly on the results acquired after the Venera, Pioneer Venus and Vega missions.     

The structure of the Venus ionosphere

Our current knowledge of the spatial structure of the Venus ionosphere and its temporal behavior is reviewed, with emphasis on the more recent Pioneer Venus measurements and analysis not covered in earlier reviews. We will stress the ionosphere structure, since other papers in this issue deal with its dynamics, and its magnetic properties. We also discuss some of the limitations that the orbit has placed on the spatial and temporal coverage of the ionosphere. For the benefit of future users of the data some of the factors which affect the measurement accuracies are discussed in an Appendix.Currently at Space Physics Research Laboratory, University of Michigan, Ann Arbor, MI 48109, U.S.A.     

Climate Variability on Venus and Titan

Venus and Titan are both slowly-rotating, approximately Earth-sized bodies with cloudy, dynamic atmospheres. Each has a complex climate system, even less well understood than the terrestrial equivalent, and the processes that appear to maintain the climate near the surface on both bodies have interesting similarities and differences with each other and with the Earth. By considering these factors and their possible evolution with the aid of elementary climate models, some interesting, albeit tentative, conclusions can be reached concerning the stability of climate on Earth-like planets, and the likely nature of past and future climate change.     

Future of Venus Research and Exploration

Despite the tremendous progress that has been made since the publication of the Venus II book in 1997, many fundamental questions remain concerning Venus’ history, evolution and current geologic and atmospheric processes. The international science community has taken several approaches to prioritizing these questions, either through formal processes like the Planetary Decadal Survey in the United States and the Cosmic Vision in Europe, or informally through science definition teams utilized by Japan, Russia, and India. These questions are left to future investigators to address through a broad range of research approaches that include Earth-based observations, laboratory and modeling studies that are based on existing data, and new space flight missions. Many of the highest priority questions for Venus can be answered with new measurements acquired by orbiting or in situ missions that use current technologies, and several plausible implementation concepts have been studied and proposed for flight. However, observations needed to address some science questions pose substantial technological challenges, for example, long term survival on the surface of Venus and missions that require surface or controlled aerial mobility. Missions enabled by investments in these technologies will open the door to completely new ways of exploring Venus to provide unique insights into Venus’s past and the processes at work today.     

The magnetosheath and magnetotail of Venus

We describe the observational history and assess the current understanding of the magnetosheath and magnetotail of Venus, stressing recent developments. We make recommendations for research that can be done using existing observations, as well as desirable trajectory and instrumentation characteristics for future spacecraft missions.     

The solar wind interaction with Venus

This paper assesses our current understanding of the solar wind interaction with Venus in light of developments since the last major reviews were published in 1983. Suggestions for making further progress in the area of solar wind interactions with planetary atmospheres and ionospheres are offered based on the available observations and techniques, and from the viewpoint of forthcoming missions to Mars.     

Ion dynamics in the Venus ionosphere

Dynamics play an important role in defining the characteristics of the Venus ionosphere. The absence of a significant internal magnetic field at Venus allows the ionization to respond freely to gradients in the plasma pressure. The primary response to a gradient in plasma pressure is the nightward flow of the ionization away from a photoionization source on the dayside. The flow is approximately symmetric about the Sun-Venus axis and provides the source of O⁺ that maintains the nightside ionosphere during solar maximum. Modelling efforts have generally been successful in describing the average nightward ion velocity. Asymmetric and temporally-variable flow is measured, but is not well described by the models. Departures from axially-symmetric flow described in this paper include ionospheric superrotation at low altitudes and an enhanced flow at high altitude at the dawn terminator. Variability that is the result of changes in the ionopause height induced by changes in solar wind dynamic pressure is especially strong on the nightside. Ion flow to the nightside is also reduced during solar minimum because of a depressed ionopause.     

Radio observations of Venus and the interpretations

The radio observations of Venus are reviewed and compared with theoretical microwave spectra computed for a variety of models of the Venusian environment. The models considered are (a) a CO₂-N₂ atmosphere, (b) an atmosphere of dust (the aeolosphere model), and (c) a cloud model with various loss mechanisms in the cloud. The effect of polarization on the surface emissivity has been included in all the computations. It is shown how the radio observations place limits upon the acceptable models, for example, the density and size of dust particles required in the aeolosphere model. It is shown how some models place severe restrictions on radar observations at short centimeter wavelengths, thereby emphasizing the importance of such experiments. These same models show that the Mariner II observations can not be interpreted in terms of surface phenomena and provide a new interpretation for the microwave phase effect.This work was supported in part by the U.S. Army, Navy and Air Force under Contract DA36-039-AMC-03200(E); and in part by the National Aeronautics and Space Administration (Grants NsG-250-62 and NsG-419).