Venus and the VRM mission PETER FRANCIS

The most profound advances in the Earth sciences during the next decade will not be made on Earth. This paradoxical situation has come about because, while it is becoming more and more difficult to find new approaches to investigating the planet we are so familiar with, by the end of the decade we will have had the opportunity to study a second planet that is remarkably similar to the Earth, but one that has evolved quite differently: Venus. Such study may help us to learn why the Earth evolved in the way that it did, with the unique combination of surface conditions that allow life to be sustained.

T h e exploration of the solar system by spacecraft over the last 20 years has advanced our knowledge of the planets further than all the studies of the previous three millennia. In a period of a few years, answers have been found to many of the problems that have confronted man since he first looked up to the heavens. One of the saddest aspects of this glorious flowering of science has been that it has made little impression on the geological community. While they may have admired the technical achieve- ments of landing men on the Moon and spacecraft on Mars, few geologists seem to think that studies of such distant bodies are of much relevance to them. Thus they have continued in much the same old way to plot endless diagrams of trace element compositions, to quarrel over

Fig. 1. Many bizarre ideas about Venus circulated in the 19th and early 20th centuries, when telescopes were the only source of information. This odd- looking chart was drawn by Percival Lowell, the American astronomer chiefly renowned for his enthusiasm for ‘canals’ on Mars. The chart says as much about Lowell as C l m i d V-. 6m by b w ~ U a oaobcr I@. 1806 (h Monfhly

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it does about Venus.

the interpretation of neodymium and strontium isotopes and to kid themselves that the best geologist is the one that has hammered the most rock.

To a degree, this attitude is understandable. Most. of the results obtained to date have been from small planetary bodies (the Moon, Mars, Mercury, the satellites of Jupiter) whose geo- logical life was short and dominated by early impacting. Little has happened on the Moon in the last three billion years, whereas terrestrial geology has traditionally focused on the last 600 million years. Venus, by contrast, is so similar to the Earth that it provides a sort of giant ex- perimental test-bed, a kind of alternative Earth, where fundamentally similar geological pro- cesses have led to startlingly different results.

Most of the planets are difficult to spot with the naked eye, unless one knows where to look. Venus is hard to avoid. It comes so close to the Earth that it often blazes as a brilliant jewel in the morning or evening sky, bright enough to cast its own shadows and so bright that it is regularly reported as a UFO. At its closest approach, Venus comes within 64 million kilometres of the Earth, still a very respectable distance. The reason for its brilliance is that it is completely shrouded with clouds, which reflect nearly 80% of the incident sunlight.

Although its surface remained permanently invisible, astronomers using classical telescope techniques were able to show that Venus shares many of the Earth’s physical characteristics, as Table 1 demonstrates. Such strong similarities encouraged the belief that the Earth and Venus were twin planets; and in the early part of the century, it was widely supposed that the clouds concealed a Venusian Eden - a planet covered with lush forests and populated by all manner of exotic wildlife. For example, Svante Arrhenius, the Swedish Nobel prize winner, announced in 1918 that Venus was covered by hot, humid swamps and that lowly forms of life flourished there. In the 1920s, a quite different view

174lGEOLOGY TODAY Nov-Dec 1985

emerged, arguing that Venus was a dry, barren desert whose surface was whipped by winds to raise impenetrable dust clouds into the atmos- phere (Fig. 1).

In the absence of reliable data, such argu- ments simmered on intermittently into the 1960s, when the first techniques not dependent on optical telescopes became available. The modern era in the exploration of Venus opened in 1962. In that year, Earth-based radar, able to ‘see’ through the clouds, was used to track features across the planet and to demonstrate that the axial rotation period was 243 days. In the same year, a spacecraft from Earth provided the first close-up views of the planet. Earth- based radar and spacecraft studies have con- tinued hand-in-hand ever since. When the first microwave radiation temperature estimates were obtained for Venus they were not believed. They suggested that the serene, cloud-shrouded planet had a surface temperature of several hundred degrees. This was so contrary to in- tuition that alternative explanations were sought. It was argued that the microwave radia- tion was generated in Venus’ ionosphere and that the surface was comfortably cooler, an anthropocentric view that was not to survive subsequent scrutiny.

Venus: the view from 1984 A positive shower of spacecraft has descended on Venus. In all, no fewer than 19 successful spacecraft missions to Venus had been accom- plished by the end of 1984, providing extra- ordinary new insights into the geology of the planet but leaving tantalising gaps that are criti- cal to a full understanding (Fig. 2).

On the broadest scale, radio altimetry obtained by the Pioneer spacecraft revealed that on Venus the total range of topography from highest mountain to deepest valley is similar to that of the Earth (13.7 km as opposed to 15.4 km). Whereas a hyposometric curve of the Earth’s topography shows that it has distinct ‘continental’ and ‘oceanic’ areas, the curve for Venus has only one peak. Sixty per cent of its surface lies within 500 m of the ‘average’ level, with few high and low spots. There are, how- ever, some distinct ‘highland’ areas which re-

Table 1: Physical characteristics of Venus and the Earth

semble terrestrial continents, but are much smaller. The largest, Ishtar Terra, is about as big as the USA and contains the highest point on Venus, Maxwell Montes, about 12 km above the mean level. Studies of the gravitational field carried out by Pioneer simultaneously with its radar work showed that there is a strong correla- tion between topography and regional gravity on Venus, higher elevations being associated with higher gravity. This contrasts sharply with the

Fig. 2. The first close-up view of Venus. This ultraviolet image was Obtained by the

Of the Mariner 10 spacecraft as it swung past Venus on its way to Mercury in 1974. The well-defined cloud structure is invisible to the naked eye.

Mass Equatorial radius Oblateness (ratio of polar to equatorial

radii) Density Surface gravity Equatorial escape velocity Axial inclination

Venus

4.9 x kg 605 1 km 00.000

5.24 x lo3 kg mP3 8.87 msC2 10.4 km s-’ -2.6 deg

Earth 5.9 x kg 6378 km 0.0034

5.52 x 103kgmP3 9.78 ms-’ 11.2 kms-’ 23.45 deg

GEOLOGY TODAY Nov-Dec 19851175

Earth, where, once all the various corrections are applied, gravity and topography are not strongly correlated owing to the effects of iso- stasy.

This simple observation raises some im- mediate questions. What produces the ‘conti- nents’ on Venus? Does plate tectonics as we know it operate? The geophysical arguments are quite difficult to grapple with, but the correla- tion between regional gravity and topography has been used to argue that the depth of isostatic compensation is much deeper on Venus than the Earth, and also that the topography of Venus may be dynamically supported by convection in the mantle rather than floating passively on top.

Cosmochemical arguments suggest that Venus should be made of the same sort of stuff as the Earth, and specifically that Venus should have a silicate mantle with a similar content of heat- producing radioactive elements. Thus Venus’ mantle should be hot and convecting. How is this manifested at the surface of the planet? Confirmation that Venus’ mantle is closely simi- lar to that of Earth came from the Venera landers that carried out analyses of surface sam-

Fig. 4. The general structure of one of the early Venera landers. (M.V. Keldysh)

Fig. 3. Mankind’s first views of the surface of another planet. Above is a panorama of the Venusian surface, taken by a camera aboard the Russian Venera 9 spacecraft; below is the one from Venera 10. Both reveal a bleak, boulder strewn monotonous terrain with little relief.

ples and thus demonstrated the presence of basalts similar to those of the Earth’s oceanic crust. While the Pioneer topographic data did demonstrate the presence of elevated areas approximating to ‘continents’, they did not re- veal anything approaching oceanic ridges, though these should be detectable at the resolu- tion of the Pioneer data. Thus at present there is no direct evidence for sea floor spreading on Venus.

The first successful Venera landing on Venus (Fig. 4) confirmed what the microwave radiation data had indicated: the surface temperature is a searing 500°C. This introduces another com- plicating factor into the plate tectonic dis- cussion; with such a high surface temperature, the Venusian lithosphere should be hotter and therefore less dense than its terrestrial counter- part. This will make the lithosphere more buoyant and therefore less likely to be sub- ducted. On these grounds, it has been argued that plate tectonic processes, as we know them on Earth, cannot operate on Venus.

Plate tectonics is a manifestation of the way the Earth loses its internal heat to the oceans, the

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176lGEOLOGY TODAY NowDec 1985

atmosphere, and finally to space. Lacking ‘ter- restrial’ type plate tectonics, where most of the heat is lost through the oceanic crust to the oceans, Venus must lose its heat some other way. An alternative mechanism to account for Venus’ heat loss is ‘hot spot’ volcanism. Instead of losing heat by conduction and hydrothermal circulation in oceanic crust generated at oceanic ridges, heat loss on Venus may be concentrated on a relatively few major volcanic centres (about 35) dotted around the planet, each one compar- able to Hawaii and each one spewing forth copious volumes of basaltic lavas which radiate their heat to the atmosphere. Such hot spot volcanoes may possibly have been very active in the very early history of the Earth, before the present system of plate tectonics became orga- nised.

While the details of Venus’ internal processes (Fig. 5) are highly controversial, there is strong evidence that there are indeed major volcanoes on Venus and that they may actually be active at the present time. The radar images obtained from Earth-based radio telescopes and from the most recent Russian spacecraft in orbit around Venus show massive edifices which look very like volcanoes with central calderas and which are ‘bright’ to the radar beam, showing that the surfaces are rough, just what one would expect from lava flows. The largest of these edifices is Beta Regio, which rises several kilometres above the surrounding rolling plains and which also corresponds to a significant positive gravity anomaly. It has been suggested that powerful uprising plumes in the Venusian mantle are responsible for the topographic elevation, the gravity anomaly and for sustained volcanic activity.

Venus’ present topography is the end result of the interplay between its internal and surface processes. Its surface processes have in turn been dictated by the evolution of its atmosphere. The searing surface temperatures on Venus and the crushing atmospheric pressure (90 times the Earth’s) have become familiar through Carl Sagan’s rapturous accounts on his celebrated voyage through the cosmos, and his explanations of the ‘greenhouse effect’ are as authoritative as they are rhapsodical. There is as much C 0 2 in the atmosphere of Venus as there is in all the carbonate rocks on Earth, and it has played a significant part (perhaps to the tune of 50%) in trapping thermal radiation. The small traces of water and the smaller traces of SO2 in the atmosphere are also remarkably effective in trap- ping thermal radiation, almost as effective as the much larger amounts of COz. It is the green- house effect that is responsible for Venus’ pre- sent lack of water; any water that existed at the surface would be rapidly vaporised, entering the atmosphere and re-enforcing the greenhouse. Once in the atmosphere, the water is eventually

Crust 10- 30 km Crust 3 77

VENUS EARTH

lost through photo-dissociation at high levels, the hydrogen escaping into space and the oxygen combining chemically with other elements. Thus while there may still be some water deep down in Venus’ mantle, none has survived on its surface or in crustal rocks.

There is a good deal of cosmochemical evi- dence to suggest that Venus should have started life with almost as much water as Earth. Thus there could in theory once have been great oceans on its surface. Indirect confirmation of this came from an American probe that pene- trated the atmosphere and found that deuterium is enriched by a factor of 200% in Venus’ atmosphere, relative to the deuterium-hydrogen ratio in the Earth’s atmosphere. The heavier isotope could only have been concentrated in the atmosphere when the lighter hydrogen isotope was lost to space through photo-dissociation. Thus there must have been large amounts of water on Venus originally in order to account for such conspicuous enrichment. If oceans of water ever existed at the surface, they may have left behind recognisable evidence of the fact - marine terraces, river deltas and so on - which should be identifiable on images of quite modest resolution.

One of the most taxing questions of all geo- logical science, then, centres on how the Earth and Venus came to end up so differently, given such similar starting points. The critical factor was probably that Venus receives nearly twice the amount of solar radiation as the Earth. Thus much more energy was pumped into its atmos- phere from the beginning, rapidly assembling the components of the greenhouse. On Earth we may owe our salvation from incineration to the slower pace of things. Water vapour certainly existed in the atmosphere from an early date, warming the surface, but primitive life forms were able to evolve in the oceans in time to deal with the increasing amount of C 0 2 exhaled from volcanic sources, replacing it with oxygen and thus putting a brake on the greenhouse effect.

Fig. 5. Venus and Earth are closely similar in size and mass. Their internal structures are thought to be closely similar, although much remains to be learned about Venus,

GEOLOGY TODAY Nov-Dee 19851177

Fig. 6. An example of the recent Russian radar imagery of Venus. The picture is a mosaic from a number of strips, each corresponding to a single orbital pass. Picture width is about 1500 km; height 1800 km. The area covered is Metis Region, at lat. 72” near Venus’ north pole, and west of Ishtar Terra. A few small impact craters are seen, but the area is dominated by curious ring-like structures 200-500 km in diameter termed coronae. Their origin is uncertain, but radar- bright features in and around them may be lava flows and hence the coronae may be forms of large volcanic caldera. VRM imagery will have much higher resolution and may provide the key to interpreting these enigmatic features.

VOIR and VRM At the time of writing (early 1985), two Russian spacecraft are in orbit around Venus. They will eventually obtain radar imagery of about 30% of the planet at a resolution of between 1 and 2 km. To date, the Russians have been rather reticent about their results and have released only a few images showing volcanic calderas and rift-like valleys. Although the Russian radar imagery

known as VOIR (Venus Orbiting Imaging Radar), but the idea did not survive President Reagan’s accession to the White House. The scientific community, however, fought back against the President’s emasculation of what had been an extraordinarily successful programme of research in planetary science, and managed to get a more modest mission funded, with launch planned for 1988. This will be the Venus Radar Mapping (VRM) mission. An impression of the extent of the scrimping and saving needed to get this mission off the ground may be gained from the fact that the main radar antenna on the spacecraft - its raison d’etre - was not purpose- built but was merely a left-over from the hard- ware manufactured for the earlier Voyager mis- sions to Jupiter and Saturn. Notwilhstanding such miserly economies, the VRM mission is likely to be a great success scientifically, not least because it is being mounted by the same groups of uniquely talented and resourceful scientists and technicians that have obtained such remark- able results from the earlier US planetary mis- sions in the face of severe technical and financial restraints. The spacecraft will image over 90% of the surface area of the planet at a resolution better than 500 m.

In short, although the VRM spacecraft will not be as sophisticated or as expensive as it might have been, it will still be extremely power- ful and capable of revolutionising our knowledge of our sister planet. Our understanding of the Earth’s internal processes took a great leap forward during the 1960s when we were able to see the Earth for the first time in a plate tectonic framework, a concept that illuminated the inter- relationships of the whole range of geological processes. The opportunity to see how another, similar planet works will be another step to- wards understanding the evolution of the Earth. That step may also prove to be a great leap.

Suggestions for further reading Francis, P. 1981. The Planets: the Geography,

Geology and Meteorology of Eight ‘ N e w Worlds’. Penguin Books.

Hunten, D.M., Colin, L., Donahue, T.M. & Moroz V.I. (eds). 1983. Venus. University of Arizona Press.

James, W. (ed.) V-gram (A newsletter published by the Jet Propulsion Laboratory, Pasadena, California, USA, containing up-to-date in- formation on Venus and the status of the VRM mission.)

represents a substantial technical achievement,

of the outstanding geological questions (Fig. 6). With typical Californian bravado, scientists at

the Jet Propulsion Laboratory in Pasadena prop-

much better resolution is required to solve most ~- ~~ ~

Peter Francis is Reader in Earth Sciences at the Open University.

osed a highly sophisticated mission to Venus, ~ ~ ~~ ~ ~~

178IGEOLOGY TODAY Nov-Dec 198.5