The first glimpses of the surface using radar from Earth

The surface of Venus has long been hidden from human eyes by the thick veil of clouds, leading, as we have seen, to all kinds of speculation about what lies beneath. However, radar can be bounced from the surface and the recorded echoes synthesised into a picture, and today most of our detailed knowledge and mapping of the surface has been obtained in this way. To get good resolution, and to cover the polar regions, the radar equipment needs to be on a spacecraft orbiting Venus.

However, before the first mission to carry radar flew to Venus in 1978, remarkable progress had been made in obtaining pictures of the surface using the same technique all the way from the Earth. This requires a very large dish antenna to transmit and receive the pulses, and those at Goldstone in California and at Arecibo in Puerto Rico were the first to be pressed into service.

Now we are mapping the Moon. Unlike past times when mapping meant sailing or walking to geographic features to mark them on a map, or peering through a telescope from Earth, our robots orbit tens of kilometers overhead and remotely characterize each lunar surface parcel (pixel) in optical light, infrared, particle flux, radar, elevation, and a dozen other techniques. It may seem we will soon describe the Moon completely. It is secure science: if a scientist discovers something, it will still be there on the next orbit in two hours, or when the spacecraft passes overhead two weeks or a month later, or next year, or when another lunar craft orbits with a similar instrument. What we discover depends critically on not only how we look with our choice of instrumentation (like Oersted’s magnetic needle) but also how observers are trained and what they acknowledge.

We will soon enter a new phase. Since Soviet robotic lunar rover Lunokhod 2 crashed and overheated in May 1973, and the last lunar sample reached Earth on Luna 24 in August 1976, no spacecraft and certainly no human has operated significantly on the Moon’s surface. (Recently the Chinese Chang’e 3 and Yutu rover worked on the Moon, only briefly; several lunar craft have crashed landed, too.) NASA shut down Apollo’s lunar surface ALSEP instruments on September 30, 1977. Soon robotic spacecraft will once again be roving and sampling the Moon, and eventually humans will leave new trails of nearly eternal footprints. Gene Cernan and Jack Schmitt left the lunar surface more than 40 years ago, and are in some sense the most recent human space explorers, certainly of an alien world. The Apollo lunar astronauts’ experience is key to understanding how exploration is possible, especially by humans – one of the key questions in the next few chapters.

Centuries of speculation about the nature of the clouds on Venus, and many theories and guesses about their composition, some quite exotic, finally led to the definite identification of concentrated sulphuric acid in the 1970s. The clouds were considered likely to have a quite simple vertical and global structure because they looked so featureless through a telescope. Now that they have been studied and analysed, from Earth, from orbit, and by probes parachuting through them, are they still ‘mysterious’?

Yes, for a number of reasons. Consider some of the problems we are still pondering:

1. The clouds form distinct layers. What produces this vertical structure? How much does it vary, across the planet and in time, and what produces the variations?

2. Only the higher layers are ‘definitely’ composed of sulphuric acid. The clouds form layers over a range of depths in the atmosphere, and there is evidence that the deeper layers have some different composition, possibly solid particles.

3. Even the higher layers have impurities mixed in them that we have not yet identified, including the absorber that gives rise to the dark ultraviolet markings. Some sort of variable chemistry is going on, producing localised concentrations and patterns of absorber.

4. The horizontal structure in the upper cloud layer is produced by waves and by weather that can be at least partially understood through its resemblance to terrestrial meteorology. But the deep cloud also shows dramatic, variable contrasts, and these have a different character. Before these were discovered, they would not have been expected at all, let alone on such a dramatic scale, and the mechanism responsible is still unclear. In Chapter 11 we suggested that they could be dust plumes from erupting volcanoes, but this is not yet confirmed.

5. As on Earth, the clouds on Venus are a central part of a hydrological cycle of evaporation, condensation and precipitation that is at the heart of a highly interactive climate system. Any variation in cloud density or distribution is going to be linked to changes in the energy balance in the atmosphere, and any long-term changes will affect the atmospheric ‘greenhouse’ and the surface temperature. We need to understand this in detail.

There is still a lot to do then. Let’s see how these puzzles fit with what we already know.

The motivation to continue the quest

Having arrived at an up-to-date position on missions to Venus, we can now summarise what we know, and ponder what we still wish we knew, about this close-by and Earthlike world. To begin with, as we have seen, in some ways it is not much like the Earth at all, and we seek to find out why.

Since 1962, there have been no fewer than forty-four spacecraft launched towards Venus, to orbit, land, float or just fly past and make observations in transit (although not all of them were successful, see Appendix A). This programme, along with advanced Earth-based observations over a wide range of wavelengths, and some imaginative theory and comparative planetology studies (mostly treating Venus as an analogue of the Earth, recognising that Mars and Mercury are siblings too), has painted a fairly complete general picture of our mysterious neighbour at last.

In some ways, this vision is not too encouraging, at least for those who see planetary exploration as a search for Earthlike, habitable environments. Where Mars turned out to be a cold place with a thin atmosphere, Venus is now seen to be the opposite, with a surface environment far worse, for human survival, than the conditions found in a pressure cooker in any kitchen on Earth. In fact, if a tin of beans was placed on the surface of Venus (and if the tin were really made of tin, which they are not these days) the atmosphere would not only cook the beans but the tin would melt as well. Human expeditions are not therefore on the cards for Venus for quite some time, and the prospects for any kind of life there have faded (but not quite vanished) after centuries of raised expectations based on early twentieth-century predictions of tropical forests and warm, soda-water oceans.

By necessity this book concerns not just next year or 10 years, but maybe the next 20 or 40. People yet unborn will fulfill (or forsake) these efforts and decisions. What will their world be like, and who will they be? For now we depend on others’ educated guesses. However, as we choose, space exploration can play important roles, in ways useful to current politics but also influencing youth central to realizing these and other dreams.

Exploration takes time, generations. It can inspire generations, one of the few strong motivations demonstrated to attract young people into science, engineering, and mathematics (over-succinctly acronymed as STEM: Science, Technology, Engineering and Mathematics). Modern economies, especially America’s, require vital influx of inspired scientists, engineers, and mathematicians. These workers and innovators maintain the main seminal foundation of America’s economic production (besides its agricultural base). Innovative products (and food) are what America creates from scratch that people in other nations want to buy.

Twin planets?

When considering the solid planets and not their climate at the surface (which is the subject of Chapter 10), Earth and Venus seem very similar and really are twin planets. But they are not identical twins. We have already seen that Venus is 5 per cent smaller in diameter and quite different in its orbital dynamics, with slow retrograde rotation and near-zero obliquity. The absence of a magnetic field is one of the few really clear indications that we have that the deep interiors are not altogether the same. The absence, or at least the difference in character, of plate tectonics on Venus suggests major differences in the solid crust, as do substantial differences in the geography of the two surfaces.

It is a crucial part of our outlook on the world, and not just as scientists, to try to understand to what extent Venus and Earth are the same and to what extent they differ, and why. To a very great degree, we still lack the experimental evidence to answer this, although some progress has been made. When considering the solid body, progress will continue to be slow because it is much harder to make surface and, especially, interior measurements of the kind we need. This is in contrast to the atmosphere, which is now quite well studied because it is relatively accessible. Unless there is a surprising increase in the amount of effort and money devoted to planetary exploration and prospecting, it will be a long time until we have data from deep-drilled cores and seismological measurements on Venus that are comparable to those that are responsible for so much of our knowledge about the earth below our feet.

The Soviet Union launched the first space probe towards Venus on 4 February 1961. However, this failed, and so did their next several attempts. The Americans, too, came unstuck on their first attempt. It was not to be expected that such a sophisticated endeavour as the first flight to another planet would be achieved easily, and both teams soon tried again. In the end, it was the Americans who got a working spacecraft to Venus first.

The Venus Mariners: the first close-up views

The US space agency NASA was set up in 1958 and among its first tasks was the development of the Pioneer series of small spacecraft to explore the interplanetary medium near the Earth. These were followed by the Surveyor series, which targeted the Moon. A larger spacecraft than these would be needed to go on to even the closest planets, and NASA gave the job to its newly acquired centre in Southern California, the Jet Propulsion Laboratory (JPL). Before this, JPL had been an Army Air Corps facility for the development of rocket engines, with the name dating back to 1943. The new series of spacecraft was called Mariner, and Venus was its first target.

The key to any discussion of the past and future state of the climate on Venus is an understanding of the production and loss processes for atmospheric gases, from the interior, at the surface, and at the top of the atmosphere where it merges into space. The balance between all of these determines the total mass of the atmosphere, and hence the surface pressure. This, in turn, is the principal factor controlling the temperature and hence the habitability and other characteristics of the surface environment.

At the heart of the problem is an almost complete lack of understanding of the various factors, summarised in Figure 11.1, related to volcanic activity on Venus. Volcanic emissions not only contribute to the mass and composition of the atmosphere, they also fuel cloud formation as part of a complicated cycle of atmospheric and surface chemistry involving various sulphur compounds. The surface itself is mainly of volcanic origin, although this leaves plenty of scope for puzzling about its composition and its capacity for absorbing, as well as emitting, atmospheric gases. The crust is dry and thin but evidently supports some huge volcanic mountains despite apparently being too weak to do so without convective upwelling which, if present, should also drive plate tectonics, although the observational evidence for the latter is elusive.

1. “Beside the Mare Crisium, that sea

2. Where water never was, sit down with me

3. And let us talk of Earth, where long ago

4. We drank the air and saw the rivers flow

5. Like comets through the green estates of man,

6. And fruit the color of Aldebaran

7. Weighted the curving boughs.”

A century ago water on the Moon began as an idea with the worst possible intellectual pedigree. In 1894 Hans Hörbiger, a successful engineer (who invented a valve to control blast furnaces’ airflow), had a curious vision of the Universe. His Glacial-Kosmogonie, published in 1912, propounded the Welteislehre (“World Ice”) theory, with the Moon, our Galaxy, and even space itself dominated by water ice, apparently inspired by the icelike appearance of the Moon in the night sky.

Hörbiger’s book was championed by respected German amateur selenographer Philipp Fauth, aided by Hörbiger and family. Public extravaganzas promoted the theory to common knowledge. Its cold, northerly tenor in opposition to Einstein’s relativity (and even Newtonian physics) attracted Nazi leaders. Welteislehre became party doctrine, and Fauth was promoted by S. S. Reichsführer Heinrich Himmler to university professor (having never taught at that level or conducted sufficient research). Fauth named a lunar crater Hörbiger (following his death in 1931). During and after the Third Reich, Hans Schindler wrote several books expanding the World Ice theory, soon discredited. In 1948 Hörbiger’s name was stripped from the crater (although a crater Fauth remains).

Long before the Cold War and the Space Race, people dreamed of traveling to places beyond Earth, like islands in a mythical sea, and they persist now that the Cold War is over. Dreams change, however, with the centuries’ achievements and the day’s technology and society. An odd mixture of myth, politics, and scientific knowledge sets the goals. More than five decades after the first robotic Moon missions, we have seen enough cycles of interest and dispassion about space exploration to see why we sometimes advance and at other times fail. To grasp the difference between the times in which Apollo was born versus later thrusts into space (and to understand why some succeed), momentarily let us return to the beginning.

Humans have dreamed of space flight for thousands of years. In second century AD Syria, Lucian of Samosata wrote of a sailing ship blown hundreds of kilometers skyward to an inhabited, cultivated, and luminescent island: the Moon. After war between the kings of the Moon and of the Sun, the ship and sailors return home to the Mediterranean. Before rocketry’s importance was realized, imaginative means were proposed to reach the Moon. In eleventh-century Persia, Firdausí wrote of King Kai-Kaus who “fetched four vigorous eagles and bound them firmly to the throne” to ride them to the Moon, much like the hero in Lucian’s other space fiction (Icaro-Menippus), whereas in 1630 the protagonist of Johannes Kepler’s Somnium is transported by demons. In Francis Godwin’s 1638 The Man in the Moone, the traveler exploits a flock of magic geese to find himself on the Moon with its human inhabitants (Christians, no less). In 1657 Cyrano de Bergerac’s hero reached the Moon propelled by rockets (fireworks, actually). In 1870 Edward Hale described a “brick moon” to be built in Earth’s orbit as a navigational aid for shipping.

Krafft Ehricke not only helped pioneer some of the earliest modern, liquid-propellant rockets, but also lived to develop workhorse boosters for the space age and concepts for lunar mining and planetary exploration now in the works. He envisioned the Moon as a stepping-stone, a role it played in several ways throughout humanity’s development starting long ago. He had a clever way of stating the profoundly obvious.

Ehricke’s life-span saw astounding human achievements: harnessing amazing new energy sources, traveling hundreds of times faster than ever before, probing scales millions of times larger and thousands of times smaller than imagined before, and transforming the Moon and planets from dreamlands to mapped worlds. We once ascribed romantic notions to the Moon; now we see how alien worlds differ from Earth and distant worlds of our imagination.