Is there life on Venus? Probably not.

We could leave it there, except for the fact that there are several good scientists who do not dismiss the possibility, so there is definitely something to discuss. Also, we have to admit that we do not really understand the conditions under which life can evolve, or at least survive, and accept that there is life on Earth in niches (inside nuclear reactors, for example, or deep down in cold, dark frozen lakes) where no one expected to find it until it was discovered recently. Certainly, if we want to know whether we are alone in the Universe, and most of us do, we should leave no stone unturned.

Possible habitats on Venus

There are two very different environments where we might look for life on Venus: in the clouds and on the surface. Most of the research to date has focused on the former, for the obvious reason that the temperatures and pressures near the cloud tops are close to those at the Earth’s surface where we know that conditions suit life forms of many kinds. There is also a supply of liquid water, widely accepted as a prerequisite for life of any kind. There is also plenty of energy, as solar radiation beats down on the cloud tops with only moderate amounts of attenuation during its passage through the upper atmosphere. The vigorous dynamical activity, providing mixing and a strong diurnal effect, is all to the good. Finally, we already have evidence for chemical activity, for instance that which converts sulphur dioxide to sulphate, and this is no doubt only the tip of the iceberg as far as chemistry is concerned, and in a basicallyCO2 atmosphere some of it will be pre-organic (at least), providing further sources of energy and the compounds that might be the building blocks of life.

Preface

There is a new Moon. Many of the most dramatic recent discoveries in planetary science are lunar. They transform our understanding of the Moon and lunar exploration’s prospects for exciting work there. Yet in many minds the Moon is an old story. NASA sent a dozen men to its surface in 1969–1972, and most people do not remember those events personally. Only in the past few years has lunar exploration accelerated again, and many, including policy makers deciding about the space program, do not realize how rapidly our knowledge of the Moon is changing.

This ignorance is unfortunate, since from 2004 to 2010 we were headed to the Moon but changed our minds. Some of these recent discoveries about the Moon might have changed our minds back again. By “our” in this case I refer to the United States, since many other nations’ space programs still see the Moon as an essential objective.

Larger, cheaper payloads: aerobraking and aerocapture

In the slightly longer term, missions to Venus are likely to benefit from various technical improvements that have been made or are in an advanced stage of development. For instance, there are two new techniques that will make it much cheaper to place a substantial mass in Venus orbit without using a very large and expensive launch vehicle. Both involve interaction with the atmosphere to slow the spacecraft down on arrival, thus eliminating some of the heavy rocket motors and the fuel that would otherwise be required.

Aerocapture is the approach in which the spacecraft is targeted accurately to the right pressure level so that atmospheric drag reduces its velocity relative to the planet to the point where it is captured into orbit. There is a risk, of course, that erroneous targeting, or poor knowledge of the atmospheric conditions, can result in the spacecraft crashing into the planet, or missing it altogether. In either case the margin available is not great.

The pre-space age planetary astronomers knew that the albedo (reflectivity) of Venus is higher than that of Earth, which must partially offset the extra heating that comes from being closer to the Sun. It was widely expected that Venus would turn out to be a more tropical version of the Earth, but no one pictured a climate as extreme as the reality that was first detected by ground-based radio astronomers and confirmed by Mariner 2 and Venera 9. This was a big surprise at the time.

In fact, the modern value for the albedo, integrated over wavelength, is more than two and a half times that of Earth, at about 76 per cent rather than 30 per cent, so that Venus actually absorbs less radiative energy than Earth, despite the Sun appearing twice as large in the sky. Thus, it could be argued that Venus should be not warmer but actually cooler overall. This, however, does not take into account the huge difference in atmospheric thickness. On Venus the surface temperature enhancement through the blanketing ‘greenhouse’ effect is similar to, but more than ten times larger, than the corresponding effect on Earth.

(Throughout this appendix we will refer to a large number of transient events reportedly observed in the vicinity of the Moon, and will refer to them as they are listed in the works by Middlehurst et al. 1968, Cameron 1978, and Cameron 2006.)

Most TLP reports are visual and in recent decades originated from amateurs. Before 1900, however, many reports came from reputable, professional astronomers, even famous ones: Wilhelm Herschel in 1783–1790 with six TLPs (he also discovered Uranus, several moons, and infrared light); Edmond Halley in 1715 (Astronomer Royal, of Halley’s comet fame); Edward Barnard with several TLPs in 1889–1892 (showed novae are exploding stars, discovered 17 comets and a moon of Jupiter); Ernst Tempel in 1866–1885 (discoverer of 21 comets); Johann Bode in 1788–1792 (famous celestial cartographer); George Airy in 1877 with a TLP confirmed independently (famous Astronomer Royal); Heinrich Olbers in 1821 (confirmed asteroid belt); Johannes Hevelius in 1650 (pioneering lunar topographer); Jean-Dominique Cassini in 1671–1673 (director of l’Observatoire de Paris); Camille Flammarion in 1867–1906 (founded Société Astronomique de France); William Pickering in 1891–1912 (co-founded Lowell Observatory); Johann Schröter in 1784–1792 (first noticed the phase anomaly of Venus); Friedrich von Struve in 1822 (founded Pulkovo Observatory); Francesco Bianchini in 1685–1725 (measured Earth’s axis precession); and Etienne Trouvelot in 1870–1877 (noted astronomical observer). In the twentieth century noted astronomers reporting TLPs included Dinsmore Alter (in 1937–1959), Zdeněk Kopal (in 1963), and Sir Patrick Moore (in 1948–1967). Franz von Gruithuisen in 1821–1839 reported changing luminous and lunar obscured spots, yet also described the Moon inhabited and dotted by cities. (He also was first to conclude that craters result from meteorite impacts.)

When, if ever

When plans for exploring the Solar System with manned spacecraft are discussed, Venus tends to get short shrift. In the near term, of course it is natural to talk about a return to the Moon and the establishment of a manned base there. The short journey times and low gravity are just two of many reasons this is the easiest and least expensive option for human exploration in the near term. It is also a good place to practise survival techniques and develop procedures for living successfully in space before venturing further afield. Also, of course, there is much about the Moon that is of scientific and practical interest that makes exploration still a valid objective 40 years after the first Apollo landing.

Once humans are permanently established on the Moon, almost everyone thinks of pressing on to Mars. Often the two are programmatically linked, with the lunar landings seen explicitly as a stepping stone on the way to the red planet. This was the case for the NASA initiative started under President George W. Bush, and recently terminated by President Obama. Mars remains a long-term goal in Europe under the Aurora programme. The reasons for favouring Mars over Venus are pretty much taken for granted: men and women can land there and explore in the traditional manner, driving buggies, using hammers and drills, climbing mountains and cliffs or descending into deep valleys. It is fairly easy to see, in outline at least, how they could build permanent bases and live in them, becoming self-supporting by growing food and mining ice deposits for water, possibly even making their own rocket fuel. No one would think of trying any of that on Venus.

As a result of the labyrinthine processes described in Chapter 16, we arrive at a ‘best guess’ for the near-term (next two decades, say) future of Venus exploration in the form of some combination of entry probes, balloons and landers. These will come from NASA and the European Space Agency, possibly in tandem but more likely not; and Venera-D from the Russians. Japan may try again to orbit Venus, and something from the Chinese and Indians cannot be ruled out, although they are more likely to focus on the Moon and Mars.

Such a programme is by no means assured, of course; there could be no new mission to Venus for 20 years, at the end of which time everything will have changed. It would be nice to think that several of the world’s space agencies might get together and pool their resources in the future, to mount a single large mission, perhaps sample return. However, history suggests a more fragmented approach can be expected, at best. Despite all of the uncertainty, we now look, in a spirit of optimism tempered with realism, more closely at the plans as they stand and will likely evolve in this possible multi-pronged attack on the remaining mysteries of Venus.

The circulation and dynamics of Venus’s atmosphere behave in ways that sometimes remind us of terrestrial meteorology, but mostly seem quite bizarre. Yet we routinely compute the dynamical behaviour of Earth’s atmosphere, for weather forecasting and other reasons, and it should be possible to do the same for Earth’s twin. However, even the most basic behaviour on Venus, the four-day ‘super-rotation’, is proving hard to diagnose or to replicate. Despite a great deal of attention by groups using some of the most sophisticated computers and models, Earthlike simulations with Venusian parameters inserted have tended to circulate too slowly.

A lot of meteorological activity – weather – has been observed in Venus’s atmosphere by orbiting spacecraft and measured in situ by descent probes, but understanding and interpreting what is going on is still at an early stage. Researchers continue to argue about whether lightning occurs, and although it probably does, there is no clear picture of how or where it is generated. Some of the small-scale and transitory features, such as the waves seen in the ultraviolet images of the cloud tops, and some of the global and semi-permanent behaviour, for example the Hadley circulation, have some recognisable relationship to similar behaviour on the Earth, although they may have to be scrutinised closely to see it. Other important phenomena seem fairly unique to Venus, not just the fast winds that circle the equator, but the complexity of the giant vortices at each pole, and the behaviour in the upper atmosphere, where the circulation seems to change to a completely different regime.

Tennyson reminds us that despite the scope for humanity of exploration, personal fulfillment rests on immediate accomplishment: on commerce, personal opportunity, and peace versus conflict – measured on a human scale. How can outer space fit into human lives?

From the start, Columbian-era Europeans set out to explore the world for profit. Although investors often lost their stakes and crew-members their lives, both dreamed of riches, and some were rewarded. In intervening centuries ships sailed the globe to exploit and trade in natural resources: spices, whale oil, and more. In the twentieth century, exploration of Earth’s polar regions and outer space was a contest for the prestige of nations and individuals more than profit. Lunar explorers need not fear unreasonably for their lives given precaution (Chapter 11), but what have investors to gain? Can distant space including the Moon be monetized for profit?

Views of Venus, from the beginning to the present day

Observations of Venus with the naked eye as a prominent planet or ‘wandering star’ were recorded by the Babylonians around 3000 bc, and have continued ever since. All the major civilisations have contributed knowledge and myth to a recondite and, until recently, quite abstruse concept of our nearest companion in space beyond the Moon. With the invention of the telescope in about 1610 it became clear to Galileo that Venus shone in reflected light from the Sun and had phases like the Moon, leading eventually to an understanding that Venus is not any kind of star, but an Earthlike object, one that orbits closer to the Sun than we do. The presence of an atmosphere on Venus, filled with cloud that veiled the entire planet at all times and prevented the observation of surface features, was recognised and refined from the 1760s onwards, and the principal composition of the atmosphere was established in the 1930s.

The ‘evil twin’ syndrome

After Magellan, there was a long hiatus in the exploration of Venus by spacecraft that extended from the end of the radar-mapping mission in October 1994 until the arrival of Venus Express in April 2006. The campaign to kick-start that European mission relied in no small part on pointing out that our nearest neighbour had become the ‘Forgotten Planet’, rather as Mars did for a time after the successful landing of the Viking surface stations.

However, at the turn of the millennium, interest in comparative planetology was at an all-time high and it might have been logical to focus our available resources on Earth’s nearest neighbour and closest twin. The Venus Express advocates were also at pains to point out that one of the key comparative aspects was global climate change, with Venus as the ultimate example of a greenhouse-warmed, Earthlike planet.