Venus is the brightest 'star' in the night sky and it has been observed since ancient times. Often dubbed Earth's 'twin', it is the planet most similar to the Earth in size, mass and composition. There the similarity ends: Venus is shrouded by a dense carbon dioxide atmosphere, its surface is dominated by thousands of volcanoes and it lacks a protective magnetic field to shield it from energetic solar particles. So why isn't Venus more like Earth? In this book, a leading researcher of Venus addresses this question by explaining what we know through our investigations of the planet. Venus presents an intriguing case study for planetary astronomers and atmospheric scientists, especially in light of the current challenges of global warming, which supports, and potentially threatens, life on Earth. Scientifically rigorous, yet written in a friendly non-technical style, this is a broad introduction for students and astronomy and space enthusiasts.

Overview page xi

Prologue xiii

A note on scientific units xvii

Acknowledgements xix

Part I Views of Venus, from the beginning to the present day

1 The dawn of Venus exploration 3

      The Evening Star and the Morning Star 3

      Transits: Venus crosses the disc of the Sun, but rarely 6

      Early observations: another planet with an atmosphere 8

      Observations of surface features 9

      The ashen light 10

      The ultraviolet markings 12

      Speculation on the nature of the clouds and conditions at the surface 13

      The search for water vapour in the atmosphere 14

      A hot and arid surface? 15

2 Mariner and Venera: the first space missions to Venus 17

      The Venus Mariners: the first close-up views 17

      Venera: the first landings 24

      Venera atmospheric measurements 29

      The first Venus orbiters 29

3 Pioneer Venus and Vega: orbiters, balloons and multiprobes 32

      NASA plans multiple-entry probes and an orbiter 33

      Pioneer Venus 33

      Vega: the first balloon mission 40

      A picture of the planet emerges 41

4 Images of the surface 48

      The first glimpses of the surface using radar from Earth 48

      Pioneer and Venera: early radar maps from orbit 51

      Magellan: high-resolution radar images of the surface 54

5 The forgotten world 64

      The 'evil twin' syndrome 64

      Unfulfilled objectives 67

      Technical studies 68

      Phantom missions 69

      Broken dreams 74

6 Earth-based astronomy delivers a breakthrough 75

      New windows on Venus 77

      The leaking 'greenhouse' 78

      Mapping the deep cloud structure and dynamics 79

      Composition measurements in the deep atmosphere 81

7 Can't stop now: Galileo and Cassini fly past Venus 87

      Deep cloud structure 88

      Ashen light revisited 89

      Atmospheric composition 90

      Atmospheric dynamics and meteorology 93

      Temperature maps of the surface 94

      Observation of Venus by Cassini /VIMS 95

8 Europe and Japan join in: Venus Express and Akatsuki 97

      The Venus Express mission proposal 97

      The scientific payload 99

      The flight to Venus 103

      Venus Express achievements 105

      Venus Express lives on 113

      Japan's climate orbiter, Akatsuki 113

Part II The motivation to continue the quest

9 Origin and evolution: the solid planet 119

      Twin planets? 119

      What is Venus made from? 119

      Composition of the surface rock 121

      Messengers from the interior: the noble gases 122

      Why no magnetic field? 125

      Venus's surface features: Earthlike but different 125

      On the surface: mountains, continents and river valleys 127

      Tectonic features 133

      Impact craters 135

      Lava plains 138

      Metallic snowcaps 139

10 Atmosphere and ocean 141

      Not-so-identical twins 142

      Atmospheric composition and the greenhouse effect 145

      Energy balance and entropy 146

      Why so much CO2? 147

      Venus's primordial ocean 150

      Water on Venus now 152

      Exosphere and escape 152

11 A volcanic world 156

      Images of volcanic features on the surface 158

      Surface composition and temperature 160

      Volcanic gases in the atmosphere: sulphur dioxide 161

      The water budget 163

      Noble gases and isotopic ratios 166

      Heat flux from the interior 167

      Bright plumes and ash clouds 169

      Sulphur dioxide and water vapour fluctuations above the clouds 173

      Comparison with terrestrial volcanism 175

      Models for the current level of volcanic activity 176

      Consolidating the models 180

12 The mysterious clouds 182

      Vertical structure of the clouds 183

      Partide sizes and composition 185

      Cloud chemistry 188

      Horizontal structure of the clouds 190

      The ultraviolet absorber 193

      Carbon dioxide clouds? 194

      The polar clouds 195

      Diurnal and long-term variation 195

13 Superwinds and polar vortices 197

      Meteorological measurements on Venus 198

      Probe wind measurements 199

      Cloud images and infrared remote sounding 200

      The Hadley circulation 202

      Explaining the zonal super-rotation 204

      The polar vortices, collar and 'dipole' 208

      A 'strange attractor' on Venus 210

      Waves and cloud features 211

      Deep cloud motions and upper atmosphere dynamics 213

14 The climate on Venus, past, present and future 215

      Early Venus 216

      Volcanic Venus 217

      Present Venus 217

      Future Venus 218

      Climate change models 220

      Model forecasts and hindcasts 221

      Summing up 223

15 Could there be life on Venus? 225

      Possible habitats on Venus 225

      Evolution and adaptation 226

      Terrestrial vs. Martian vs. Venusian life 227

      Possible evidence for life 227

      Astrobiological experiments: next steps 229

      The prospects for finding life on Venus 232

      Part III Plans and visions for the future

16 Solar system exploration: what next for Venus? 237

      Choosing the way forward 237

      Venus in NASA's Discovery Programme 242

      Priorities and road maps: NASA leaves Venus on the back burner 242

      The Russian revival 244

      European plans 246

17 Coming soon to a planet near you: planned Venus missions 249

      The next NASA mission to Venus 249

      European Venus mission studies 252

      Venera-D 255

      International cooperation 257

18 Towards the horizon: advanced technology 259

      Larger, cheaper payloads: aerobraking and aerocapture 259

      Surveying the surface: mappers, aeroplanes and submarines 260

      Direct sampling of the surface 262

      Instruments 263

      Rovers and drilling 263

      Sample return: bringing material back to Earth 265

19 Beyond the horizon: human expeditions 270

      When, if ever 270

      NASA's 1967 manned flyby 271

      Floating research stations 273

      Terraforming Venus: building an Earthlike climate 274

Epilogue 277

References and acknowledgements 279

Appendix A: Chronology of space missions to Venus 289

Appendix B: Data about Venus 291

Index 293

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