_Venus Revealed_ by David Harry Grinspoon is a well-written, witty, thoroughly researched book on our nearest planetary neighbor, the planet Venus, often thought of as Earth's twin due to its roughly same size and mass. Grinspoon covered the history of human perception of the planet, the observation of Venus by scientists from the ground through the centuries, what the amateur astronomer can see and learn about the planet, the saga of the numerous probes to orbit the planet as well as it enter its atmosphere and even land on its surface, current understandings of the atmosphere and geology of Venus, and speculations on whether or not Venus has or had life and the future of human exploration of the planet. There are two inserts in the book, one a color insert that included a color image of the surface of Venus made by the Soviet _Venera 13_ lander in March 1982 as well as several global and regional topographic maps made by the _Pioneer Venus Orbiter_ and _Magellan_, and a black and white insert which included more Soviet lander images of the ground of Venus as well as numerous close-ups taken by _Magellan_ of a wide variety of Venusian surface features. In the text of the book itself I really liked the various diagrams included, including schematics of the sulfur cycle on Venus and a diagram of typical cloud structure. _Magellan_ images are dominant in the book, an extraordinary space probe that peeled back the "bright, unyielding clouds" with "gentle radar fingers," revealing massive amounts of new information for Venus scientists to ponder and debate over.
Venus has long attracted human attention, as it is the brightest object in the night sky after the full moon. Though the planet was noticed by virtually every human culture, no civilization paid it more mind than the Classic Maya (A.D. 300-900). They felt they owed their very existence to Venus (whom they called Kukulcan) - a debt that they paid back in human sacrifices - and based their entire calendar on the 260-day Venus appearance interval. Mayan astronomers were able to chart the appearance, disappearance, and reappearance of Venus in the night skies with incredible accuracy, so much that the Mayan Venus Calendar has an error of only two hours in five hundred years of elapsed time.
The "solid citadel of clouds" that protected Venus from observation made it into a "tabula rasa," a blank slate that was inscribed by the wishes and dreams of observers for centuries. Grinspoon documented the many speculations about Venus being a swamp or ocean world, referencing both the serious speculations of astronomers such as Percival Lowell and the flights of fancy of popular literature and film. So little was known about the planet that even its rate of rotation wasn't resolved until 1962, when Earth-based radar images established that one day on Venus equaled 117 Earth days (and that it rotated in a backward or retrograde direction, with the sun rising in the west and setting in the east). Passive radio observations in 1956 that showed the planet emitting massive amounts of microwave radiation lead to the first real understanding of just how hot Venus was, as researchers began to infer that this was heat radiation from the surface, eventually establishing the surface temperature at 900 degrees Fahrenheit (so hot that an observer on the Venusian surface at night could see thanks to the glowing of the red-hot ground).
I enjoyed his coverage of the Venusian atmosphere the most of anything in the book. Though the planet-wide cloud cover looks basically bright and featureless even from orbit, images taken with ultraviolet filters have revealed that the atmosphere is dynamic and volatile, an intricate and complex swirl of high-contrast, fast-moving tiny splotches and huge, planet-wide streaks. The identity of this material, so dark in the ultraviolet portion of the spectrum that its it responsible for absorbing nearly half the solar energy received by Venus, is still unknown and is simply called the unknown ultraviolet absorber. Its existence though has allowed scientists to study and model patterns of atmospheric circulation, an atmosphere that at the upper levels circles the planet at 200 miles per hour, circling the planet in four days (dubbed superrotation), while at the same time is virtually motionless at the surface. Explaining this phenomenon has presented another major challenge offered us by Venus, one not yet answered.
The atmosphere is unlike anything seen on Earth; immense cloud banks of sulfur dioxide and sulfuric acid cover the planet, not very dense and relatively transparent but incredibly vast, towering up to an altitude of 44 miles from the cloud base at 33 miles. They are concentrated into three discrete layers - each layer with a different mixture of droplet sizes - and separated by relatively clear air between. The nature of the droplets in the lowest cloud layer (called Mode 3 droplets) is uncertain, as they are not spherical in shape, may be crystalline, and appear to contain far more chlorine than sulfur (as well as perhaps other substances).
Grinspoon gave the reader a tour of the surface, from the "continents" of Africa-sized Aphrodite Terra along the equator and Australia-sized Ishtar Terra near the north pole to the wide plains to the great variety of volcanoes on the planet, some of which are probably active. Volcanic landforms cover some 90% of the surface, ranging in size from small shield volcanoes (often less than 12 miles across), so numerous that they gather in clusters of a hundred or so in immense shield fields, to odd six to forty mile across pancake dome volcanoes to still larger ones. Many features appear unique to Venus, such as ticks (volcanoes with flanks scalloped by landslides such that the ridges appear to be the jutting legs of an insect), arachnoids (volcanic domes surrounded by spider-web like patterns of fractures and ridges), and anemonae (volcanoes with petal-like lava flows extending outward from them). Other features include the odd circular coronae and intensely deformed areas called tessera.
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