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The planet Venus, blanketed by a thick atmosphere of carbon dioxide and nitrogen containing reflective, high-altitude clouds of sulfuric acid, has been extremely mysterious to astronomers until very recently in history. Prior to investigation of Venus by cloud-penetrating radar in 1961, astronomers knew absolutely nothing about its surface. Some writers speculated that a warm tropical world lie beneath its clouds.
They were right about the warm part. Microwave and infrared radiometers mounted on Mariner 2, a space probe which made a fly-by of Venus in 1962, revealed that the surface was incredibly hot — 425 °C (797 °F), hot enough to melt lead. This crushed all speculation of life on the surface. On the plus side, the cloud-tops of Venus were found to be relatively cool, comparable to temperatures on Earth. The pressure at the surface was found to be about 92 times greater than at sea level on Earth, similar to the pressure 1 km (0.62 mi) under the ocean.
Throughout the 1970s and 1980s, intense investigations were carried out on Venus, both using Earth-based radar and space probes. Earth-based radar only reveals surface features larger than about 5 km (3 mi), so further investigation requires probes.
The surface of Venus was found to be very flat, resulting from the massive weight of its atmosphere and the lack of tectonic activity. The tallest mountain range are the Maxwell Montes, which the highest point 12 km (7.4 mi) above the surface. Because of this altitude, Maxwell Montes is the coolest and least pressurized location on the surface of Venus, but it would still kill any human in seconds. The distance between the highest and lowest point on Venus is only 13 kilometers (8.1 mi), while on the Earth the difference is about 20 kilometers (12.4 mi). 51% of the Venusian surface is located within 500 meters (1640 feet) of the median radius of the planet (Venus' equivalent of "sea level").
About 10% of the planet's surface consists of two main "continents" called highlands. These include Aphrodite Terra, about the size of Africa, and Ishtar Terra, which contains the Maxwell Montes. Along with these highlands, there are several large uprisings created through volcanic action, including Beta Regio, Phoebe Regio, Themis Regio, Alpha Regio, Eistla Regio, Bell Regio, and Tholus Regio. The largest of these volcanoes were discovered in the first radar surveys of the early 1960s.
Venus has about 900 impact craters, almost all of which are larger than 30 kilometers (18 mi) in diameter. The reason why is because smaller asteroids burn up in the thick atmosphere before they manage to hit the surface. If an asteroid has the momentum to make it through the atmosphere, it is certain to leave a large crater. A few well-known craters include Danilova, Aglaonice and Saskja. All have been given names, generally after female figures from history and mythology.
Other features on Venus include volcanoes which may be active, as well as mysterious structures called arachnoids found nowhere else. Arachnoids consist of concentric ovals surrounded by a complex network of fractures, and may have a diameter as large as 200 km (124 mi). They may have a volcanic origin, or be formed through some other process.