----- The following copyright 1991 by Dirk Terrell ----- This article may be reproduced or retransmitted ----- only if the entire document remains intact ----- including this header Lecture #13 "Love is Hell" I'm sure most of you have seen Venus in the sky. In the nightime sky, only the moon surpasses it in brightness. In fact, Venus' brilliance often catches people by surprise. A large number of UFO reports are merely sightings of this dazzling planet. Thus it is not difficult to understand why our ancestors named this planet after the goddess of love. But appearances can be deceiving, and in this case greatly so. Before the age of spacecraft exploration, very little was known about the surface of the planet because it is completely enshrouded by thick clouds. It is amusing to read about old speculations as to the surface conditions. Some described it as a humid, swampy planet teeming with life. Others wrote that it was a hot, dry desert. One wonders if they weren't describing two different planets. (Because of its orbit, Venus is seen either in the morning or evening sky, and it was once thought to be two planets -- Hesperus and Phosphorus.) Probes of the planet by spacecraft (the first by Mariner 2 in Dec., 1962) revealed it to be covered by a mainly carbon dioxide atmosphere with a surface temperature of 750 Kelvins and pressures as high as 1300 psi, or 90 times that here on Earth. And the principle component of the clouds is sulfuric acid. Certainly no place for life as we know it! Venus is very similar to Earth in size and chemical composition. The equatorial diameter of the planet is 12,104 km, which makes it about 95% the size of the Earth. Its mass is 4.9 x 10^24 kg, which is about 82% the mass of the Earth. Putting those two numbers together shows that the two planets have very similar average densities, and thus similar chemical compositions. Soil samples taken by the Soviet Venera landers show that the crust density is about 3 gm/cm^3, like that of Earth. Its orbit is nearly circular (e=0.007) and it orbits the sun in 224.7 earth days. One unusual characteristic about the planet is its rotation -- it rotates very slowly, spinning once every 243 days, and it rotates in a retrograde sense (opposite to its orbital motion, only Uranus shares that characteristic). Since its orbital period is shorter than its rotation period, a "day" on Venus is longer than a "year." Venus has been explored extensively by spacecraft from the USA and the USSR: Spacecraft Encounter Date Type Country ------------------------------------------------------------------ Mariner 2 12/62 Flyby USA Venera 4 10/67 Hard Lander USSR Mariner 5 10/67 Flyby USA Venera 5 5/69 Hard Lander USSR Venera 6 5/69 Hard Lander USSR Venera 7 12/70 Soft Lander USSR Venera 8 7/72 Soft Lander USSR Mariner 10 2/74 Flyby USA Venera 9 10/75 Orbiter/Lander USSR Venera 10 10/75 Orbiter/Lander USSR Pioneer Venus 1 12/78 Orbiter USA Pioneer Venus 2 12/78 Hard Landers USA Venera 11 12/78 Soft Lander USSR Venera 12 12/78 Soft Lander USSR Venera 13 3/82 Soft Lander USSR Venera 14 3/82 Soft Lander USSR Venera 15 10/83 Orbiter USSR Venera 16 10/83 Orbiter USSR Vega 1 6/85 Flyby/Balloon USSR Vega 2 6/85 Flyby/Balloon USSR Magellan 8/90 Orbiter USA ------------------------------------------------------------------ Some notable achievements in that crowd: Venera 7 was the first spacecraft to land on another planet, Venera 9 gave us the first photograph of the surface, Mariner 10 gave us the first pictures of the planet from a spacecraft, and Venera 13 returned the first color pictures from the surface. Just to show you how harsh the surface conditions are, Venera 7 survived only 23 minutes after landing! Venera 8 was tougher. It lasted almost an hour! Because of the thick clouds, imaging of the surface from Earth or from space must be done by radar. Radar can penetrate the clouds and let us probe the surface. Practically the entire planet has been mapped by radar. These maps have revealed that there are two continent-sized landforms on Venus, and several deep basins. Most of the planet, however, consists of rolling planes. Over 70% of the surface lies within +/- 1 km of the mean radius of the planet. The same figure for Earth is only 20%. The Venera lander photographs show the surface to be a rocky desert. The chemical composition determined from soil samples show it to be very similar to solidified lavas here on Earth. Magellan images have unmistakably shown volcanic flows. Being mainly carbon dioxide, Venus' atmosphere is the cause of its extremely high surface temperature. The planet has a runaway greenhouse effect because the carbon dioxide, sulfuric acid, and sulfur dioxide trap the solar energy that reaches the surface. Even though only about 2-3% of the solar radiation incident on the planet reaches the surface, the greenhouse gases trap this energy very efficiently, causing the surface temperature to be elevated by hundreds of Kelvins. So, as you can see, Venus is not the kind of place where humans will be running around in the near future. Although it is a beautiful sight in the sky, the planet named after the goddess of love is probably not like what the ancients had in mind. Dirk