Venus is often known as Earth's sister planet, because it’s the planet closest to us in space and the most similar in size. Its diameter is 95% of Earth's, and its mass is 82% of Earth's. Both planets have atmospheres and whitish clouds. But at that point, the resemblance ends.

If you view Venus through a backyard telescope, you can see the disk of Venus go through phases, but you won’t see any surface details. The entire surface is hidden by nearly featureless clouds. For many years, astronomers assumed that these were clouds of water vapor, like the Earth's clouds. Science fiction writers pictured the Venusian surface as a swamp with perpetual rain, or as a rainforest with giant tropical plants and dinosaurs. In 1932, data replaced speculation. Astronomers from the Mt. Wilson observatory in California were able to identify features in the spectrum of Venus’ atmosphere. Surprisingly, the gas was not water vapor, oxygen, or any of the other gases that are important in the Earth's atmosphere, but carbon dioxide. Venus' atmosphere is about 96% CO2 by volume, with nitrogen the largest trace constituent.

In the 1960s, scientists learned another surprising fact about Venus when they first detected its thermal infrared radiation. They applied Wein’s law to calculate the temperature of Venus. The temperature was not slightly warmer than Earth, as you might expect for a cloudy planet72% as far from the Sun as Earth, but a hellish 750 K (891 °F) - even hotter than Mercury! Moreover, because the atmosphere is so insulating, the temperature is about the same on the night side as on the daytime side. Soon after this discovery, astronomers used Earth-based telescopes for spectroscopic studies of the clouds, and they found that the clouds are not composed of water droplets. They are made primarily of sulfuric acid (H2SO4) droplets! With its acid rain and blazing temperatures, Venus differs markedly from the tropical paradise envisioned by science fiction writers. The planet’s eponym is the ultimate irony: Venus is the Roman goddess of love and beauty.

Astronomer Carl Sagan made his scientific name by correctly deducing that the dense CO2 of Venus' atmosphere might create a very strong greenhouse effect - much stronger than the one we worry about on Earth. This work was one of the earliest and most noteworthy of Sagan's scientific contributions. He became known to many people through his Cosmos book and TV programs, and through his work as a spokesperson and popularizer of science. It’s a rare and happy occurrence when a scientist has the enthusiasm and ability to convey his or her subject to a wider audience.

The cloud-covered surface of Venus was a mystery until the 1970s, when the former Soviet Union sent a series of robotic probes to the surface of the planet. In 1967, the Soviet Venera 4 ("Venus 4") probe became the first human artifact to reach another planet. Unfortunately, it crashed on the surface without returning data. Three years later, Venera 7 became the first human artifact to land successfully on another planet, transmitting data for 23 minutes from Venus' surface. This was six years before the initial Mars landing, making Venus the first planet to be successfully reconnoitered by human devices. The Venus lander confirmed high temperatures previously measured remotely, and showed that the atmospheric pressure is an incredible 90 times as great as the air pressure at sea level on Earth. This is equivalent to the pressure endured by a diver nearly a kilometer (3,300 feet) below the terrestrial ocean surface!
 
From the mid-1970s to the 1980s, Soviet scientists landed a number of additional Venus probes that took photos and measured soil compositions. The photos showed a barren, lifeless planet covered with rocks and gravel, similar to barren volcanic terrain on Earth. Out of all the Soviet probes and landers, only four returned images. None of them could survive very long in Venus’s harsh environment.
 
Starting in the late 1970s, American and Soviet probes began a new kind of study: radar mapping of Venus. Space probes in orbit around Venus bounced radar signals down through the clouds and off the surface. Astronomers then used the returned waves to construct radar images of the entire planet. The radar measured altitudes all over Venus, revealing that most of the planet is covered by low, rolling plains, with only about a kilometer of relief in these areas. These plains may be analogous to the low elevation, basaltic sea-floor crust of Earth. The other 40% of Venus is covered by highlands, including a few Australia-sized, continent-like plateaus standing a few kilometers above the surface. The largest of these are Ishtar Terra and Aphrodite Terra, named after the Babylonian and Greek goddesses of love and beauty. These highlands contain a few huge volcanic peaks like Maxwell Montes (the only Venusian feature named after a man). Maxwell towers 10.6 kilometers (35,000 feet) above the average elevation - higher than our Mount Everest rises above sea level.
 
Radar studies of the planet revealed another strange feature: not only does Venus have an extremely long rotation period - one Venus “day” is equivalent to about 243 Earth days - but it also has retrograde motion. Venus rotates in the opposite direction of the rest of the planets! 
 
Venus' surface conditions would make it extremely difficult for humans to land there in the near future. Because the atmosphere is so thick and insulating, the entire planet stays at the same high temperature and never cools off. So any space suit, for example, would not only have to be rigid enough to withstand the crushing pressure of the atmosphere, but would also have to withstand temperatures of 900 °F. The forbidding carbon dioxide atmosphere and clouds of sulfuric acid further complicate exploration.
Author: Chris Impey
Editor/Contributor: Ingrid Daubar-Spitale
Last modified: Monday, August 30, 2021, 10:23 AM