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!
Soviet Venera 4 probe. Click here for original source URL
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, where 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! This anomaly begs for an explanation. All the other planets rotate in the same way, which is also the way the Sun rotates. In the current explanation for how the Solar System formed, tthis rotation reflects the the slight rotation of the large cloud of gas and dust that formed the Sun and planets. In science, explaining exceptions is tricky; how do you know when the exception is just happenstance and how do you know when it means the theory is flawed. In this case, planetary scientists speculate that Venus suffered a collision with a large planetesimal early in the history of the Solar System, a collision violent enough to knock Venus "on its head." Notice that this is similar to the explanation for the Moon being an unusually large satellite for a planet to have. In the case of the Moon impact hypothesis there’s evidence to back it up. In the case of the Venus impact hypothesis there’s no direct evidence so it remains speculation.
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. Technology has improved since the 1970s and a lander could be made to survive the Venusian surface, at least for a few weeks. NASA has a mission concept to visit the surface, but it has not yet been funded. The goddess of love still beckons.