Venus: Earth's Blazing-hot and Bizarre Twin

The two worlds are quite similar in size. Earth has a surface area of around 197 million square miles (510 million square kilometers). By comparison, Venus' surface area is closer to 177 million square miles (460 million square kilometers). And if you were to stuff Venus inside our planet like a Matryoshka doll, it would occupy roughly 86 percent of Earth's total volume.

Venus has Earth beaten in some key regards, though. Earth displays a slight midsection bulge, being wider around its equator than it is from one pole to the other. Conversely, Venus is an almost perfect sphere.

When a massive celestial body (like a star or planet) spins quickly around its axis, centrifugal force will give it a more dramatic bulge around the equator. However, Venus has an ultra-slow rotation speed.

It takes the equivalent of 243 Earth days for Venus to complete one full rotation around its axis — and just 225 Earth days to finish a new lap around the sun. So in other words, a day on Venus lasts longer than a Venusian year does.

And get this: From our self-centered perspective, Venus rotates backward. Most of the planets in this solar system rotate from west to east. The Uranus and Venus orbits buck the trend. On those two worlds, the sun appears to rise in the west and set in the east.

Nobody knows how that came to pass. Astronomers think the Venus orbit used to move in a counterclockwise direction like Earth. But at some point, its spin might have reversed. Alternatively, perhaps the sun's gravitational influence — or a collision with a large object — caused the entire planet to flip upside-down.Either way, Venus rotates backward now.

In December 1962, Venus became the first planet to get a flyby visit from a man-made spacecraft. Exploiting a brief window of opportunity, NASA's Mariner 2 probe studied this world up close, from distances as near as 21,606 miles (34,773 kilometers).

The onboard instruments taught us a great deal. Mariner 2 confirmed that Venus doesn't have an Earth-like magnetic field, and it recorded surface temperatures of 300 to 400 degrees Fahrenheit (149 to 204 degrees Celsius).

By the time Mariner 2 launched, scientists already knew there were high levels of CO₂ in the Venusian atmosphere. And that composition should give us pause.

Carbon dioxide makes up a whopping 96 percent of the Venus' thick atmosphere. Scientists attribute this to a runaway greenhouse effect. Theoretically, the planet used to have a more temperate climate that could've remained stable for billions of years. Back then, oceans of liquid water may have covered its surface (although we don't know for sure).

Things changed as our growing sun became hotter. Any oceans would've evaporated during this time — and astronomers think much of the carbon in Venusian rocks leached out and traveled skyward. While Venus' atmosphere changed, it got better at trapping heat, creating a vicious cycle that worsened the problem. Inevitably, temperatures spiked.

Since our own planet has a major greenhouse gas problem, Venus could offer us some important insights regarding climate change. But sending probes to explore it has always presented major challenges.

On Venus, the surface gravity is comparable to what you and I experience on Earth. What's not comparable is the atmospheric pressure, which is 92 times greater on the face of Venus than it is here.

Faced with the extreme temperatures and high pressure of this second planet from the sun, it's no wonder man-made objects don't last long in the planet's environment. When the Soviet Venera 13 probe landed on Venus in 1982, it stayed intact for a record-setting 127 minutes before it was destroyed.

Mind you, this wasn't the USSR's first rodeo. Previous Venera spacecrafts had successfully visited the thick atmosphere of Venus and touched down on its outer crust. Brief though their visits were, these probes captured the first-ever photographs of the Venusian surface. NASA's Magellan spacecraft provided further insights, as it mapped 98 percent of the planet's face.

Volcanoes on Venus

All in all, Venus boasts more than 16,000 volcanoes and volcanic features — but we don't know if any of these are still active. Highland plateaus, deep canyons and meteorite impact craters have also been discovered there. Although Venus is about 4.6 billion years old, its crust is thought to be much younger, with an estimated age of just 300 to 600 million years.

Venus lacks tectonic plates as we know them on Earth. Nevertheless, some geologists think upwellings of magma occasionally recycle sections of the crust.

Somehow, knowing that Venus is a stifling hothouse doesn't diminish its allure. With every new discovery, it inspires curiosity and awe.