The Earth and Venus do often get called planetary twins, and this is largely because they are very close to being the same mass. Both the Earth and Venus are rocky planets, which means that they're effectively the same density (which can not be said of the Earth and, say, Neptune), and so they are also very nearly the same physical size. They also both have significant atmospheres surrounding their surfaces. However, their evolutionary pathways since the time of the early solar system have taken both planets down dramatically different tracks, in spite of all their similarities.
The Earth clocks in at a very respectable 5.97 x 10^24 kilograms of mass. Venus sits at 81.5% of this, at 4.867 × 10^24 kg. Venus' radius is only a few hundred kilometers smaller than the Earth's - ~6050 km instead of ~6370, a difference of only about 5%. If we were only looking at their size, rock type, and mass, you might expect that Venus and Earth should be pretty similar, and this is precisely where the “twin” styling comes from.
It doesn’t take much further examination to reveal that Venus is a very different place to our home, and much stranger than we originally thought. Unlike most of the other planets in the solar system, which rotate counterclockwise (as viewed looking down from the North Pole), Venus rotates clockwise. (You could equally fairly say that Venus is simply upside down.) It also does so extremely slowly -- Venus is the slowest rotator in the entire solar system. Each ‘day’ lasts for approximately 117 Earth days - in other words, the Earth will have spun 117 times in the time it took Venus to rotate once around its own axis. Considering that Venus’ year is shorter than the Earth’s (it is closer to the Sun, after all), Venus’ year takes about two Venusian days.
Venus is shrouded with a thick, dense atmosphere, far thicker than our own. On Earth, our atmosphere is thick enough to produce a significant amount of pressure on the surface, but our planet is not totally cloud-covered. Our Earth-monitoring satellites are regularly able to see the ground from space, without the interference of the clouds. There’s no such break in the clouds on Venus. Venus is permanently clouded over, and its atmosphere is so thick that the surface pressure on Venus is 92 times the pressure here on Earth. An unshielded human would fare very badly in this environment.
The atmosphere of Venus is a Level 99 Expert at trapping heat. It’s almost entirely carbon dioxide, which, as we know from our own planet, is a greenhouse gas. With such a high concentration of heat-trapping gas in the atmosphere, the surface temperature of Venus sits at a temperature hotter than 850 degrees Fahrenheit.
The crushing pressure and searing heat makes Venus a particularly difficult place to explore. The electronics in most robotic missions generally do not handle these kinds of temperatures very well; to function at all, they need to be reinforced against the pressure. This is the space equivalent of sending your delicate machinery to about 900 meters below the ocean, but where the ocean is 50% of the way to being as hot as lava. We’ve sent relatively few landers to Venus, and the most recent of them (Vega 2) dates back to 1985, operating for just under an hour before succumbing to the heat.
There is another similarity between the Earth and Venus, though not one that makes Venus a more hospitable place to go visit: both planets have volcanoes. Because Venus is so hot and the pressures are so great, the volcanoes on Venus’ surface aren’t quite as vertically imposing as they can be on Earth. These Venusian volcanoes are much larger, in that they cover a much greater area, but they're extremely flat. Sapas Mons, in the image above, covers 250 miles from edge to edge, but only rises to 4752 feet. (The image above has had its vertical scale exaggerated by a factor of twenty two). This volcano covers roughly the same area as the entire state of New York. Venusian volcanoes dwarf most of their Earthly counterparts in area. I'm biased, but I think you'll agree that Earth has it grandly outdone for scenery.
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