How do Venus and Earth differ in atmosphere composition?

Venus and Earth are often called “sister planets” because they’re similar in size and structure, but their atmospheres couldn’t be more different. Earth’s atmosphere is a comfortable mix of 78% nitrogen, 21% oxygen, and small amounts of carbon dioxide, water vapor, and other gases—perfect for life. Venus, on the other hand, is wrapped in a thick blanket of carbon dioxide (about 96.5%) with clouds of sulfuric acid. 

This dense atmosphere traps heat like an oven door shut tight. The pressure on Venus’s surface is about 90 times that of Earth, equal to being nearly a kilometer underwater here. Unlike Earth’s sky-blue look, Venus’s clouds reflect sunlight so intensely that the planet appears bright and yellowish-white. These differences explain why Earth supports life while Venus remains a toxic, scorching world. In short: Earth’s atmosphere breathes life; Venus’s suffocates it.