Earth and Venus are almost twins in size, mass and gravity. Earth's highest mountains are capped with frozen water; Venus's may be coated in metallic frost containing lead and bismuth sulfides. Somehow, two remarkably similar planets took radically different paths—and scientists still don't know exactly when or why.

Earth and Venus are almost twins in size, mass and gravity. Earth's highest mountains are capped with frozen water; Venus's may be coated in metallic frost containing lead and bismuth sulfides. Somehow, two remarkably similar planets took radically different paths—and scientists still don't know exactly when or why.

Put Earth and Venus side by side and the family resemblance is unusually close. Venus is about 95 percent of Earth’s diameter, 81.5 percent of its mass, and has roughly 90 percent of Earth’s surface gravity. A person who weighed 75 kilograms on Earth would press on the ground with about the same force as a 68-kilogram person on Venus. Everything else has gone in another direction. Earth’s high mountains carry snowfields and glaciers. Radar obser…