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The twin sister that went astray
Venus has a radius only 5% smaller than Earth's and 82% of its mass. If one were to look only at those two figures, one would assume the two planets are identical. In reality, it is quite the opposite.
The atmosphere of Venus consists of 96% CO₂, so thick that the surface pressure is equivalent to diving 900 meters deep into Earth's oceans. A layer of sulfuric acid clouds reflects 75% of incoming light, making Venus the third brightest object in the sky after the Sun and the Moon.
Runaway greenhouse effect
Light penetrates the clouds, heating the surface, but the emitted infrared radiation is trapped by the CO₂. As the temperature rises, water evaporates, and water vapor is itself a potent greenhouse gas—this self-amplifying loop has pushed temperatures to 464°C, hot enough to melt lead, and uniform from the equator to the poles.
Backward and ultra-slow rotation
Venus rotates in the opposite direction compared to most other planets, and so slowly that one full rotation relative to the background stars takes 243 Earth days, longer than its year (225 days). But that is the sidereal rotation period, not the length of a day: because Venus rotates in the opposite direction to its orbit around the Sun, the solar day there—from one sunrise to the next—is only about 117 Earth days, which is shorter than a year. The cause is still under debate, between the giant impact hypothesis and the atmospheric tide hypothesis.
Why it is still worth studying
At an altitude of 50–60 km, the temperature and pressure in the Venusian atmosphere are close to Earth's surface conditions. Upcoming missions such as NASA's DAVINCI and VERITAS will survey this very region along with the planet's water history.
Further reading
References
- [1]Planetary Fact Sheet — NASA NSSDC (2024)
- [2]Venus Facts — NASA Science (2024)
Image: NASA or Ricardo Nunes — Wikimedia Commons, Public domain.
