Atmosphere of Venus - Structure and Composition - Troposphere

Troposphere

The atmosphere is divided into a number of sections depending on altitude. The densest part of the atmosphere, the troposphere, begins at the surface and extends upwards to 65 km. At the furnace-like surface the winds are slow, but at the top of the troposphere the temperature and pressure reaches Earth-like levels and clouds pick up speed to 100 m/s.

The atmospheric pressure at the surface of Venus is about 92 times that of the Earth, similar to the pressure found 910 metres below the surface of the ocean. The atmosphere has a mass of 4.8×1020 kg, about 93 times the mass of the Earth's total atmosphere. The density of the air at the surface is 67 kg/m3, which is 6.5% that of liquid water on Earth. The pressure found on Venus's surface is high enough that the supercritical carbon dioxide is technically no longer a gas, but a supercritical fluid. This supercritical carbon dioxide forms a kind of sea that cover the entire surface of Venus. This sea of supercritical carbon dioxide transfer heat very efficiently, buffering the temperature changes between night and day (which last 56 terrestrial days).

The large amount of CO₂ in the atmosphere together with water vapour and sulfur dioxide create a strong greenhouse effect, trapping solar energy and raising the surface temperature to around 740 K (467°C), hotter than any other planet in the solar system, even that of Mercury despite being located farther out from the Sun and receiving only 25% of the solar energy Mercury does.K The average temperature on the surface is above the melting points of lead 600 K (327°C), tin 505 K (232°C), and zinc 693 K (420°C). The thick troposphere also makes the difference in temperature between the day and night side small, even though the slow retrograde rotation of the planet causes a single solar day to last 116.5 days on Earth. The surface of Venus spends 58.3 days of darkness before the sun rises again behind the clouds.

The troposphere on Venus contains 99% of the atmosphere by mass. Ninety percent of the atmosphere of Venus is within 28 km of the surface; by comparison, 90% of the atmosphere of Earth is within 10 km of the surface. At a height of 50 km the atmospheric pressure is approximately equal to that at the surface of Earth. On the night side of Venus clouds can still be found at 80 km above the surface.

The area of the troposphere most similar to Earth is near the tropopause—the boundary between troposphere and mesosphere. It is located slightly above 50 km. According to measurements by the Magellan and Venus Express probes, the area from 52.5 to 54 km has a temperature between 293 K (20 °C) and 310 K (37°C), and the area at 49.5 km above the surface is where the pressure becomes the same as Earth at sea level. As manned ships sent to Venus would be able to compensate for differences in temperature to a certain extent, anywhere from about 50 to 54 km or so above the surface would be the easiest area in which to base an exploration or colony, where the temperature would be in the crucial "liquid water" range of 273 K (0°C) to 323 K (50°C) and the air pressure the same as habitable regions of Earth. As CO₂ is heavier than air, the colony's air (nitrogen and oxygen) could keep the structure floating at that altitude like a dirigible.