In mass and density, Venus is nearly the Earth's twin. Venus' atmosphere is very thick and opaque. The atmosphere is about 95% carbon dioxide. Other components are also present: Nitrogen, water vapor, sulfuric acid

Heat is trapped near the planet's surface by a runaway Greenhouse Effect, that produces a surface temperature of nearly 470C.

The planet has been imaged from orbit using the Magellan radar mapping system. The surface was photographed by the Russian Venera spacecraft.

• Mars has a mass of about 10% that of Earth. 
• Its density is 3.9 g/cm³.  
• The peak surface temperature is nearly 20 C (but minimum is -140 C). Mars also has polar ice caps like that of Earth. 
• The atmosphere is 95% CO₂, but it is much thinner than atmospheres of Venus and Earth.