As a result its surface is marked by isolated regions of intense volcanic uplift. In fact, Venus is probably as active geologically as the Earth. Mercury, Venus, and Mars have very different atmospheres. Mercury was too small to either make or retain an atmosphere and is now essentially airless. Venus and Mars both have carbon dioxide atmospheres, but Venus is about 100 times denser than Earth s, while Mars is about 100 times less dense.
Astronomers can nevertheless study the Venusian surface with radar telescopes.
Mars atmosphere is too thin now for a strong greenhouse effect, but the old river channels, cut presumably by running water, imply that Mars once had a denser and warmer atmosphere, perhaps even resembling Earth s. The great differences between the atmospheres of Venus, Earth, and Mars can probably be explained by the differences in their water content and the fact that life evolved on Earth. The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use and Privacy Policy .