Size comparison of Earth and Mars. Click here for original source URL.

Global view of Mars as seen by the Viking 1 orbiter in 1980, showing the Valles Marineris (center). Click here for original source URL.

In many of its physical properties, Mars is similar to the Earth. Our neighboring planet is only about half the size of ours, but it has many Earth-like features, such as clouds, sediment layers, sand dunes, and volcanoes. Mars also has polar ice caps that grow and recede with the changing of the seasons. The tilt of the axis of Mars is 25°, just slightly more than the 23.5° of Earth, so that Mars also has seasons like Earth's. Martian days are almost the same length as the terrestrial day. The surface of Mars is scored by both wind and water erosion - evidence that liquid water flowed on its surface in the past.  Mars’s uniquely wet past makes it a fascinating comparison with Earth, the only planet to currently have liquid water on its surface.

The similarities to Earth, and hints of a drastically different climate in Martian history, have made scientists wonder whether Mars harbors life, or perhaps did so in the past. Such questions have excited the imaginations of scientists and philosophers, as well as science fiction writers. These unanswered questions continue to inspire anyone with an interest in the "red planet."

Author: Chris Impey
Editor/Contributor: Ingrid Daubar-Spitale