Introduction

Mars, the fourth planet out from the sun in Earth's solar system, is about half the size of Earth and has a rotation period just slightly longer than one Earth day. Since it takes Mars 687 Earth days to orbit the sun, its seasons are about twice as long as ours. Mars has two polar caps. The northern one is larger and colder than the southern. Two small moons, Phobos and Deimos, orbit the planet.

Who discovered the polar caps on Mars?

Gian Domenico Cassini (1625-1712) made detailed observations of Mars, the only planet whose surface can be seen clearly from Earth. He discovered that Mars has polar caps that spread during the Martian winter and shrink in the summer. These seasonal variations caused him to consider the possibility of life on Mars.

What are the so-called canals seen on Mars?

Mars is marked by what appear to be dried riverbeds and flash-flood channels. These features could mean that ice below the surface melts and is brought above ground by occasional volcanic activity. The water may temporarily flood the landscape before boiling away in the low atmosphere pressure. Another theory is that these eroded areas could be left over from a warmer, wetter period in Martian history.

What are conditions like on Mars?

Spacecraft sent to Mars revealed a barren, desolate, crater-covered world prone to frequent, violent dust storms. They found little oxygen, no liquid water, and ultraviolet radiation at levels that would kill any known life form. The high temperature on Mars was measured at -20 degrees Fahrenheit (-29 degrees Celsius) in the afternoon, and the low was -120 degrees Fahrenheit (-84 degrees Celsius) at night.

What's notable about the topography of Mars?

The two most distinguishing features of the northern hemisphere of Mars are a 15-mile-high (24-kilometers-high) volcano called Olympus Mons, larger than any other in the solar system, and a 2,000-mile-long (3,220-kilometers-long) canyon called Valle Marineris, twenty-six times as long and three times as deep as the Grand Canyon. The southern hemisphere is noteworthy for Hellas, an ancient canyon that was long ago filled with lava and is now a large, light area covered with dust.

How has our understanding of the planet Mars changed over time?

Mars was named for the Roman god of war. It was long believed to hold life, perhaps even intelligent, human-like life. Early astronomers peered through telescopes and saw dark seas on the planet, connected by lines. Some imaginative scientists theorized that the dark spots were seas and that the lines were channels dug by Marthian engineers to bring water to populated areas. A series of space probes launched by the United States and the former Soviet Union in the 1960s and 1970s, however, put an end to such speculation.