Is Earth the only planet in our solar system that could support life? The most Earth-like of the planets is Mars, one of our closest planetary neighbors. Rotating on its axis once every 24.5 hours, Mars's day is nearly the same length as Earth's. Mars also experiences seasons, due to the 28.7° tilt of its axis.

We have long been fascinated with the possibility of life on Mars. Through early telescopes, astronomers saw light and dark areas that they believed might be continents and seas. In 1877, the Italian astronomer Giovanni Schiaparelli observed what he thought were linear features on Mars's surface, calling them "canali", or channels. When this was mistranslated as canals, many people pictured large artificial structures. Someone had to have built these structures, they reasoned, so they imagined the planet populated by intelligent life—the Martians. When the quality of telescopes improved sufficiently to see Mars's surface, it was shown that these canals did not exist.

In science fiction books and movies, Mars is home to many fascinating creatures. So far, however, no evidence of any kind of Martian life has been found. Scientists continue to search for signs of life, current or in the past, on Mars.

  !   Compare these views of Earth and Mars from space. The planets' sizes are shown to scale. Click the image to see a larger version.

2. List at least two similarities and two differences between Earth and Mars.