c. 4.5 Billion BCE

Mars

We may have to go no farther than the next planet out to find out if life exists—or ever existed—beyond Earth. Mars has seemingly always been the subject of fascination, from ancient times, when it was seen as a cosmic incarnation of the Roman god of war, to the twentieth century, when many imagined the planet to be the abode of Percival Lowell’s desperate canal builders.

Mars is a small planet, about half the diameter of Earth and only about 15 percent of its volume. For further reference, the surface area of Mars is about the same as the surface area of all of the continents on Earth. On average, the planet orbits about 50 percent farther from the Sun than we do. The thin Martian carbon dioxide atmosphere (only 1 percent as thick as Earth’s) can’t trap much heat, so the surface is very cold. Daytime temperatures near the equator rarely rise above the freezing point of water, and nights near the poles routinely drop down to the freezing point of carbon dioxide (which is 150 kelvins, or about -190°F). Today Mars is a dusty world in a deep freeze.

And yet, spacecraft images, meteorites from Mars, and other data over nearly 50 years have shown that Mars is the most Earthlike place in the solar system (besides Earth itself), and that during its first few billion years, the Red Planet may have been a much warmer and wetter world. What happened? Possibilities include gradual cooling of the planet’s core and solar wind or catastrophic impact destruction of the atmosphere. Determining how and why the planet’s climate changed so dramatically is a hot topic of research.

We’ve learned enough about the Mars of 3 or 4 billion years ago to know that parts of the surface and subsurface were habitable to life. The next 50 years of Mars exploration will be all about expanding the search for habitable environments there and finding out if any were—or still are—inhabited.

This Hubble Space Telescope photo of Mars was taken during the Red Planet’s close approach to Earth in 1999. Dusty, more oxidized areas are orange; volcanic rocks and sand are brownish-black. The north polar water ice cap is at top; wispy bluish water-ice clouds and a polar storm system provide evidence of the planet’s thin atmosphere.