NASA's Cosmos

8. Mars: the red planet

Mars now has a thin, cold and dry atmosphere that is composed almost entirely of carbon dioxide.

The low surface temperature and pressure of the Martian atmosphere are comparable to those in the Earth’s stratosphere.

The Martian atmosphere contains virtually no oxygen, so it has no ozone layer. The planet’s surface is therefore exposed to the full intensity of the Sun’s ultraviolet rays.

It cannot now rain on Mars, and liquid water is now unstable on its surface. If any liquid water was now released on the Martian surface, it would survive for just a brief time before freezing or evaporating.

Powerful and pervasive winds roar across Mars, sweeping up vast dunes of sand and fine-grained dust, creating tornado-like dust devils, and occasionally producing global dust storms that hide the entire planet from view.

Polar caps of frozen carbon dioxide, or dry ice, wax and wane with the seasons at both the north and south poles of Mars. Because winter in the southern hemisphere is colder and longer than winter in the northern hemisphere, the southern winter cap is the largest.

The northern residual polar cap, which remains in the summer heat, is composed of water ice, but the residual cap in the south consists of frozen carbon dioxide, or dry ice. If melted, the water in the northern cap would amount to much less than a planetary ocean.

Mars is divided into two strikingly different hemispheres; in the south there are the older, elevated, heavily cratered highlands that resemble the lunar highlands. In the north there are the younger, lower-lying, smoother volcanic plains.

The oldest terrain on Mars exhibits bands of magnetized material with alternating directions. They might have originated long ago when the red planet might have had a global magnetic field.

Mars does not currently have a global magnetic field to deflect lethal energetic cosmic and solar particles, and it does not have a thick atmosphere with an ozone layer to absorb solar ultraviolet radiation. As a result, the surface of Mars is sterile and life cannot survive there unless protected in some other way.

Volcanoes have persisted on Mars for at least 2 billion years, and the planet may still be volcanically active.

The dry tracks of past flowing water are etched into the surface of Mars, marking the site of ancient rivers and flash floods that occurred billions of years ago.

Water might have lapped the shores of long-vanished lakes and seas on Mars.

Huge amounts of water once flowed on the Martian surface, but exactly where all that water came from and what happened to it are still uncertain.

Though cold and dry today, Mars might have been wetter and warmer long ago, with a thick, dense atmosphere.

The Martian climate may go through huge swings triggered by periodic variations in its orbit and spin axis.

Liquid water may have been seeping out of the walls of canyons and craters on Mars in recent times, creating small gullies and depositing the debris in fanlike deltas.

Three spacecraft have landed on the reddish-brown surface of Mars. The Viking 1 and 2 landers failed to detect any unambiguous evidence for life on Mars. The Mars Pathfinder lander and its Sojourner Rover found that the landing site has been untouched by water since it flowed across the region more then two billion years ago.

There are no detectable organic molecules in the Martian surface examined by the Viking 1 and 2 landers, which means that the surface now contains no cells, living, dormant or dead. The highly oxidized material in the soil has also rusted the Martian surface red.

Cosmic impacts with Mars are capable of ejecting surface rocks into space, and some of them eventually arrive at the Earth. One such meteorite from Mars, named ALH 84001, exhibits several signs that bacteria-like microorganisms could have existed on the red planet billions of years ago. The evidence includes carbonates, organic molecules, mineral features, and microscopic fossils. Nevertheless, most scientists now think that there is nothing in the meteorite that conclusively indicates whether life once existed on Mars or exists there now.

If life did once exist on the surface of Mars, it might have survived when things got tough, not on, but beneath the surface, within rocks or deep underground in the wet and more temperate part of the planet’s hot interior. In support of this conjecture, we know that terrestrial microbes live in complete darkness within rocks, inside the Earth’s crust or at the bottom of its oceans, energized by the Earth’s internal heat.

The future search for life on Mars may include fossil or extant evidence of tough and tenacious, one-celled microbes that can survive in hostile environments, perhaps energized from the planet’s hot interior by chemical processes that may involve inorganic compounds.

Mars has two small moons, named Phobos and Deimos. Phobos is heading towards eventual collision with Mars.