NASA's Cosmos

8. Mars: the red planet

Like the Earth, the planet Mars has an atmosphere, white clouds, polar caps and seasons.

Mars has a partially liquid core, probably containing molten iron and perhaps surrounding a solid iron core, as within the Earth.

Mars does not now have a global, dipolar magnetic field to deflect lethal cosmic rays and energetic solar particles.

The oldest terrain on Mars exhibits bands of magnetized material with alternating polarity, most likely originating about 4 billion years ago when the red planet might have had a global, dipolar magnetic field.

In the early 20^th century it was thought that seasonal water melting from the polar caps in spring and summer produced a dark band of vegetation on Mars, and that intelligent Martian inhabitants had constructed canals to transport water across the planet.

The seasonal dark regions on Mars are now attributed to winds, and the canals are now known to be an illusion caused when the eye arranges small-disconnected features into lines.

Mars now has a thin, cold and dry atmosphere that is composed almost entirely of carbon dioxide, with a surface pressure of about one hundred fifthtieth of the Earth’s atmosphere and a mean surface temperature that is well below the freezing temperature of water.

Because of the low surface pressure and temperature of the Martian atmosphere, it cannot now rain on Mars. If any liquid water were now released on the planet’s surface, it would survive for just a brief time before freezing or evaporating.

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.

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.

Seasonal polar caps of frozen carbon dioxide, or dry ice, wax and wane with the seasons on Mars. They lie on top of extensive caps of frozen water in both hemispheres. In the north, a residual or permanent cap of water ice remains in the summer heat, but carbon dioxide ice also remains in the southern winter.

Layers in the polar caps of Mars suggest climate changes on time scales of 10 thousand to 100 thousand years, perhaps triggered by periodic variations in the planet’s orbit and rotation axis.

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.

Towering volcanoes and immense canyons are found on Mars.

The dry tracks of past flowing water are etched into the surface of Mars, marking the site of ancient rivers and floods that occurred 3 to 4 billion years ago. Water networks are found in the heavily cratered southern highlands, and outflow channels are located in the equatorial regions running downhill from the highlands to the northern plains.

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

Instruments aboard orbiting spacecraft have detected water-related minerals on the surface of Mars, especially in the ancient southern highlands

Landers and rovers have obtained evidence for a watery past on Mars, but the Mars Pathfinder lander and its Sojourner Rover found that their landing sites have been untouched by water since it flowed across the region more than 2 billion years ago. The two Mars Exploration Rovers, Opportunity and Spirit, have found evidence of past water flow on Mars, but no signs of recent running water.

Huge amounts of water once flowed on the Martian surface, but exactly where all that water came from and where it all went are still uncertain. Colliding asteroids and comets could have deposited the water in the early history of the planet, or Mars might have been warmer long ago, with a thick, dense atmosphere, rain and flowing water. Most of the water that once flowed on Mars is now frozen into ice on or below the surface.

Subsurface water ice is suggested by flow-like patterns of material ejected from impact craters on Mars, and substantial amounts of frozen water have been inferred from spectrometers aboard the 2001 Mars Odyssey spacecraft.

The Phoenix lander obtained evidence for subsurface water ice and past water flow in the northern polar plains of Mars, and detected snow falling from Martian clouds of water ice.

Liquid water may have been seeping out of the walls of canyons and craters on Mars in the past few million years, creating small gullies and depositing the debris in fanlike deltas. Some of the gullies have been attributed to landslides of sand or loose dust.

The lack of a global dipolar magnetic field and a thick atmosphere with an ozone layer, which would respectively divert cosmic rays or solar energetic particles and absorb ultraviolet rays, suggest that Mars’ surface is now a sterile place where life cannot survive. If life did once exist there, it might have survived beneath the surface, within rocks or deep underground in the possibly wet and more temperate part of the planet’s interior.

Five spacecraft have successfully landed on the reddish-brown Martian surface and revealed no signs of life on Mars.

There are no detectable organic molecules or cells in the Martian surface examined by the Viking 1 and 2 landers. They found no unambiguous evidence for biological life at their landing sites. The chemically reactive, highly oxidized soil will destroy cells, while also rusting 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, was once thought to exhibit evidence for ancient, microscopic bacteria-like fossils, but non-biological explanations are now accepted for these features.

The search for life on Mars is now focused on the hunt for liquid water, which might indicate that Mars is habitable and could have sustained life either in the remote past or recently.

Methane has recently been found in the Martian atmosphere; it could be due to geochemical processes or to bacteria-like microorganisms.

NASA has plans to launch a Mars Science Laboratory that would analyze the surface material near its landing site on Mars for proteins, amino acids and other molecules that are essential to life as we know it.

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