Mars

Until very recently it appeared that Percival Lowell was wrong about the existence of life on Mars. However, in August 1996, a team of scientists at the National Aeronautics and Space Administration's (NASA) Johnson Space Center and at Stanford University announced the discovery of evidence that strongly suggests primitive life may have existed on Mars over 3.6 billion years ago. This evidence is contained in a 4.2–lb, potato-sized meteorite discovered in Antarctica in 1984, named ALH84001. This meteorite is one of twelve found on Earth to date that match the unique Martian chemistry measured by the Viking spacecraft when they landed on Mars in 1976. The meteorite contains detectable amounts of polycyclic aromatic hydrocarbons (PAHs), the first organic molecules thought to be of Martian origin; several mineral features (i.e. carbonates) characteristic of biological activity; and possible microscopic fossils of bacteria-like organisms.

Mars is now a cold, dry, almost airless world, but between 3.6 and 4 billion years ago water flowed across the Martian landscape. The planet had a thicker atmosphere and was also much warmer than it is today. The rock that eventually fell to Earth as a meteorite was located underneath the Martian surface and fractures in the rock were penetrated by water and carbon dioxide from the planet's atmosphere. (The rock is debris from an asteroid that collided with Mars millions of years ago, scattered into space before eventually falling on Earth about 13,000 years ago.) Carbonate minerals were deposited in the meteorite's fractures.

Scientists studying the meteorite initially argued that living organisms may have assisted in the formation of the carbonate, organisms that were eventually fossilized much like fossils are formed in limestone rock on Earth. The largest of these possible fossils are less that 0.01 the diameter of a human hair. In appearance and size, these structures are quite similar to microscopic fossils of the tiniest bacteria found on Earth. The presence of PAHs in the meteorite provides further evidence that life may have existed on Mars because PAHs are frequently formed by the degradation of the complex organic molecules contained in microorganisms after the microorganisms die.

However, debate over the interpretation of the tiny structures in ALH84001 has been active since the initial announcement. No similar markings have appeared in other meteorites known to be from Mars. Two independent chemical studies in 1998 gave evidence that at least some signs in the rock are the of contamination from Earth. One study looked for amino acids, the building blocks of proteins that play an essential role in biology, and were detected in only small amounts that appeared to be terrestrial in origin. Another found no sign of organic matter in the tiny globs of carbonate on the meteorite, but did not look for PAHs. Given the intense interest in finding life outside the Earth, together with the rigorous scientific demands of any such remarkable claim, the controversy is sure to continue for years. As of 2003, most planetary scientists argue that the evidence is, at best, inconclusive with regard to supporting evidence of biological processes.

In the last half of the 1990s, NASA sent several probes to Mars that were unlike anything seen before. The Mars Pathfinder mission landed on the red planet on July 4, 1997, on the rocky flood plain Ares Vallis. After landing the Pathfinder craft unfurled and a 23-pound, six-wheeled remote roving vehicle, named Sojourner, crawled off the platform and onto the planet's surface, while Pathfinder itself raised a camera arm to a height of five feet.

Pathfinder returned over 10,000 color pictures from Mars, painting a picture of the surface as one that over a billion years ago had once been scoured by huge floods of liquid waters, with salty residues left from puddles that once slowly evaporated. At the same time, Pathfinder took pictures of Sojourner roaming about the planet's surface, sometimes nestling against rocks for analysis. Sojourner was equipped to chemically analyze the rocks it encountered—the first two of which were nicknamed Barnacle The largest volcano on Mars, Olympus Mons, is much larger than the largest volcano on Earth. Olympus Mons is over 15.5 miles (25 km) tall, three times as tall as Mt. Everest, and has a base the size of the state of Arizona. This photo shows how large Olympus Mons is compared to Arizona. U.S. National Aeronautics and Space Administration (NASA). Bill and Yogi—with an alpha proton x-ray spectrometer that bounces particles or x rays off rocks and analyzes what returns. Barnacle Bill was quartz-like, indicating it had been heated and reheated somewhere in the planet's crust. The more-primitive Yogi was most likely of volcanic origin. Pathfinder also found wild fluctuations of temperatures—as much as 20 degrees up or down in a few seconds—and evidence of towering dust devils up to a half-mile high winding across the desert plain. And it found evidence that, like Earth, Mars is not merely a solid rock ball, but has a crust, a mantle, and an iron core.

Later in 1997 the Mars Global Surveyor went into orbit around Mars and began a mapping survey of the planet. It found that Mars had a weak magnetic field, about 1/800 that of Earth, but one stronger than scientists had expected. (By comparison, Jupiter's magnetic field is 10,000 times that of Mars.) This field is important in the geological history of Mars, and helps determine the nature of its rock below the surface. At some points in the orbit, the spacecraft was able to descend to between 105 and 75 mi (170-120 km) in altitude, beneath the ionosphere, low enough to detect remnant magnetic fields of material on the surface. These results suggested that Mars once had a magnetic field comparable to that on Earth, which would have protected the surface from the cosmic rays and energetic particles from the Sun. The Global Surveyor also found evidence of hematite, an iron-bearing mineral that forms only in high-temperature aqueous systems—compelling evidence for hydrothermal vents on Mars. Magnetic stripes discovered on the surface in 1999 hint that early-on the surface of Mars may have been formed by tectonic plates, much like that on Earth.

There is intensive interest in Mars as a site for eventual human exploration. Several robotic missions are planned for the near future, and public debate concerning manned space exploration as well as long-term goals for NASA has renewed in the wake of the tragic loss of the space shuttle Columbia in February 2003.