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MARS FAST FACTS

Distance from Sun

Maximum 249 million km

Minimum 206 million km

Distance from Earth

Maximum 399 million km

Minimum 56 million km

Rotational Period 24.6 hours

Equatorial Diameter 6,786 km

Equatorial Inclination to Ecliptic 25.2º

Gravity 0.38 of Earth’s

Atmosphere

Main Component Carbon Dioxide

Pressure at Surface -8 millibars (vs 1,000 on Earth)

Temperature Range 143ºC to +17ºC

Moons (2) Phobos (Fear), 21 km diameter

Deimos (Panic), 12 km diameter

Rings None

Orbital Eccentricity 0.093

Orbital Inclination to Ecliptic 1.85º

Magnetic Field Density To be determined.

Very weak, if any.

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Of all the planets, Mars has long been considered the solar system's prime candidate for harboring extraterrestrial life. Astronomers studying the red planet through telescopes saw what appeared to be straight lines crisscrossing its surface. These observations later determined to be optical illusions led to the popular notion that intelligent beings had constructed a system of irrigation canals on the planet. In 1938, when Orson Welles broadcast a radio drama based on the science fiction classic War of the Worlds by H.G. Wells, enough people believed in the tale of invading martians to cause a near panic.

Another reason for scientists to expect life on Mars had to do with the apparent seasonal color changes on the planet's surface. This phenomenon led to speculation that conditions might support a bloom of martian vegetation during the warmer months and cause plant life to become dormant during colder periods.

So far, six American missions to Mars have been carried out. Four Mariner spacecraft, three flying by the planet and one placed into martian orbit, surveyed the planet extensively before the Viking Orbiters and Landers arrived. Mariner 4, launched in late 1964, flew past Mars on July 14, 1965, within 9,846 kilometers (6,118 miles) of the surface. Transmitting to Earth 22 close-up pictures of the planet, the spacecraft found many craters and naturally occurring channels but no evidence of artificial canals or flowing water. Mariners 6 and 7 followed with their flybys during the summer of 1969 and returned 201 pictures. Mariners 4, 6 and 7 showed a diversity of surface conditions as well as a thin, cold, dry atmosphere of carbon dioxide.

On May 30, 1971, the Mariner 9 Orbiter was launched on a mission to make a year-long study of the martian surface. The spacecraft arrived five and a half months after liftoff, only to find Mars in the midst of a planet-wide dust storm that made surface photography impossible for several weeks. But after the storm cleared, Mariner 9 began returning the first of 7,329 pictures; these revealed previously unknown martian features, including evidence that large amounts of water once flowed across the surface, etching river valleys and flood plains.

In August and September 1975, the Viking 1 and 2 spacecraft, each consisting of an orbiter and a lander, lifted off from Kennedy Space Center. The mission was designed to answer several questions about the red planet, including, is there life there? Nobody expected the spacecraft to spot martian cities, but it was hoped that the biology experiments on the Viking Landers would at least find evidence of primitive life - past or present.

Viking Lander 1 became the first spacecraft to successfully touch down on another planet when it landed on July 20, 1976, while the United States was celebrating its Bicentennial. Photographs sent back from Chryse Planitia ("Plains of Gold") showed a bleak, rusty-red landscape. Panoramic images returned by Viking Lander 1 revealed a rolling plain, littered with rocks and marked by rippled sand dunes. Fine red dust from the martian soil gives the sky a salmon hue. When Viking Lander 2 touched down on Utopia Planitia on September 3, 1976, it viewed a more rolling landscape than the one seen by its predecessor, one without visible dunes.

The results sent back by the laboratory on each Viking Lander were inconclusive. Small samples of the red martian soil were tested in three different experiments designed to detect biological processes. While some of the test results seemed to indicate biological activity, later analysis confirmed that this activity was inorganic in nature and related to the planet's soil chemistry. Is there life on Mars? No one knows for sure, but the Viking mission found no evidence that organic molecules exist there.

The Viking Landers became weather stations, recording wind velocity and direction as well as atmospheric temperature and pressure. Few weather changes were observed. The highest temperature recorded by either spacecraft was -14 degrees Celsius (7 degrees Fahrenheit) at the Viking Lander 1 site in midsummer.

The lowest temperature, -120 degrees Celsius (-184 degrees Fahrenheit), was recorded at the more northerly Viking Lander 2 site during winter. Near-hurricane wind speeds were measured at the two martian weather stations during global dust storms, but because the atmosphere is so thin, wind force is minimal. Viking Lander 2 photographed light patches of frost, probably water-ice, during its second winter on the planet.

The martian atmosphere, like that of Venus, is primarily carbon dioxide. Nitrogen and oxygen are present only in small percentages. Martian air contains only about 1/1,000 as much water as our air, but even this small amount can condense out, forming clouds that ride high in the atmosphere or swirl around the slopes of towering volcanoes. Local patches of early morning fog can form in valleys.

There is evidence that in the past a denser martian atmosphere may have allowed water to flow on the planet. Physical features closely resembling shorelines, gorges, riverbeds and islands suggest that great rivers once marked the planet.

Mars has two moons, Phobos and Deimos. They are small and irregularly shaped and possess ancient, cratered surfaces. It is possible the moons were originally asteroids that ventured too close to Mars and were captured by its gravity.

The Viking Orbiters and Landers exceeded by large margins their design lifetimes of 120 and 90 days, respectively. The first to fail was Viking Orbiter 2, which stopped operating on July 24, 1978, when a leak depleted its attitude-control gas. Viking Lander 2 operated until April 12, 1980, when it was shut down due to battery degeneration. Viking Orbiter 1 quit on August 7, 1980, when the last of its attitude-control gas was used up. Viking Lander 1 ceased functioning on November 13, 1983. Despite the inconclusive results of the Viking biology experiments, we know more about Mars than any other planet except Earth.