The Planet Mars
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What is Mars?

Mars is the fourth planet from the Sun and is commonly referred to as the Red Planet. The rocks, soil and sky have a red or pink hue. The distinct red color was observed by stargazers throughout history. It was given its name by the Romans in honor of their god of war. Other civilizations have had similar names. The ancient Egyptians named the planet Her Descher meaning the red one. Before space exploration, Mars was considered the best candidate for harboring extraterrestrial life. Astronomers thought they saw straight lines crisscrossing its surface. This led to the popular belief that irrigation canals on the planet had been constructed by intelligent beings. In 1938, when Orson Welles broadcasted 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. In July of 1965, Mariner 4, transmitted 22 close-up pictures of Mars. All that was revealed was a surface containing many craters and naturally occurring channels but no evidence of artificial canals or flowing water. Finally, in July and September 1976, Viking Landers 1 and 2 touched down on the surface of Mars. The three biology experiments aboard the landers discovered unexpected and enigmatic chemical activity in the Martian soil, but provided no clear evidence for the presence of living microorganisms in the soil near the landing sites. According to mission biologists, Mars is self-sterilizing. They believe the combination of solar ultraviolet radiation that saturates the surface, the extreme dryness of the soil and the oxidizing nature of the soil chemistry prevent the formation of living organisms in the Martian soil. The question of life on Mars at some time in the distant past remains open. Other instruments found no sign of organic chemistry at either landing site, but they did provide a precise and definitive analysis of the composition of the Martian atmosphere and found previously undetected trace elements. Atmosphere The atmosphere of Mars is quite different from that of Earth. It is composed primarily of carbon dioxide with small amounts of other gases. The six most common components of the atmosphere are: Carbon Dioxide (CO2): 95.32% Nitrogen (N2): 2.7% Argon (Ar): 1.6% Oxygen (O2): 0.13% Water (H2O): 0.03% Neon (Ne): 0.00025 % 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. At the Viking Lander 2 site, a thin layer of water frost covered the ground each winter. 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. Temperature and Pressure The average recorded temperature on Mars is -63 C (-81 F) with a maximum temperature of 20 C (68 F) and a minimum of -140 C (-220 F). Barometric pressure varies at each landing site on a semiannual basis. Carbon dioxide, the major constituent of the atmosphere, freezes out to form an immense polar cap, alternately at each pole. The carbon dioxide forms a great cover of snow and then evaporates again with the coming of spring in each hemisphere. When the southern cap was largest, the mean daily pressure observed by Viking Lander 1 was as low as 6.8 millibars; at other times of the year it was as high as 9.0 millibars. The pressures at the Viking Lander 2 site were 7.3 and 10.8 millibars. In comparison, the average pressure of the Earth is 1000 millibars. The Interior of Mars The current understanding of the interior of Mars suggests that it can be modeled with a thin crust, similar to Earth's, a mantle and a core. Using four parameters, the Martian core size and mass can be determined. However, only three out of the four are known and include the total mass, size of Mars, and the moment of inertia. Mass and size was determined accurately from early missions. The moment of inertia was determined from Viking lander and Pathfinder Doppler data, by measuring the precession rate of Mars. The fourth parameter, needed to complete the interior model, will be obtained from future spacecraft missions. With the three known parameters, the model is significantly constrained. If the Martian core is dense (composed of iron) similar to Earth's or SNC meteorites thought to originate from Mars, then the minimum core radius would be about 1300 kilometers. If the core is made out of less-dense material such as a mixture of sulfur and iron, the maximum radius would probably be less than 2000 kilometers. It is a view similar to that which one would see from a spacecraft. The center of the scene shows the entire Valles Marineris canyon system, more than 3,000 kilometers (1,860 miles) long and up to 8 kilometers (5 miles) deep, extending from Noctis Labyrinthus, the arcuate system of graben to the west, to the chaotic terrain to the east. Many huge ancient river channels begin from the chaotic terrain and north-central canyons and run north. Many of the channels flowed into a basin called Acidalia Planitia, which is the dark area in the extreme north of this picture. The three Tharsis volcanoes (dark red spots), each about 25 kilometers (16 miles) high, are visible to the west. Very ancient terrain covered by many impact West Candor Chasm (Enhanced Color) This picture (centered at latitude 4 S, longitude 76 W) shows areas of central Valles Marineris, including Candor Chasm (lower left), Ophir Chasm (lower right), and Hebes Chasm (upper of great interest for future searches for fossil life on This NASA Hubble Space Telescope view of Mars is the clearest picture ever taken from Earth, surpassed only by close-up shots sent back by visiting space probes. The picture was taken on February 25, 1995, when Mars was at a distance of approximately 103 million kilometers (65 million miles) from Earth. Because it is spring in Mars' northern hemisphere, much of the carbon dioxide frost around the permanent water-ice cap has sublimated, and the cap has receded to its core of solid water-ice several hundred miles across. The abundance of wispy white clouds indicates that the atmosphere is cooler than seen by visiting space probes in the 1970s. Morning clouds appear along the planet's western (left) limb. These form overnight when Martian temperatures plunge and water in the atmosphere freezes out to form ice-crystal clouds. Towering 25 kilometers (16 miles) above the surrounding plains, volcano Ascraeus Mons pokes above the cloud deck near the western or limb. Valles Marineris is in the lower left. Martian Atmosphere This oblique image taken by the Viking orbiter spacecraft shows a thin band of the Martian atmosphere. This image looks northeast across the Argyre basin. The Argyre basin is about 600 kilometers across with a rugged rim of about 500 kilometers in width..

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