The Planet Mars

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• Mars - In Depth - For Teens and Adults
• All About Mars - For Kids

Mars is the last planet of the inner four terrestrial planets in the solar system at an average distance of 141 million miles from our Sun.  It revolves around the Sun every 687 days and rotates every 24.6 hours (nearly the same as Earth).  Mars has two tiny satellites, named Deimos and Phobos (shown below).  They are  most likely small asteroids drawn into Mars' gravitational pull.  Deimos and Phobos have diameters of just 7 miles and 14 miles, respectively.  An interesting side note; the inner moon, Phobos, makes a revolution around Mars in slightly more than seven hours.  This means since it orbits Mars faster than the planet rotates, the satellite rises in the west and sets in the east if observed from the Martian surface. 

  Atmosphere and Weather:  The Martian atmosphere is composed primarily of carbon dioxide.  However unlike Venus, the Mars atmosphere is very thin, subjecting the planet to a bombardment of cosmic rays and producing very little greenhouse effect.  Mariner 4, which flew by Mars on July 14, 1965, found that Mars has an atmospheric pressure of only 1 to 2 percent of the Earth's.  Temperatures on Mars average about -81 degrees F.  However, temperatures range from around -220 degrees F. in the wintertime at the poles, to +70 degrees F. over the lower latitudes in the summer.  

Various probes over the past few decades have found the surface of Mars to be rather desert like.  A fascinating panoramic view of the martian surface was taken (picture below) in 1997 by the Pathfinder mission.  The surface is cratered, but not as much as our Moon or Mercury.  The craters have probably been weather worn over the years by fierce windstorms, some of which can cover the entire planet.  These windstorms are common on the red planet, lifting rust-colored dust well up into the atmosphere encircling the entire globe.  Mars' red color comes from its reddish rock, sand and soil which encompasses about 5/8 of the surface.  The rest of Mars has patches of green.  But it is not clear what is producing this green color as it certainly is not vegetation.  Evidence does exist in the terrain that water has eroded some of the soil.  No flowing water is present today, but NASA announced on March 2, 2004 that the two rovers, Spirit and Opportunity confirmed liquid water once flowed on Mars.  In addition, a NASA research team in 1984 found a meteorite in Antarctica which may have come from Mars.  The meteorite was dated back 4.5 billion years and some evidence of microscopic life was left in the rock.   At present, Mars' water appears to be trapped in its polar ice caps and possibly below the surface.  Because of Mars' very low atmospheric pressure, any water that tried to exist on the surface would quickly boil away. 

Although water in Mars' atmosphere is only about 1/1000th of the Earth's, enough water vapor exists that thin, wispy clouds are formed in the upper layers of the Martian atmosphere as well as around mountain peaks.  No precipitation falls however.  At the Viking II Lander site, frost covered the ground each winter.

Seasons do exist on Mars, as the planet tilts on its axis about 25 degrees.  White caps of water ice and carbon dioxide ice shrink and grow with the progression of winter and summer at the poles.  Evidence of climatic cycles exists, as water ice is formed in layers with dust between them.  In addition, features near the south pole may have been produced by glaciers which are no longer present.

Mars does have many terrain features similar to Earth, such as canals, canyons, mountains and volcanoes.  Mars has a prominent volcano named Olympus Mons, which stands 69,800 feet above the Martian surface.  This makes it the tallest mountain known in our solar system. 

In general, Mars has highly variable weather and is often cloudy.  The planet swings from being warm and dusty to cloudy and cold.  Mars long ago was likely a warmer, wetter planet with a thicker atmosphere, able to sustain oceans or seas.

QUICK FACTS
(Data is from NASA Goddard)

DEFINITIONS:

Average distance from Sun:  Average distance from the center of a planet to the center of the Sun. 
Perihelion:  The point in a planet's orbit closest to the Sun.
Aphelion:  The point in a planet's orbit furthest from the Sun. 
Sidereal Rotation:  The time for a body to complete one rotation on its axis relative to the fixed stars such as our Sun.  Earth's sidereal rotation is 23 hours, 57 minutes.
Length of Day:  The average time for the Sun to move from the Noon position in the sky at a point on the equator back to the same position.  Earth's length of day = 24 hours
Sidereal Revolution:  The time it takes to make one complete revolution around the Sun.
Axis tilt:  Imagining that a body's orbital plane is perfectly horizontal, the axis tilt is the amount of tilt of the body's equator relative to the body's orbital plane.  Earth is tilted an average of 23.45 degrees on its axis.