Overview - page 2
AtmosphereMars has a very thin atmosphere composed mostly of carbon dioxide (95 %), while Earth's atmosphere is mostly nitrogen (77%) and oxygen (21%). The average atmospheric pressure on Mars is less than 1% of Earth's, and no human could survive there without a pressurized spacesuit. Mars' atmosphere also lacks a protective ozone layer, which allows much of the Sun's dangerous ultraviolet radiation to reach the planet's surface.
At 27,000 meters (88,600 feet) high and 600 kilometers (375 miles) in diameter, Mars' Olympus Mons is the largest known volcano in the solar system. (Image Credit: NASA/USGS)
Looking down from orbit in January 2004, the European Space Agency's Mars Express spacecraft scanned a 1700 by 65 km (1056 by 40 mile) swath across Valles Marineris with its remarkable High Resolution Stereo Camera. Valles Marineris is 7 km (4.3 miles) deep and 4,000 km (2,485 miles) wide - four times as deep as the Grand Canyon and as wide as the continental United States. [Image Credit: ESA/DLR/FU Berlin (G.Neukum)]
Mars "Wobbling" Axis
The Earth's axis is currently tilted at an angle of 23.5 degrees. This axial tilt varies between 22 and 24 degrees over time, whereas the axial tilt of Mars varies between 13 and 40 degrees. (Photo Copyright Richard Harwood, Black Hawk College)
Mars and Earth share a similar angle of axial tilt - Mars at its present 25 degrees, and Earth at a fairly constant 23.5 degrees. During the past ten million years, Earth's axial tilt has only varied between about 22 and 24.5 degrees, because our relatively large Moon helps maintain a stable tilt. But Mars, which has two tiny moons, has experienced more extreme changes in its axial tilt - between 13 and 40 degrees over timescales of about 10 to 20 million years. At times when Mars' axial tilt is high, the summer polar cap points directly towards the sun, which allows the entire polar ice cap to melt or sublime (change from a solid to a gas). These fluctuations in axial tilt lead many scientists to believe that extreme changes in climate and seasons have occurred throughout Mars' history.