Mars as imaged with MIRA's 36-inch telescope.

Mars is the fourth planet closest to the Sun. It is named after the Roman god of war, perhaps because of its red color being associated with blood. Now we know that the red color is caused by rust (iron oxide) on the surface.

Mars has intrigued humans with the possibility of life existing there. Many wonderful science fiction stories have been written about Martians, to the extent that The War of the Worlds, written by H.G. Wells in 1898 and broadcasted by Orson Welles over the radio in 1938 caused widespread panic over the possibility of Martians attacking the Earth.

Mars's density is about three-fourths of Earth's density, and is also smaller than Mercury's and Venus's densities, leading astronomers to think that perhaps the iron is distributed uniformly around the planet rather than concentrated in the core, like in the other terrestrial planets. The atmosphere in Mars is very thin compared to Earth's, the pressure at the surface is only one percent or less than on Earth's surface.

Deimos. NASA Viking 1 Orbiter image.

The moons are so small that there is not enough gravitational potential energy to form them into spherical shapes; they are irregular, shaped much like potatoes. Their densities and composition are similar to those of rocky asteroids, leading scientists to believe that they were probably asteroids captured by Mars as they passed close to the planet. In addition, in the past Mars' atmosphere was denser than it is today and could have contributed to slowing down passing asteroids.

Phobos. NASA Viking 1 Orbiter image.

Both satellites revolve synchronously around Mars: their rotation period equals their period of revolution, just like our Moon. This means that they always show the same face to Mars. This is due to tidal forces from Mars on the moons over billions of years. Phobos orbits Mars faster than Mars revolves around its axis, so if you were on Mars you would see Phobos rising in the west and setting in the east, while Deimos would rise in the east and set in the west!

Orbital and Rotational Information

Its orbit is much less circular than Earth's, its distance to the Sun varies about 19%, from approximately 210 million kilometers to 250 million km (130-155 million miles). For comparison, Earth's maximum and minimum distance to the Sun differ by only 3.3%. Mars (and all the planets outside Earth's orbit) doesn't show phases like Venus or Mercury. The angular diameter of Mars at closest approach to Earth is 17.88 arc seconds.

Both the tilt of Mars' equator is similar to that of Earth's: 25.1°; so Mars experiences seasons in much a similar way as Earth. A Martian day is longer than an Earth day by approximately 40 minutes.

A lot can be learned about the composition of a planet merely by looking at its density. Mars's density is about three-fourths of Earth's density, and is also smaller than Mercury's and Venus's densities, leading astronomers to think that perhaps the iron is distributed uniformly around the planet rather than concentrated in the core, like in the other terrestrial planets. The atmosphere in Mars is very thin compared to Earth's, the pressure at the surface is only one percent or less than on Earth's surface.

Seasons in Mars

Due to the the tilt of its rotation axis, Mars experiences seasons. These seasons are longer than Earth's because of Mars' greater distance from the Sun. In addition, the eccentricity of Mars' orbit is greater than that of Earth's: the distance from the Sun at perihelion and aphelion varies 19%. Mars is further from the Sun during southern winter, resulting in long, cold winters. The northern winters aren't as long, nor as cold. The northern summers are colder than the southern summers, so the temperature variations in the northern hemisphere are less extreme than in the southern hemisphere.

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