Seasonal Changes

Because the axis of rotation is tilted about 25° to the plane of revolution, Mars experiences seasons somewhat similar to those of the earth. One of the most apparent seasonal changes is the growing or shrinking of white areas near the poles known as polar caps. These polar caps, which are are composed of water ice and dry ice (frozen carbon dioxide). During the Martian summer the polar cap in that hemisphere shrinks and the dark regions grow darker; in winter the polar cap grows again and the dark regions become paler. The seasonal portion of the ice cap is dry ice.

Astronomical Characteristics

The mean distance of Mars from the sun is about 141 million mi (228 million km); its period of revolution is about 687 days, almost twice that of the earth. At those times when the sun, earth, and Mars are aligned (i.e., in opposition) and Mars is at its closest point to the sun (perihelion), its distance from the earth is about 35 million mi (56 million km); this occurs every 15 to 17 years. At oppositions when Mars is at its greatest distance from the sun (aphelion) it is about 63 million mi (101 million km) from the earth. It rotates on its axis with a period of about 24 hr 37 min, a little more than one earth day.

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