|ESA Science & Technology||30-Jun-2005 14:20:48|
On Earth, the seasons in the northern and southern hemispheres are very similar. Not so on Mars where the southern hemisphere has shorter, hotter summers and colder, longer winters than the northern hemisphere. This is a consequence of the shape of the planet's orbit around the Sun.
The Earth's seasons show little difference between the north and the south because the planet orbits around the Sun in a close approximation to a circle. This means that the distance from the Sun remains roughly constant throughout the orbit and the southern and northern hemispheres stay tilted towards or away from the Sun for similar lengths of time.
The Martian orbit, however, is considerably more elliptical than Earth's, which means that for part of the year the planet is quite a bit closer to the Sun than at other times. The hemisphere that is tilted towards the Sun at the time of closest approach (perihelion) will experience a hotter summer than the other hemisphere, which will be tilted towards the Sun at the furthest approach (aphelion). As planets travel fastest when closest to the Sun, the hemisphere with the hotter summer will also have a shorter summer. At present, Mars' southern hemisphere is tilted towards the Sun at the time of closest approach.
The shape of Mars' orbit, however, changes cyclically with consequences for the climate. The more elliptical or eccentric the orbit, the greater the difference in distance between the planet and the Sun at perihelion and aphelion, and hence the greater the climatic difference between the two hemispheres. The eccentricity of the Martian orbit undergoes two types of variation, one with a period of about 100 000 years, the other with a period of about 2 million years. At present, the orbit has a marked eccentricity, but it is by no means at its maximum. Climatic differences between the hemispheres can be more or less extreme than now.
Another influence on the climate is the tilt of a planet's axis of rotation to the perpendicular to the plane of its orbit, known as the obliquity. For Mars and Earth this is presently about 24o. However, the figure varies for both planets. For Mars, it ranges from about 13o to 42o over about 100 000 years, which means that the surface area around the poles exposed to total summer sunlight or winter darkness varies gradually over this period.
The axis of rotation also precesses, that is it describes a circle in the sky every 51 000 years. This means that the northern and southern hemispheres take it in turns to be the one tilted towards the Sun at perihelion. Some of the regular changes in the eccentricity of Mars' orbit, the tilt of its axis and the precession of its axis can be predicted accurately. But their effects on climate are difficult to predict in more than general terms. Mars itself, however, holds the secret. Layers of sediment deposited over aeons around the poles provide a record of climatic conditions over timescales ranging from seasonal to millions of years.
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