This document is part of the Martian Time Boneyard. It was originally located at http://www-star.stanford.edu/~frankb/seasons.html.
Author: Frank Bauregger


The data returned by the Mars Global Surveyor radio science experiment are expected to help us better understand the seasonal variations on Mars. In order to put these data into perspective, let's first try to understand how the seasonal changes on Mars compare with those on Earth.

Mars has a Spring, Summer, Autumn, and Winter like Earth, but the Martian year is about 668.599 Martian sols long (687 Earth days), which means that all the seasons will be longer than on Earth. Mars' orbit is quite eccentric compared to that of Earth. Because of this, there are major differences in the orbital speed of Mars as it orbits the Sun. Mars is slowest when it is at aphelion (the farthest point from the Sun) and fastest at perihelion (the closest point to the Sun). Since the eccentricity of Mars' orbit exaggerates the aphelion and perihelion distances, Mars' seasons vary in duration more than those on Earth, which has a nearly circular orbit. The table below helps to illustrate these effects.

Season

(Northern Hemisphere)

Length of Season on Earth

(Earth Days)

Length of Season on Mars

(Martian Days)

Spring 93 194
Summer 93 178
Autumn 90 142
Winter 89 154

Summer is the longest season on Earth because summer occurs when Earth is near aphelion. Similarly, most of Spring and early Summer occur near Mars' aphelion, resulting in a longer northern summer season than in the southern hemisphere. Although one would expect warmer summers in the northern hemisphere of Mars due to the extreme length of Spring and Summer, and the more direct radiation from the Sun reaching the surface, this is not the case. The eccentricity of Mars' orbit causes Mars to receive 44% more radiation from the Sun at perihelion (during southern Summer) than at aphelion . As a result, the southern hemisphere summers are shorter, but hotter than northern hemisphere summers. This phenomenon is demonstrated by the fact that the southern polar ice cap frequently disappears entirely during summer, while the northern polar ice cap has never been known to do so. Southern summers can be as much as 30 degrees centigrade hotter than northern summers. The southern hemisphere also experiences longer, colder winters than the northern hemisphere since Mars is near aphelion during these seasons.

Season Northern Hemisphere Southern Hemisphere
Winter Shorter and Warmer Longer and Colder
Summer Longer and Cooler Shorter and Hotter

Since the inclination of Mars' axis (25 degrees) is quite close to that of Earth (23.5 degrees), the hours of daylight and darkness over the seasons vary nearly the same as on Earth. However, temperature variations on Mars follow the hours of sunlight more closely since the atmosphere is quite thin, and there are no bodies of water to retain heat after dark.

Seasonal variations may be more precisely specified by the parameter Ls, which is the areocentric longitude of the Sun. It is an angular measure of the position of a planet in its orbit. Ls = 0 corresponds to the first day of Spring, or when the planet is on the Spring Equinox. Ls = 90 is the Summer Solstice, Ls = 180 is the Autumnal Equinox, and Ls = 270 is the Winter Solstice. So Ls provides a convenient way of measuring the changes in season. Incidentally, equinox is Latin for "equal night", and refers to the fact that on the day of the Spring or Autumnal Equinox, there are equal hours (12) of day and night everywhere on the Earth. Also, solstice means "the Sun stands still", and refers to the observation that near the Summer and Winter Solstice dates, the sunrise and sunset times do not vary appreciably.

Given the orbital parameters for both Earth and Mars, one can calculate the values of Ls over a year. Due to the eccentric orbit of Mars, the variation in Ls with each Mars day is not linear over the year. However, Earth's Ls variation with each day is nearly linear. Given these functions of Ls over the Earth and Martian years, one can develop a Martian Calendar. This calendar parallels Earth's calendar regarding seasonal changes. So a person living on Mars who is accustomed to Earth's calendar would be able to use the Martian calendar to accurately judge the seasons on Mars. For example, if it were near the end of March on Mars (say March 50), one would be experiencing early Spring in the northern hemisphere. A spreadsheet containing the pertinent data used to develop the calendar may also be viewed. It provides the values of Ls over the Earth and Martian years in tabular form.


References:

Carr, Michael H., The Surface of Mars, Yale University, 1981

Glasstone, Samuel, The Book of Mars, NASA SP-179, 1968


Copyright 1996-98 F. N. Bauregger