Debbie Maxson, age 13, of Sarasota, Florida, for her question:
Do other planets have seasons?
Oh yes, the other planets have their changing seasons. But only on earth does the springtime bring back the songbirds. Only on earth does the fall season splash rainbow colors on leafy boughs. On no other planet does a snowshoe rabbit fly like a white cloud over the winter snows. And no other planet can warm a summertime beach.
In November, 1971, outlook for the mission to Mars seemed gloomy. Mariner 9 had traveled 287 million miles in 168 days. It carried a package of devices to scan the surface and relay a Martian portrait back home. But as the little spacecraft swung into orbit, the bashful Red Planet was veiled in its worst dust storm in more than a century.
Later, the dust subsided and Mariner 9 sent back thousands of fabulous photos. In August, 1972, almost a year after the dust storm, it showed the north polar ice cap. It was shrinking as spring, returned to the Northern Hemisphere. On earth, our yearly orbit alternates the northern and southern seasons every six months. The Martian year is 687 days and the seasonal changes last almost twice as long as ours. This is because Mars is farther from the sun. It also travels a little slower around its larger orbit. Hence, Mariner 9 relayed the southern Martian spring in late 1971 and the northern spring in August 1972.
Seasonal contrasts depend on how far a planet's axis is tilted toward the plane of its orbit. On Mars, this angle almost matches the earth's. Both summer and winter are much cooler, but the contrast between them is much the same as ours. However, our four seasons are fairly equal because the earth's orbit is almost circular. Mars has an eccentric, oval orbit. The period that brings spring to the south and fall to the north lasts 146 days. The period of southern fall and northern spring lasts 199 days.
The yearly orbit of Venus takes about 225 earth days. But it is not likely that seasonal changes penetrate the dense atmosphere above the hot, gloomy surface. Jupiter takes 12 earth years to complete its wide orbit. Its axis tilt is merely three degrees, so we would not expect much contrast in the four Jovian seasons. However, patchy hot spots, north and south, come and go. They are not seasonal, but may be related to mysterious sources of energy in giant Jupiter's interior.
The axis of Saturn is tilted almost 27 degrees and each of its four seasons lasts 7 ~ earth years. We would expect the contrasts to be extreme, though at 886 million miles from the sun the hottest summer must be far below the endurance of earthlings. That is, unless Saturn also has mysterious sources of internal heat. It takes Uranus 84 earth years to pass through its four freezing seasons and Neptune takes almost twice as long. If little Pluto has seasonal changes, each of them must shiver and shudder for as long as 62 earth years.