Seasons
Jack Kramer
There are two reasons for seasonal changes: the tilt of a planet's axis and its orbit around the sun. Our orbit is nearly circular, so there is little variation in Earth's overall climate. The other planets also have seasons, though their seasonal effects are quite different than what we experience. The terms "summer" and "winter" tend to be Earth-oriented conventions but are applied to the other planets, as well. When the North Pole of any planet is tilted toward the sun, astronomers call it the Summer Solstice; when the South Pole is tilted toward the sun it's called the Winter Solstice. So how do the seasons stack up on other planets?
Mercury experiences odd conditions. Until the 1960's it was thought that Mercury's day was the same length as its year, keeping the same face to the Sun, much as the Moon does to Earth. But we now know that Mercury rotates three times during two of its years. Mercury is the only body in the solar system locked into an orbit-to-rotation ratio other than 1:1. This fact and the high eccentricity of Mercury's orbit would produce very strange effects for an observer on Mercury's surface. At some longitudes the observer would see the Sun rise and then gradually increase in apparent size as it slowly moves toward the zenith. At that point the Sun would stop, briefly reverse course, and stop again before resuming its path toward the horizon and decreasing in apparent size. Meantime, the stars would be moving three times faster across the sky. Observers at other points on Mercury's surface would see different but equally bizarre motions. That makes it impossible to ascribe consistent "seasons" to this planet. Temperature variations on Mercury are the most extreme in the solar system, ranging from 90oK at night to 700oK during the day.
Venus has a very small axial tilt - 3o versus Earth's 23.5o. Its dense, acidic atmosphere produces a runaway greenhouse effect that keeps the surface at 750oK year-round, which is hot enough to melt lead. The smaller orbit makes the seasons very short. The onset of the seasons on Venus are:
Spring - Feb 24, 2000
Summer - Apr 1, 2000
Fall - May 28, 2000
Winter - July 22, 2000
Mars has the highest orbital eccentricity of any planet in our solar system other than Pluto - its distance from the Sun varies between 1.36 and 1.64 AU over the Martian year. This large variation, combined with an axial tilt greater than Earth's, gives rise to seasonal changes far greater than we experience anywhere on Earth. One of the strangest effects of seasons on Mars is the change in atmospheric pressure. During winter the global atmospheric pressure on Mars is 25% lower than during summer. This happens because of the eccentricity of Mars's orbit and a complex exchange of carbon dioxide between the Martian dry-ice polar caps and its CO2 atmosphere. Around the winter solstice when the North Pole is tilted away from the sun, the northern polar cap expands as carbon dioxide in the polar atmosphere freezes. At the other end of the planet the southern polar cap melts, giving CO2 back to the atmosphere. This process reverses half a year later at the summer solstice. But Mars is 10% closer to the Sun in winter than it is in summer. At the time of the winter solstice the northern polar cap absorbs more CO2 than the southern polar cap absorbs half a year later. The difference is so great that Mars's atmosphere is noticeably thinner during winter. Its orbital motion is slowest when it is at aphelion (the farthest point from the Sun) and fastest at perihelion (the closest point to the Sun). This makes Martian seasons vary in duration more than those on Earth:
Spring - May 31, 2000
Summer - December 16, 2000
Fall - June 12, 2001
Winter - November 2, 2001
Jupiter, like Venus, has an axial tilt of only 3o, so there is literally no difference between the seasons. Its seasons:
Spring - August 1997
Summer - May 2000
Fall - March 2003
Winter - March 2006
Saturn has an axial tilt of 26.75o, which is similar to Earth's. But when we're talking about a gas giant in the outer reaches of the solar system, the concept of seasonal change is sort of meaningless. Note that because of the distance from the Sun and how long it takes for one revolution, the length of the seasons is measured in several years, rather than months.
Spring - 1980
Summer - 1987
Fall - 1995
Winter - 2002
Uranus, like Earth, has a nearly circular orbit, so it remains at the same distance from the Sun throughout its long year. But the axis of Uranus is tilted by 82o! This gives rise to 20-year-long seasons and unusual weather (though anyway you look at it, it's always c-c-c-cold). For nearly a quarter of the Uranian year (equal to 84 Earth years), the sun shines directly over each pole, leaving the other half of the planet plunged into a long, dark winter. Uranus is a ball of mostly hydrogen and helium. Absorption of red light by methane in the atmosphere gives the planet its bluish color. Early visual observers reported Jupiter-like cloud belts on the planet, but when the Voyager 2 spacecraft flew by in 1986, Uranus appeared virtually featureless. In the intervening years the planet has moved far enough along its orbit for the Sun to shine at mid-latitudes in the Northern Hemisphere. By the year 2007, the sun will be shining directly over Uranus' equator. Its seasons, if you can call them that, began in these years:
Spring - 1922
Summer - 1943
Fall - 1964
Winter - 1985
Neptune has an axial tilt of 28.5o, which isn't too much different than Earth's. But this farthest of the gas giants doesn't really experience appreciable seasonal changes.
Spring - 1880
Summer - 1921
Fall - 1962
Winter - 2003
Because Pluto is so far away, virtually nothing is known about its seasons.
So you are now prepared to plan a trip to another planet at a particular season of their year. But you know, I think I like the change of seasons on Earth just fine!
