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Planetary Seasons
07.09.04
 
"Fly me to the Moon,
Let me sing among those stars,
Let me see what spring is like,
on Jupiter and Mars."


-- Fly Me to the Moon written by Howard Bart

Every planet in our solar system has seasons. But the seasons that occur on other planets are extremely different from the traditional spring, summer, fall and winter that we experience here on Earth. Despite what may seem like great variations in temperature, weather and climactic conditions in different places around the globe, in reality, there actually is little variation in Earth's overall climate. Why?

Color image map showing the parts of Alaska frozen and thawed in April 2000. Most of the southern half of the state are thawed.
Image to right: NASA's SeaWinds satellite tracks how spring thaws in places like Alaska affect the global climate. Credit: NASA

There are two reasons that seasons occur on the planets: 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. But, other planets have more elliptical - egg-shaped - orbits, and therefore their seasons are very different than what we experience. The terms "summer" and "winter" tend to be Earth-oriented terms, but can be applied to the other planets as well. When the North Pole of any planet is tilted toward the Sun, astronomers call it the Winter Solstice; when the South Pole is tilted toward the Sun it's called the Summer Solstice.

So... how do the seasons stack up on other planets?

Mercury
Mercury experiences some of the most bizarre conditions. Until the 1960s, 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 in two of its years. The high eccentricity of Mercury's orbit also causes very strange effects. If an observer were to stand on Mercury's surface, he or she 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. In addition, 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 really tell when one season ends and the next one begins. And, to add to the uniqueness of the planet, temperature variations on Mercury are the most extreme in the solar system, ranging from 90°K at night to 700°K during the day.

Venus
Venus has a very small axial tilt -- 3° versus Earth's 23.5°. Its dense, acidic atmosphere produces a runaway greenhouse effect that keeps the surface at 750°K year-round, which is hot enough to melt lead. Venus also has a very small orbit, which makes its seasons shorter, and variations in temperature and conditions extremely slight. Its seasons last a mere 55-58 days, as opposed to 90-93 days on Earth. When the Magellan spacecraft made its historic dramatic final descent into Venus' fiery atmosphere on October 11, 1994, it was springtime on Venus, while in the peak of autumn here on Earth.

Black and white image of sand dunes seen from orbit above Mars. Light highlights indicate frost on the dunes.
Mars
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, causes seasonal changes that are far greater than we experience here 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.

Image to left: Spring frost on Mars lightens a field of dunes. The dune field is much darker in summertime. Credit: NASA

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 the Martian 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. Seasons change roughly every seven months, with spring beginning around the end of May, summer beginning around the middle of December, fall beginning in June, and winter beginning in November.

Jupiter
Jupiter, like Venus, has an axial tilt of only 3°, so there is literally no difference between the seasons. However, because of its distance from the Sun, seasons change more slowly. The length of each season is roughly three years.

Saturn
Saturn has an axial tilt of 26.75°, which is similar to Earth's. But when talking about a gas giant in the outer reaches of the solar system, the concept of seasonal change doesn't quite mean the same as on Earth. There is virtually no seasonal variation on a gas giant such as Saturn, but the length of each season is roughly 7 years. That's a long winter! When the Cassini spacecraft began its revolutionary mission to Saturn and its largest moon, Titan, Saturn was two years into its fall season. When it arrives at Saturn's orbit on July 1, 2004, it will have just become winter on the ringed planet.

Uranus
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 82°. This causes 20-year-long seasons and unusual weather, although one thing that is for certain is that it is always 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. The Northern Hemisphere of Uranus is just now coming out of the grip of its decades-long winter. As the Sunlight reaches some latitudes for the first time in years, it warms the atmosphere and triggers gigantic springtime storms comparable in size to North America with temperatures of 300° below zero. By the year 2007, the Sun will be shining directly over Uranus' equator, which will produce more evenly distributed Sunlight and the ability to see features on Uranus.

Neptune
Neptune has an axial tilt of 28.5°, which isn't too much different than Earth's. But this farthest of the gas giants doesn't really experience appreciable seasonal variation, although scientists say images from the Hubble Space Telescope showing changes in the brightness of clouds in the Neptune's southern hemisphere may be a sign of spring.

Seasons on Neptune last more than 40 years!

Three color images of blue-green planet Neptune taken from 1996 to 2002. Increased clouds in 1998 and 2002 are a sign of seasonal change on the gas planet.
Image above: Increased brightness of clouds in Neptune's southern hemisphere is a sign of changing seasons on Neptune. Credit: NASA


Extraterrestrial seasons are harsh and may be rather unpredictable. So, as we contemplate the different times of year on Earth, and may lament about the coming of a long, hot summer or, a cold harsh winter, take a quick mental tour of the extreme world of our solar system.

Suddenly, watering a garden or scraping ice off of a windshield doesn’t seem so bad.

 
 
Samantha Harvey, Office of Space Science