Interplanetary Seasons

Every planet in the solar system has seasons. Most have four like the Earth -- called Winter, Spring, Summer and Fall -- but that's where the similarities end. Extraterrestrial seasons are hardly noticeable on some planets (Venus), mindbogglingly extreme on others (Uranus) and in some cases simply impossible to define (Mercury).

The table below gives the dates of the seasons for 8 of the 9 planets in the solar system. Only Pluto is missing. It's so far away that we don't know much about seasons on that distant world.

In the table the equinoxes and solstices are named after the corresponding season in the northern hemisphere. This is the convention that astronomers often use to discuss planetary seasons. When the north pole of a planet is tilted toward the sun, astronomers call it the Summer Solstice; when the south pole is tilted toward the sun it is called the Winter Solstice. Nevertheless, the seasons are always opposite in the two hemispheres. On Earth, for example, when it is summer in New York, it is winter in Sydney. On a spring day in Paris, autumn leaves are falling in Argentina, and so on...

When the Vernal Equinox takes place on March 20, Earth will join Venus and Jupiter as the only planets in the solar system where it is now northern Spring.

Planetary seasons are caused by two factors: axial tilt and variable distance from the sun (orbital eccentricity). Earth's orbit is nearly circular and so has little effect on climate. It's our planet's axial tilt that causes almost all seasonal changes. When the north pole is tilted toward the Sun, it's northern summer. Six months later the north pole tilts away from the Sun and we experience northern winter.

The other two planets where it is northern spring, Jupiter and Venus, have very small axial tilts -- just 3 degrees compared to Earth's 23.5 degree tilt. Seasonal changes on those planets are correspondingly small. Spring on Venus isn't much different from autumn. The planet's dense, acidic atmosphere produces a runaway greenhouse effect that keeps the surface at 750 K year-round -- that's hot enough to melt lead. Spring fever on Venus is really hot!

Our second-nearest planetary neighbor Mars has the highest orbital eccentricity of any world except Pluto. Its distance from the Sun varies between 1.64 and 1.36 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 even in Antarctica.

Right: Over the past six months, the southern hemisphere of Mars has passed through spring and into summer. Spring started in early August 1999 and summer arrived toward the end of December 1999. Mars Global Surveyor is in a polar orbit, thus the spacecraft's camera has had an excellent view of seasonal changes. Shown here are three views of the same portion the layered terrain near the Martian south pole. They show how the landscape thaws and defrosts as summer approaches. [more information]

From the point of view of an Earth-dweller, 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 Mars's dry-ice polar caps and its CO2 atmosphere. Around the summer solstice when the Martian 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 winter solstice.

At first it might seem that these events occurring at opposite ends of Mars would simply balance out over the course of the Martian year, having no net effect on climate. But they don't. That's because 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.

Seasons on Mars vs. Seasons on Earth

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

Above: The orbit of Mars is very eccentric, unlike Earth's which is more nearly circular. 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 effect, combined with the planet's axial tilt, makes Martian seasons vary in duration more than those on Earth. The length of the seasons in this table are give in Earth days and Martian days. The two are almost exactly the same duration. An Earth day is 24 hours long, a Martian day is 24.6 hours long. [more information]

Martian seasons are peculiar by Earth standards, but they probably pale in comparison to seasons on Uranus. Like Earth, the orbit of Uranus is nearly circular so it keeps the same distance from the Sun throughout its long year. But, Uranus's spin axis is tilted by a whopping 82 degrees! This gives rise to extreme 20-year-long seasons and unusual weather. 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, frigid winter.

Left: A dramatic time-lapse movie by NASA's Hubble Space Telescope shows seasonal changes on Uranus. Once considered one of the blander-looking planets, Uranus is now revealed as a dynamic world with the brightest clouds in the outer Solar System. more info.

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 degrees below zero. In the animation pictured left the bright clouds are probably made of crystals of methane, which condense as warm bubbles of gas well up from deep in the atmosphere of Uranus.

Uranus does not have a solid surface, but is instead a ball of mostly hydrogen and helium. Absorption of red light by methane in the atmosphere gives the planet its cyan color. Uranus was discovered March 13, 1781, by William Herschel. Early visual observers reported Jupiter-like cloud belts on the planet, but when NASA's Voyager 2 flew by in 1986, Uranus appeared as featureless as a cue ball. In the past 13 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.

Mercury's seasons -- if they can be called that -- are also remarkable. 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 the Earth. This was shown to be incorrect by Doppler radar observations. We now known that Mercury rotates three times during two of its years. Mercury is the only body in the solar system tidally locked into an orbital-to-rotational resonance with a 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. [ref] At some longitudes the observer would see the Sun rise and then gradually increase in apparent size as it slowly moved 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. All the while 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.

Temperature variations on Mercury are the most extreme in the solar system ranging from 90 K at night to 700 K during the day.