Weather, Weather, Everywhere?
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 weather 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?
There are several factors that affect the weather on the planets: the tilt of a planet's axis (which causes the seasons), the shape of its orbit around the sun, the presence or absence of a significant atmosphere, its average distance from the Sun, and the length of its day.
Earth's axis is tilted about 23 degrees, causing the latitude of the Sun to vary from 23 degrees north of the equator at the beginning of northern summer to 23 degrees south of the equator at the beginning of northern winter. On Earth, that tilt is the primary reason for the differences in weather we observe between summer and winter. Planets with smaller tilts might have smaller weather variations; planets with larger tilts could have more extreme variations.
Our orbit is nearly circular, so there is little variation in Earth's overall climate, averaged over both northern and southern hemispheres. But other planets have more elliptical orbits, and therefore their seasonal variations in weather are much different than what we experience. We are much further from the Sun than Mercury or Venus, but closer than the other six planets. Generally, weather variations are more pronounced for those planets closer to the Sun.
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 Summer Solstice; when the South Pole is tilted toward the sun it's called the Winter Solstice.
So, how do seasons and weather conditions stack up on other planets?
Mercury experiences some of the most bizarre 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. With this bizarre 2:3 ratio, 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 cause very strange effects if an observer were to stand 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 decrease in apparent size. It is the motion of the Sun against the background stars, which varies for an observer on Mercury. 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 -280?F at night to 800?F during the day for parts of the surface. And yet near Mercury's poles there is almost no change in temperature, because, in spite of its very long duration between sunrises (176 Earth days), the tilt of its axis is zero, meaning that there are no seasonal variations on Mercury. Furthermore, with essentially no atmosphere, Mercury's weather changes are displayed not as storms in the atmosphere, but as wide swings in surface temperature.
Venus has a very small axial tilt -- 3 degrees versus Earth's 23.5 degrees. Its dense, acidic atmosphere produces a runaway greenhouse effect that keeps the surface at around 865?F year-round, which is hot enough to melt lead. Venus also has a smaller orbit than Earth, 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. In fact, circumstances combine to make Venus' seasons shorter than its days: successive sunrises on Venus are separated by 117 Earth days, and Venus rotates backwards, causing the Sun to rise in the west and set in the east. When the Magellan spacecraft made its historic dramatic final fiery descent into Venus' atmosphere on October 11, 1994, it was northern springtime on Venus, while in the peak of northern autumn here on Earth.
Mars has one of the highest orbital eccentricities of any planet in our solar system (other than Mercury and Pluto) -- its distance from the Sun varies between 1.38 and 1.67 AU over the Martian year. This large variation, combined with an axial tilt slightly greater than Earth's, gives rise to seasonal changes far greater than we experience here on Earth. On Mars, dramatic dust storms are common due to solar heat, which warms the Martian atmosphere and causes its air to move rapidly, lifting dust off the ground. Because the Martian atmosphere is thin -- about 1% as dense as Earth's at sea level -- only the smallest dust grains hang in the air, and the grains are as fine as smoke. 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 southern summer than it is in northern summer. At the time of the winter solstice the northern polar cap absorbs less CO2 than the southern polar cap absorbs half a year later. The difference is so great that Mars's atmosphere is noticeably thicker during northern 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 greatly in duration than those on Earth. Seasons change roughly every six months, with northern spring and fall lasting 171 Earth days, northern summer being 199 days in length, and northern winter being only 146 days. Because of these variations, Martian seasons do not start at the same Earth day each Martian year.
Jupiter, like Venus, has an axial tilt of only 3 degrees, 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. Jupiter is the fastest spinning planet in our Solar System, which causes the planet to flatten at the poles and bulge at the equator. Also, like Saturn, Uranus, and Neptune, Jupiter has a very dense, turbulent atmosphere that can cause dramatic storm activity. For example, the Hubble Space Telescope took a detailed look at a unique cluster of three immense oval-shaped storms that occurred below Jupiter's Great Red Spot, in 1995. Scientists believe that Jupiter's Great Red spot is actually a hurricane that has been raging for more than 400 years within the planet's atmosphere. The temperature on Jupiter also varies widely, because of the different chemical compositions that make up its atmosphere. For example, the highest white clouds are made of crystals of frozen ammonia, and the temperature here is about -220 F (-140 degrees C). Measurements made by ground instruments and spacecraft show that Jupiter's temperature increases with depth below the clouds, and the temperature reaches 70 degrees F (21 degrees C) -- "room temperature" -- at a level where the atmospheric pressure is about 10 times as great as it is on Earth. As you travel further into the interior of Jupiter, the temperature becomes even hotter -- at the planet's core, it is even hotter than the Sun!
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Saturn has an axial tilt of almost 27 degrees, which is slightly larger than that of Mars. 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 Saturn, but the length of each season is more than 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 northern fall season. When it arrives at Saturn's orbit on July 1, 2004, it will have just become northern winter on the ringed planet.
Uranus has a relatively circular orbit, so it remains at about the same distance from the Sun throughout its long year. But the axis of Uranus is tilted by 98 degrees! This causes 21-year-long seasons and unusual weather, although one thing that is certain: 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 has a deep atmosphere 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? F 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 at all latitudes on Uranus.
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Neptune has an axial tilt of 28.5 degrees, which isn't too much different than Earth's. But this gas giant doesn't really experience appreciable seasonal variation, although its seasons last for more than 40 years!
Because Pluto is so far away, virtually nothing is known about its seasons. However, because of its tilt of nearly 120 degrees and the highest eccentricity of any planetary orbit even at its great distance from the Sun, weather variations are expected to be significant -- perhaps enough to result in far greater changes in its atmospheric pressures than even that of Mars. It is possible that the very thin atmosphere of Pluto may entirely freeze and fall as snow to Pluto's surface as it gets farther and farther from the Sun.
Extraterrestrial weather and 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 our solar system. Suddenly, having to water the garden or scrape ice off of a windshield doesn't seem so bad.
For more information, please contact Samantha Harvey at NASA's Jet Propulsion Laboratory, Samantha.K.Harvey@jpl.nasa.gov.
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