Earth has four seasons – spring, summer, fall, and winter. Other planets (including Jupiter and the other giant planets) can experience seasons, too, but they might be wildly different from the ones that we are used to. Find out about how seasons work and what role the Sun plays in causing them.
Seasons on Earth, Mars, and Beyond
Why Do We Have Seasons?
Do you notice the days getting longer and the weather getting warmer? For those of us in Earth’s northern hemisphere, spring is in full swing. As we enjoy this better weather, it’s worth reflecting on why we have seasons, and the ways in which the dynamics of the Sun-Earth system influences this yearly cycle.
Earth has seasons because its axis of rotation (the line going through the North and South geographic poles around which it rotates) is tilted about 23 degrees off of pure North and South as compared to the plane of the ecliptic (the flat “plane” going through the Sun’s equator, along which most planets’ orbits can be roughly traced). As Earth orbits the Sun, the northern and southern hemispheres are tilted either toward or away from the Sun. When a hemisphere is tilted toward the Sun, it experiences summer; when it’s tilted away, it experiences winter.
Spring and autumn arrive when the hemispheres are roughly equal in their tilt toward the Sun, with spring following winter as a hemisphere starts tilting toward the Sun and autumn following summer as it starts to tilt away. Earth’s tilt means that places farther away from the equator receive more or less sunlight during the summer and winter months, as well. For example, the High Arctic can have almost 24 hours of daylight in the middle of summer, or almost 24 hours of darkness in the middle of winter.
The Sun Has Seasons, Too
The Sun also experiences changing seasons, but they are not caused by things like axial tilt. Most of its cyclic changes have to do with the solar cycle. This roughly 11-year cycle sees more sunspots and other solar activity at solar maximum and less at solar minimum, with a range of up-and-down differences as the cycle revolves.
Missions like SDO (Solar Dynamics Observatory) and SOHO (Solar and Heliospheric Observatory) help NASA to track the Sun’s “seasons,” including the 11-year solar cycle. This includes tracking details about its magnetic field, its atmosphere (also called the corona), and its interior, to understand solar variability and its impact on Earth. They also monitor solar flares, coronal mass ejections, and the solar wind to aid in predicting space weather. All of this data helps us to exist more comfortably in the company of our powerful host star.
The Parker Solar Probe’s record-breaking journeys through the inner reaches of the corona, have also provided data about the Sun’s environment and the ways in which it impacts the rest of the solar system. In particular, it has helped gather information about the solar wind, a feature of the heliosphere predicted by its namesake, Dr. Eugene Parker, and which affects virtually every other place in the Sun’s domain, including Earth’s biosphere.
Does Mars Have Seasons?
As we have seen, Earth’s four seasons are the result of Earth’s axial tilt as its orbits around the Sun. But many worlds do not have axial tilt, and even those that do may have vastly different circumstances than we see here. Mars, for example, has a very similar axial tilt (25.2 degrees compared to Earth’s 23.4). This gives rise to a similar cycle of four seasons (spring, summer, autumn, and winter) that occur as they do on Earth.
But the eccentricity of Mars’ orbit (that is, how far from a perfect circle it is) means that the seasons there are all of different lengths. Northern spring is 194 sols (a sol is a Martian solar day, equal to about 24.7 Earth hours), and northern fall is only about 142 sols. Since a Martian year is about 668 Martian days (or 687 Earth days) long, this means northern spring is about 29 percent of a Martian year, while northern fall is only about 21 percent of the year. Mars’ orbit also takes it farther from the Sun during northern summer, meaning that southern summer is warmer than northern summer. Imagine if Earth had seasons that varied so widely!
Mars also has a dust season. When Mars is closer to the Sun, its thin atmosphere heats up more, but this heat is transferred unevenly because of the thin atmosphere. This causes intense updrafts that bring a lot of its famous rusty surface dust into the atmosphere. Many of these storms are as big as continents on Earth, and some of them get large enough to wrap around the entire planet. Storms like these can cause trouble for Mars surface missions that rely on solar power, as the dust they throw around leads to less efficient solar panels.
Mars’ thin atmosphere also traps very little heat, meaning that direct heat input from the Sun accounts for a lot of the day-to-day temperature variation. This is why daytime temperatures on Mars, which can be almost like winter on Earth (about 32 degrees Fahrenheit or 0 degrees Celsius), are so different from those at night (which can get as cold as -200 degrees Fahrenheit or -129 degrees Celsius). It also causes lots of variation at ground level. For example, if you were standing on Mars holding a thermometer, it might feel almost like springtime at your feet and more like the dead of winter up near your face!
Seasons on Other Worlds
Gas giants like Jupiter and Saturn also have seasonal variation. By studying Saturn with the Hubble Space Telescope from 2018 to 2020, scientists found that the wind speed near the equator had sped up to about 1,000 miles per hour (roughly 1,600 kilometers per hour) from about 800 miles per hour (roughly 1,300 meters per hour) as compared to when they were measured by Cassini in 2004-2009. This change is believed to be an indication of seasonal changes on Saturn, with the differing solar energy at different points in Saturn’s orbit causing differences in the atmospheric conditions sort of like what happens on terrestrial planets.
But not all worlds are so similar in their strangeness. Uranus, the seventh planet from the Sun, experiences seasons very different from Earth’s. With an axial tilt of about 98 degrees, Uranus basically orbits tipped over on its side. We don’t know how Uranus got this way (one theory is that a massive object smashed into Uranus sometime in the ancient past), but it does mean that the Sun shines directly over each of Uranus’ poles for about a quarter of its year. Uranus is about 19 times farther from the Sun than Earth is, and it takes it about 84 years to orbit the Sun, so each pole gets about 21 years of winter and summer!
Seasons Beyond the Sun
Planets orbiting other stars could have even more exotic kinds of seasons. Red dwarf stars are thought to be the most numerous stars in the universe. Many of the planets discovered orbiting red dwarf stars are thought to be tidally locked, meaning they orbit in such a way that only one hemisphere ever faces the star. Planets like these are expected to have no axial tilt, so they will not experience seasonal variation in the same way that Earth does. The side facing the star will have that temperature for its whole year.
What about planets with more eccentric orbits? If a planet’s perihelion (its closest approach to its star) was one astronomical unit (AU–the average Earth-Sun distance, or about 93 million miles) and its aphelion (when it’s farthest from its star) was 10 AU, how would that affect seasonal changes? What if the perihelion was only half of an AU? With so many variations in orbit possible, the possibilities for seasonal shifting seem endless.
This is something we don’t see in our solar system, since most planetary bodies here have largely circular orbits. But comets do have highly elliptical orbits. They spend most of their time out in the deep cold of the outer system, and they experience evaporation when they get close to the Sun. Some of them, sungrazing comets, come close enough to vaporize entirely! Exoplanets with highly elliptical orbits could see a less extreme version of this phenomenon, with atmospheric changes that swing wildly throughout the year.
Planets in multiple star systems, which outnumber systems with solo stars like the Sun, could also have very strange orbits. A planet going around Alpha Centauri A and B (the nearest sunlike stars) in a circumbinary orbit (that is, an extra long orbit that takes a planet around both stars in a binary pair) could have seasons that vary greatly as it gets closer to and further away from both stars over the course of its long track around them.
What Makes a Season?
All of this raises the question of just how we define seasons in the first place. Seasons not only depend on the energy deposited into an atmosphere, but also on atmospheric composition and density. If a planet’s atmosphere is denser, like with Venus, heat will have an easier time transferring across the planet through the process of convection, smoothing out atmospheric variations. For planets with a lighter atmosphere (Mars, for instance), it would be harder to transfer heat around the planet, allowing for more extreme atmospheric variations. This is why you might see two different “seasons” on Mars just in the handful of feet between the ground and eye level.
When we think of Earth and seasons, we often think of different weather and precipitation. These things change depending on the local chemistry, and not all exoplanets planets will have this factor in play. Seasonality as we know it is tied largely to how water changes across the year on a given planet. But if a planet is composed of other gases that aren’t water vapor, it might have very different temperatures needed to condense or precipitate those molecules. On Saturn’s moon, Titan, hydrocarbons like methane and ethane are supercooled into liquid that falls from the sky as rain, and changes in seasonal conditions are key to this eventuality.
Then there are exomoons (moons of exoplanets). In our solar system, the only moon known to have a substantial atmosphere is Titan. But research suggests that some exomoons could have diverse and maybe even pleasant atmospheres. Certain exomoons could also see tidal heating that generates subsurface oceans, similar to those believed to exist on Europa and other moons in our solar system. Worlds like these might experience seasonal variation on the order of days and weeks as a result of slight changes in the orbital dynamics of these worlds.
While many exoplanets are thought to be tidally locked, their moons might get around that limitation as a result of orbiting around a planet that orbits around a star. This would bring variation in how much stellar radiation they receive over time, but it could also potentially cause seasonal changes which have durations on the order of days or weeks (in other words, the length of an exomoon’s orbital period). We have not discovered any exomoons yet, but there is nothing to suggest that they aren’t possible, and it is certainly possible that one with the right conditions might even have Earth-like seasons.
Resources for Educators ↓
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