When is It Winter on Uranus?

This episode is brought to you by Curiosity Stream. Earth isn't the only planet with seasons. All a planet needs to have them is a tilted rotational axis and an orbit. More direct sunlight on one hemisphere - that's summer. Less direct sunlight - that's winter. Those changing seasons make us change our wardrobe and keep an eye on the weather forecast, but they also keep our planet fresh. Our yearly temperature cycles power air and water currents that pump nutrients from deep oceans, drive winds, transport water and a million other things. But do seasons on other planets work like they do here? (Interlude music) Let's start with our sun. No seasons. Just nuclear fusion and hot plasma everyday for billions of years. Mercury doesn't have much of an axial tilt or much of an atmosphere, so no seasons for you either. Instead Mercury's got more of an inhospitable death-scape thing going on burning on the side facing the sun and freezing on the other. Although if you find yourself in the shadow of certain craters, you can still find ice there. Venus's tilt is also very small or very big, depending on how you look at it. It rotates on its axis backwards relative to the other planets and so slowly that a day on Venus is longer than its year. And you thought Mondays were long on Earth. You'd think this would give it a super hot sunward side and a super cold dark side like Mercury. But Venus's atmosphere is so thick with everyone's favorite greenhouse gas CO2, at such high pressure it traps heat all over. A summer hot enough to melt lead all the time. Mars' tilt and distance from the Sun give it the most Earth-like pattern of seasons but in only a tenth the mass of Earth, the Red Planet's atmosphere is so thin that temperatures swing to extremes. The Martian poles can get cold enough to freeze carbon dioxide straight out of the atmosphere. But these dry ice caps still grow and shrink with the seasons. As solid CO2 sublimates in summer, it powers 400 kilometer per hour winds. But near the equator, Mars' summer can reach 30ºCelsius - warm enough to melt underground ice and make liquid water flows. It's just the kind of thing that may support Martian potato farms, or other life. Jupiter technically has seasons due to its orbit and slight tilt, but they're basically indiscernible due to all those sweet looking gas storms so...moving right on. Saturn's seasons are like no other. Its rings tilt back and forth from the Sun throughout the year, along with the planet itself. At the Equinox those rings are lit edge on and they're fully illuminated at the solstices. Saturn's famous hexagonal polar storm changes color with these seasons and in the evening of a Saturnian winter you can even see light from Saturn's polar auroras. Saturn's moon, Titan, is the only moon in our solar system with an atmosphere and the only known body besides Earth with a liquid surface, although the lakes are filled with methane rather than water. Titan's seasons drive a hydrological or methanological cycle where spring rains grow the lakes and evaporation shrinks them. But seasons in the Saturn system don't happen very frequently since its orbit does take 30 years. By far the strangest seasons in our solar system happen on Uranus. This cold gas giant orbits almost 90 degrees on its side. In the summer when its axis points towards the Sun, it's a pristine blue ball. Due to this tilt, the northern hemisphere warms up while the southern stays locked in dark night. Uranus spends twenty Earth years like this, heating the deeper planet in one hemisphere while chilling the other, like charging a thermal battery. When the spring finally comes the axis has rotated and the planet gets more even sunlight. Deep heat starts to flow between the two hemispheres bringing Uranus to life. I just realized how bad that sounds. (LMAO ;) With this heat transfer the stripes and winds and storms so typical of a gas giant appear for a limited time only. Neptune is so far away and orbits so slowly, its seasons take 40 Earth years to change. But Hubble telescope images showed us that Neptune's stripes change from winter to summer. Always cold though. Way out on the dwarf planet Pluto, seasons hit their most extreme. Pluto has a hugely elliptical orbit, so its summer and winter are determined by its distance to the Sun, not just its tilt. Scientists call these super seasons. When tilt summer lines up with a close approach, Pluto has a super summer and if tilt winter comes at maximum distance, that's super winter. We don't know what kind of effects these super seasons have on Pluto, but figuring that out would be *wait for it* super cool. And that's just our solar system. With astronomers now studying exoplanets by the thousands, we may soon find planets with huge tilts like Uranus and super season orbits like Pluto, orbiting binary star systems or other things we haven't even thought of yet. Who knows what seasons could be like way out there? Stay curious. Hey guys this episode was brought to you by Curiosity Stream. They've been supporting 'It's ok to be smart' for a long time. It's an awesome place to dig deeper and learn even more about the stuff that we talk about. Curiosity Stream is a subscription streaming service that offers documentaries and other nonfiction titles from some of the world's best filmmakers, including a bunch of exclusive originals. Now, thinking about the seasons on other planets, I've been watching destination Mars. It's a series about what it would take to send humans to the Red Planet and how scientists are getting ready to go there. You can get unlimited access today and for our audience the first two months are free if you sign up at curiositystream.com/smart and use the promo code 'smart' during the signup process. Thanks for watching. Hit that subscribe button, and I'll see you next time when we learn something new together. You remember New Horizons? Scientists are analyzing data from that spacecraft to see how Pluto's super seasons affect everyone's favorite almost planet. For more almost planet science check out this Pluto video we did a while back.