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SYFY WIRE Bad Astronomy

Summer comes to Saturn

By Phil Plait

On Wednesday, May 24, 2017, summer arrived for Saturn. Well, its northern hemisphere, at least.

Like Earth, Saturn has seasons. And like Earth, it’s because of the planet’s tilt — in fact, both are tipped by about the same amount; Earth’s axial tilt is 23.5° and Saturn’s is 26.7°. We measure the tilt of a planet relative to its orbit: A tilt of 0° means the planet’s axis is straight up-and-down relative to its orbit.

Right now, Saturn’s north pole is tipped toward the Sun as much as it can be during its year. What does this mean? Well, for any Saturnians who might exist and who live in the planet’s northern hemisphere, the days are at their longest for the year (a Saturnian year, which is about 29.5 Earth years), and the nights are shortest. In fact, north of 63.3° (90 – 26.7) — think of this as Saturn’s Arctic Circle — the Sun never sets! For them, the Sun is up 10.55 hours per day.

Oh, right: I should mention that Saturn spins once every 10.55 hours, so, for Saturnian penguins, the Sun is up all day. At the north pole, the Sun won’t set again until the first day of northern autumn, which won’t be for about 8 more years (Saturn’s orbit is elliptical, so the seasons aren’t spread out evenly in time).

Saturn's hex pattern

Like Earth, the seasons affect the weather. This animation from the Cassini spacecraft shows Saturn’s north pole in June 2013 and in April 2017; note how the colors have changed. That’s likely due to haze forming in the past few years as the north pole warms up. Sunlight breaks up simple carbon-based molecules like methane, and the atoms rearrange themselves into more complex molecules created that are yellowish. I’ll note that Saturn has deep banding in its atmosphere like Jupiter does, but it’s more subdued on Saturn, because the haze blocks it somewhat, fading it out.

To give you a sense of scale of these images, the blue circular patch in the middle is essentially a permanent hurricane 2000 kilometers across, which is vast: It would easily stretch from New York City to Tampa, Florida! By the way, it looks blue because the atmospheric composition scatters blue light from the Sun (the same reason the Earth’s sky is blue).

The hexagonal pattern is similar to Earth’s jet stream, and though it looks almost manufactured, it’s a natural outcome of winds blowing on a rapidly spinning planet. All the white blotches between the pole and the hexagon are huge storms; I wonder if there are more or if they are stronger now that it’s summer there? The Sun doesn’t drive the weather on Saturn as strongly as it does on Earth (almost all our heat comes from the Sun, but Saturn is so far from the Sun it only gets about 1% of the light and heat we do; most of its heat comes from its interior). I expect there will be lots of research papers coming out over the next couple of years cataloging the seasonal atmospheric changes, and trying to explain them.

The change in the rings we see over the course of Saturn’s seasons is even more dramatic. Cassini arrived at Saturn in 2004, just before the northern spring. Equinox arrived at Saturn in 2009, and that meant Saturn’s spin axis was perpendicular to the Sun; a better way to say it is that the Sun shone straight down on Saturn’s equator.

Equinox at Saturn

But Saturn’s rings orbit directly above the equator, so instead of the Sun shining down on them, it was precisely to the side; they were edge-on to the Sun. This gave the rings an eerie glow, lit only by light reflected off Saturn’s atmosphere. Moreover, the shadow of the rings on the planet was a black line, a knife’s edge cutting across the equator.

Now, at the height of summer, sunlight is shining at the steepest angle it can on the rings, and they cast their shadows wide across the face of the planet. Conversely, Saturn’s shadow on the rings is getting shorter (just like shadows on Earth get shorter in summer):

Saturn's shadow on the rings

Sadly, these views of Saturn’s northern summer days won’t last long. Cassini’s mission is nearly over, and will fall into the planet itself in September. But it has been orbiting the gas giant for 13 years, enough to see it through an entire season on Saturn. Mind you, the original mission was only for four years, but has been extended more than once due to its phenomenal success. But the spacecraft is running out of maneuvering fuel, so the end is near.

Over the next few months Cassini will continue to pass over the planet’s north pole, and then inside the rings; we’ve seen amazing things from this , and there’s more to come. This mission ain’t over yet!

Image Credit: NASA/JPL-Caltech/Space Science Institute