Seasonal quirks on the solar system's giant planets


In this second of two parts, Phil Ritchie looks at how seasons transpire at the gas giants, Jupiter and Saturn, and the ice giants, Uranus and Neptune.


Saturn's striking polar hexagon is thought to be seasonal.
NASA / JPL-Caltech / Space Science Institute

In the inner solar system, a few tens or millions of kilometres from the sun, planets are bathed in plenty of solar energy. But in the far reaches – around a billion kilometres away or more – this heat source drops considerably.

Let's take Jupiter – the largest planet in our solar system. Its orbit is elliptical and its closest point to the sun is around five times that of Earth's orbit. This doesn't mean it receives a fifth of the sun's intensity on Earth, though – rather, just a paltry 25th (five squared).

So how does distance affect seasons and weather on the gas and ice giants?

The Hubble Space Telescope snapped this ultraviolet view of aurorae at Jupiter's north pole.
NASA / ESA / J Nichols (University of Leicester)

Jupiter

Jupiter’s seasons change every three years, but because it’s around 780 million kilometres from the sun and its tilt is a measly 3.1°, they’re pretty much indistinguishable from each other.

So in its seasonal stasis, the gas giant – mostly heated from within – unleashes storms thousands of kilometres wide in both hemispheres, as well as lightning, strong winds and auroras.

These two natural color images from NASA's Cassini spacecraft show the changing appearance of Saturn's north polar region between 2012 and 2016.
NASA / JPL-Caltech / Space Science Institute / Hampton University

Saturn

Saturn isn’t the only planet with rings, but it has the largest – and thanks to the planet’s tilt of 26.7 °, their shadows can stretch over the wintery hemisphere.

Still, given the planet’s distance from the sun – around 1.4 billion kilometres – it’s not like they’re blocking huge amounts of heat.

Seasons last around seven years and internal heating drives the gas giant’s weather patterns. In the 1980s, winds whipped around the equator at around 1,700 kilometres per hour. But two decades later, they’d slowed to 1,000 kilometres per hour.

At the moment, it’s summer in the northern hemisphere. NASA scientists think the colour-changing hexagon stretched across the north pole is also seasonal.

What little sunlight reaches the planet is enough to produce a haze that gradually morphs from blue to gold in the atmosphere.

A Hubble Space Telescope false-colour view reveals Uranus surrounded by its four major rings and by 10 of its known satellites.
NASA / Erich Karkoschka (University of Arizona)

Uranus

With the largest planetary tilt of the bunch – 97.8 ° – Uranus lies almost parallel with its plane. It essentially rolls around the sun, nearly three billion kilometres away.

This means extreme Uranian seasons last 21 years. While one pole soaks up the feeble sunlight, the other is plunged into an equally long, dark winter – until its orbit switches the sun-facing side.

As autumn or spring approach, Uranus bathes in sunlight spread more evenly by its quick rotations (with each day lasting seven hours and 14 minutes).

The planet is usually a still blue until the sun hits regions that haven’t seen light in decades. The sudden spike in temperature can warm the ice giant’s atmosphere enough to generate colossal storms.

These thermal images taken by the Very Large Telescope in Chile show a 'hot' south pole on the planet Neptune.
VLT / ESO / NASA / JPL / Paris Observatory

Neptune

Spinning on an axial tilt of 28.3 °, Neptune’s seasonal changes are similar to Earth – except they last more than 40 years.

The sun is, on average, 4.5 billion kilometres away. At that distance, its rays are 900 times weaker than they are at Earth. But it’s enough to catalyse change on Neptune.

While the rest of the ice giant is buffeted by storms where temperatures average -200 °C and winds howl at 2,400 kilometres per hour, the south pole is warmer by around 10 °C.

This ‘hotspot’ is right where the sun hits Neptune and is one of the biggest seasonal effects known about the planet. Not only does the area glow brighter in the summer sun, it thaws methane that’s normally frozen in the frigid conditions.

Neptune’s hotspot will make its way up to the north pole as the planet orbits around the sun.

  1. https://www.nasa.gov/image-feature/jpl/pia21049/changing-colors-in-saturns-north
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