NASA recently released a most amazing image. It had been sent back by the Cassini spacecraft, which is currently exploring the planet Saturn and its many moons. The image shows a huge storm, like a hurricane here on Earth but much bigger, about 20 times larger than the ones we get. Unlike ours, this storm is sitting right over one of Saturn’s poles and seems to have been there for years.
Big, long-lived storms are business as usual for Jupiter and Saturn, the largest planets in our Solar System. The best-known and biggest storm is on Jupiter, the fifth planet out from the Sun. We live on the third. This storm is known as the Great Red Spot. It is a storm vortex large enough to drop the Earth into. It is also very long-lived compared with the storms we have. It was there when we first pointed telescopes at the planet. With a small telescope you can see it for yourself.
Jupiter is getting low in the west in the evenings but is easy to find. Your telescope will show you a tan disc crossed by belts of darker cloud, and the Great Red Spot as a large blob. If conditions are good enough you will see other storms too, as lighter or darker spots. Saturn, the sixth planet from the Sun, is high in the eastern sky these evenings and is easy to observe. If you can draw your eyes away from the spectacular system of rings surrounding the planet, you’ll see a somewhat fainter version of Jupiter, a tan disc crossed by cloud belts.
Since weather, including storms are all driven by solar energy, we would expect them to be smaller and weaker on planets further from the Sun. Certainly this is the case for the next two planets further out from the Sun: Uranus and Neptune, respectively the seventh and eighth planets. In this case, why do Jupiter and Saturn have such dramatic weather compared with ours, on the third planet from the Sun? The reason is that in addition to the energy input from the Sun, weather patterns and storms are moulded by a planet’s rotation.
It’s easy to visualize how here on Earth the heat of the tropics leads to warming of the air, making it rise. Cooler air flows in from the north and south to replace it. High above the ground, the warm air flows northward and southward, where it cools and sinks, and then flows along the ground back towards the tropics to repeat the cycle. This simple idea is upset by our planet’s rotation. At the equator the rotation of the Earth leads to the land and atmosphere moving eastward at about 1,670 km/hr. At the poles that velocity is zero. This changes the movement of the atmosphere completely, generating spiralling weather systems and storms.
Jupiter and Saturn are larger than the Earth, and they rotate faster; their days are only about ten hours long. A point on Jupiter’s equator is moving at 46,000 km/hr and on Saturn, 36,000 km/hr. This rapid rotation stretches the cloud into belts and spawns enormous storms. Moreover, with no land masses to disrupt the air flows, these storms can persist for a long time. If our world had the sort of weather we see on Jupiter and Saturn, we would probably not be here to comment on it.
Jupiter is still unmissable, but is getting low in the western sky in the evening. Saturn rises before sunset and is well up in the east by dark. The Moon will reach first quarter on the 18th.
Ken Tapping is an astronomer with the National Research Council’s Dominion Radio Astro-physical Observatory, Penticton.