Mohawk Valley Astronomical Society

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The Evil Eye

by Perry Pezzolanella

There is no storm in the Solar System that is larger or more intense than Jupiter’s Great Red Spot. When it comes to weather, Jupiter is the King of Storms. The Great Red Spot is the Perfect Storm - nothing else compares to it and no other storm has been observed to last for centuries.

Jupiter is more than eleven times wider than Earth at 88,846 miles in diameter and spins very fast with its day just under ten hours. This rapid rotation drives the storms and winds of the upper atmosphere creating dark belts, bright zones, eddies, and ripples. The heat from below creates convection and turbulence, which develops into small storms. These often merge into larger storms that are a common occurrence on Jupiter and have been observed by telescope and spacecraft. Ammonia is the main ingredient in the cloud features and the reason why Jupiter’s clouds do not thoroughly mix into a bland ball. The rising blobs of warm ammonia cool and condense, which releases energy that causes the surrounding gas to rise even higher. This feedback persistently drives the smaller storms and prevents them from mixing together to create a bland Jupiter. The Great Red Spot, at 10,000 miles across and 200 miles deep, takes this to the extreme as it is one highly efficient weather machine.

Juno is a NASA spacecraft that has been orbiting Jupiter since July 4, 2016. The main goal of Juno is to study Jupiter’s atmosphere, especially deeper down, as it orbits Jupiter once every 53 days, coming as close as 2700 miles above the cloud tops. Juno has so far revealed a Jupiter rich in dynamic storms that swirl around the planet, but the most impressive close encounter occurred on July 10, 2017 when Juno flew only 2700 miles above the Great Red Spot. This storm has been observed since the invention of the telescope, over 400 years ago, and rotates on the edge of the South Equatorial Belt. It gobbles up smaller storms and eddies which fuel it, but sometimes it spits them out again. Juno zoomed in to reveal structure within the Great Red Spot including ripples and waves of high level ammonia ice clouds above it. South of the Great Red Spot are small white clouds that erupt like thunderheads rich with ammonia that may fuel it. The red color is most likely a photochemical byproduct such as phosphine that has accumulated through the years of exposure to the ultraviolet radiation from the Sun.

The Great Red Spot is dynamic and steadily shrinking. It was once a huge oval over twice as wide as Earth and truly red. It can vary in redness, appearing brilliant brick red to dull salmon pink, depending on the atmospheric conditions. It can be easily visible through a small telescope when red and especially when the nearby South Equatorial Belt fades or disappears. It can be hard to see if it fades to salmon and the belt darkens, often the only clue to it presence will be a dent in the belt. The Great Red Spot is now only a little wider than Earth and growing rounder and shrinking. It is not known if we are living in an era when it will disappear. Jupiter has no solid surface to rip apart the Great Red Spot unlike the hurricanes on Earth, and the heat from below also helps to maintain it, but its existence may be nearing an end. The Great Dark Spot was the most prominent feature on Neptune when Voyager 2 flew by in August 1989, but by 1994 it was gone when the Hubble Space Telescope observed Neptune. These storms of the giant planets of Jupiter, Saturn, Uranus, and Neptune are huge by Earth standards, but they do not last forever.

The Great Red Spot is like a huge eye staring back at anyone standing on the surface of its moons giving an eerie feeling of being watched. The big red eye can take on an evil appearance if the stormy South Equatorial Belt gives it an angry eyebrow. Rarer, but even scarier, is when the shadow of one of the moons casts a dark spot upon the Great Red Spot giving it a glaring pupil. Now that is a frightening sight indeed!