Jupiter's Great Red Spot
By Amanda Jermyn



Of all the wild and crazy weather on Jupiter there is none more iconic than the Great Red Spot. Though it may seem small relative to its enormous host planet, the Great Red Spot is about as tall as the Earth and almost three times as wide.

This oval-shaped feature is actually a giant storm, similar to a hurricane on Earth, that moves in a counter-clockwise or anti-cyclonic direction. With wind speeds up to 400 miles per hour, it makes a full rotation about every six Earth days. While hurricanes on Earth may seem pretty big, the Great Red Spot is about six times the diameter of the largest hurricane ever measured on Earth.

It has been around a long time, at least 350 years, and is comprised mostly of the gases hydrogen and helium. With a temperature of about minus 260 degrees Fahrenheit, it is much colder and also higher than the other clouds on Jupiter, with its cloud tops reaching about five miles above the surrounding clouds.

So why donít our hurricanes last nearly as long? We donít yet know for sure. Here on Earth hurricanes receive their energy from the moist air above warm water, and usually die down when they pass over cold water or land. However, on Jupiter there is no cold water or land, and the planetís thick atmosphere provides a continuous source of energy. So this may be why the Great Red Spot just keeps on going. Fundamentally, much of Jupiterís weather is powered by heat generated by the planetís gradual contraction of matter under its powerful gravity. As a result, there is a lot of energy to fuel the Great Red Spot.

There is, however, evidence that it is getting smaller, and itís possible that it may eventually disappear. In the late 1800ís the Great Red Spot was estimated to be about 25,500 miles across. By 1979, during NASAís Voyager 1 and Voyager 2 flybys it was measured at 14,500 miles across, and current observations by the Hubble Space Telescope indicate it is now only 10,250 miles across. Observations starting in 2012 suggested that the rate at which the Great Red Spot is shrinking has been speeding up, though more recently that rate appears to be slowing down. In the process, it is changing its shape from an oval to a circle.

A team led by Amy Simon of NASAís Goddard Space Flight Center in Green Belt, Maryland plans to study how small eddies feed into the storm to see if these might affect its size and shape. Unfortunately, as this is a region of highly complex and variable wave patterns, itís tough to figure out whatís really going on, and the mechanisms responsible for the shrinkage remain a mystery.

Another mystery is what causes the stormís reddish color. Possibilities include complex organic molecules, red phosphorous or some other sulfur compound, but there is as yet no hard evidence for these. And sometimes the Great Red Spot is not actually red, varying in color at irregular intervals, from bright red to pale pink. Currently it is more orange than red, and its center, which usually has more intense color, is less distinct than it used to be. Exactly why is unknown.

With NASAís Juno mission due to reach Jupiter in July 2016, direct investigation by its instruments may finally resolve some of these mysteries.

While there is no public Stars Club meeting in December, on Friday, January 8th at 7:30pm, the Stars Club and Springfield Science Museum will host ďStars over Springfield,Ē an astronomy adventure for the whole family. A fee of $3 for adults and $2 for children under 18 will be charged.