Red Spots on Jupiter

Chris - Most people will be aware that Jupiter, that giant planet in our solar system, has a giant red spot. We've known about this for 300 years or so. But what actually is it and why has another one, which has been named red spot junior, appeared more recently? Well I asked University College London's Steve Miller to tell me all about it.

Steve - Jupiter's a very large planet. It has a diameter of about 140 000 km compared with something like 13 000 km for the Earth. It also spins a great deal faster than the Earth does; roughly two and a half times as fast. Jupiter's day is less than 10 hours and our day, as we all know is 24 hours. So this is a huge rapidly spinning ball of gas something like five times as far away from the sun as we are. Now the great red spot is a very large storm system on Jupiter: you could fit the entire Earth into it. It's at a latitude of about 22 or 23 degrees south of the Jupiter equator. That has been known probably for 350 years and the storm is probably quite a bit older than that.

Chris - Steve, how do we know it's a storm though?

Steve - We can see that there are clouds at the northern and southern edges of it that are simply being swept round in a circle, and some winds go up to about 600 miles per hour, so you can tell that this thing must be spinning very rapidly.

Chris - Why is there just this one mega storm system? Is it because it's so powerful that it draws everything else into it?

Steve - Well that's indeed how it might have formed. It's very likely that it's formed up from a lot of smaller storms merging and then forming this enormous structure. We do know that it's changing. It's now something like 17 000 km in the west-east direction and 12 500 km north-south. When it was really catalogued in the 1880s, it was something like 39 000 km east-west, so we can see that in the past 125 years the storm has shrunk in size. It's changing. But it's almost certain that it formed from the merger of a lot of smaller storms.

Chris - We've got some evidence that this may be happening on Jupiter in the wake of the giant red spot junior, which has crept up in more recent years.

Steve - Yes, that's right. That's a storm that's a bit to the south of the great red spot and this is a storm that has resulted from the merger about five years ago of three smaller storms that basically caught each other up and then merged into one. It's really quite big; about 40% of the volume of the great red spot. So red spot junior may be junior may be junior but it's still pretty massive.

Chris - Do you know why it's red though?

Steve - Difficult question. The most likely theory is that when the three storms that formed red spot junior merged together, they became much more violent as one combined storm. This has had the effect of bringing material up from Jupiter's lower atmosphere higher up into the atmosphere. Now it's probable that this Jupiter air has quite high concentrations, or relatively high concentrations of phosphorus compounds or sulphur compounds or something like that, and bringing it up higher into the atmosphere has allowed sunlight to cause chemical reactions to occur that have turned the colour red. It's giving us an insight but we still have no idea what the actual chemical compound is that gives the great red spot and now red spot junior it's red colour. We just don't know what that is.

Chris - Steve, I have an email from Richard Wood in Columbus in Ohio and he'd like to ask a couple of questions about Jupiter. First of all, he says why do storms like Jupiter's great red spot not have eyes like hurricanes on the Earth do?

Steve - I think it's just a question of how big they are. The funnel of the great red spot and red spot junior are so much huger in size so that they don't give you the impression of an eye in the same way that a hurricane does. But then if you distance yourself a bit from them, you can actually imagine that you are looking at an eye. So there is sort of an eye but it's so much greater than anything that we see on Earth.

Chris - Richard goes on to ask, why is it that these Jovian storms don't migrate towards the poles, ie: away from the equator, like hurricanes on Earth seem to do?

Steve - Well this is something we call the coriolis force, and the coriolis force will tend to keep things rotating in the same latitude. We have coriolis forces on Earth but because the Earth is much smaller and rotating much slower than Jupiter is, they're not so effective so you do get winds that move north and south on the Earth. It's very difficult to get winds to move north or south on Jupiter simply because the coriolis force will always tend to bend anything moving north or south, it will tend to move it again in an east-west direction. That's why Jupiter has these very stable light coloured zones, these dark coloured belts on the Jupiter atmosphere. They remain very stable because they're held in place by these coriolis forces.