Neptune's dark spots

Have you ever had an annoying spot on your face that you just wish would go away? Well, the planet Neptune might give you some hope, as it seems to be able to get rid of the most enormous spots without too much difficulty.

The same cannot be said for Jupiter, which has had its “Great Red Spot” for at least 350 years. This is a massive storm that is more than 24,000 km long and 12,000 km across, making it twice the diameter of planet Earth. Despite its longevity it does appear to have shrunk slightly over the past 100 years, so perhaps the ointment is starting to have an effect.

However, Neptune’s spots are a different matter. Now that Pluto has been relegated from its former planetary status, Neptune is (as far as can be reliably known) the outermost planet of the solar system, and so far away (about 30 times further away from the Sun than is the Earth) that details of its outer atmosphere are difficult to see from Earth, although the launch of the Hubble Space Telescope in 1990 made this much easier.

It was therefore not until 1989 that the “Great Dark Spot” in Neptune’s southern hemisphere was discovered by Voyager 2. This was of similar shape to Jupiter’s spot, but smaller at only one Earth diameter. It also showed signs of being surrounded by high winds (in excess of 2000 km per hour) which pushed it around the planet in around 18 hours.

However, when the Hubble telescope was focused on Neptune in 1994 the Great Dark Spot had vanished! There was a spot in the northern hemisphere, which had not been seen by Voyager, so the new spot was designated the Great Dark Spot of 1994 as opposed to the Great Dark Spot of 1989.

Voyager 2 had seen a second, much smaller, dark spot in 1989, and this had also disappeared by 1994.

The answer to the variable nature of Neptune’s spots, as opposed to that of Jupiter, would appear to be that they are not self-perpetuating storms, in the sense that the Jupiter spot is, but holes in the outer layer of the atmosphere that permit views of a darker layer beneath. In effect, the Great Dark Spots are more like the ozone holes of planet Earth, that grow and shrink over time, than Jupiter’s Great Red Spot.

The Great Dark Spots are evidence that Neptune’s atmosphere is more dynamic than used to be thought the case.

© John Welford