|photo credit: thebadastronomer|
On February 28, 2010, Earth was exactly on the far side of the sun from Jupiter, an alignment called solar conjunction. In the months since then, Earth, on its faster orbit, has moved out of this alignment, allowing Jupiter to emerge from the sun’s glare into our morning skies. As astronomers again turned their telescopes towards Jupiter after months of being unable to see it, they made a startling discovery–one of its belts is gone!
Jupiter, our largest planet (bigger than all others combined) is a gas giant with no solid surface. When we look at Jupiter, we see the tops of its clouds. As at other gas giants, Jupiter’s clouds appear in a banded structure; light colored ‘zones’ alternate with dark colored ‘belts.’ Astronomers remain uncertain about which chemical processes make the belts darker than the zones. We have determined, however, that the zones are regions of upwelling air, while the belts are where air is descending. Zones, then, extend to higher altitudes above Jupiter than do belts. The two belts most noticeable even in small backyard telescopes are the North Equatorial Belt (NEB) and the South Equatorial Belt (SEB), with the lighter Equatorial Zone between them. The South Equatorial Belt is the one that has faded from view, making Jupiter’s changed appearance readily noticeable to virtually all telescope users. The Great Red Spot orbits in the South Tropical Zone just south of the SEB, with its northern part normally extending into that belt. With the SEB gone, the Great Red Spot now appears alone, entirely surrounded by much lighter clouds.
At the moment, we can only speculate as to why the SEB has vanished. One speculation is that the belt is now hidden under higher clouds that have formed above it. These higher clouds would be like cirrus clouds on the Earth, except that at Jupiter they’re composed of ammonia, not water.
In late 2009, astronomers noticed that the SEB was fading, though they did not expect it to fade away entirely. Although the SEB’s disappearance at this moment caught us by surprise, this belt has disappeared before. For example, that belt had faded from view when Pioneer 10 flew past Jupiter in December 1973. It also vanished in 1992. In all, 2010 marks the 17th known disappearance of Jupiter’s SEB since 1901.
Here is the abstract of a 1996 paper about this belt and its changes, written just after it last vanished and reappeared.
Each time the SEB vanished, it eventually reappeared, usually within about 2 years. How long will it take the South Equatorial Belt to return this time? Watch for yourself by observing throughout 2010. In late May 2010, Jupiter is in the southeastern sky at dawn, rising just after 3 a.m. and well placed for observing by 4. It rises earlier and earlier each night, however, until it is up all night long in September 2010. That’s when Earth again aligns with Jupiter, this time on the same side of the sun. From September 2010 to March 2011, Jupiter is visible in the evening. In late spring 2011, Jupiter is again in the morning sky. Remember that many backyard telescopes produce an inverted image, in which case the belt normally on top will be the one missing from view.
As you watch for the return of the SEB, you have the chance to participate in science. Science, after all, is not simply about delving into established stores of knowledge or observing well understood phenomena where we know what to expect. Science is most exciting when we have the chance to observe something that not even the experts understand completely, and thus contribute to the advancement of knowledge.