What Is That Spot on Jupiter

The spot has been observed continuously since about the 1830s. Astronomers in the 17th century also described a dark feature in the same band, but whether it was the same vortex or its predecessor is still debated: the gap in the records is too long.

A hurricane on Earth lasts days, or weeks at most. It draws energy from a warm ocean, and as soon as it moves over land, it loses its fuel and breaks apart against the terrain. Jupiter has no terrain at all: no shore, no mountains, and no friction against a solid surface.

Depth matters too. A hurricane on Earth has a bottom: ocean below, air above, and a whole structure about 15 kilometers thick. A Jovian vortex reaches hundreds of kilometers down and has far more mass. You can't spin up or stop something that massive quickly: it has too much momentum.

The spot is squeezed between two powerful currents flowing toward each other: one above, the other below. They spin it up like a pencil between your palms and keep it from falling apart. On top of that, the vortex feeds on small storms that come near it, taking their energy for itself.

Even so, it is shrinking. In the late 19th century, the spot was about three times wider than it is now, and its contraction has sped up over the past few decades. No one can say where this is headed: possibly toward disappearance, possibly toward a smaller, stable size.

The red color, by the way, is the least well explained. Most likely, it has to do with the chemistry of the upper cloud layer, which is illuminated by the Sun's ultraviolet radiation. The shade changes over decades: the spot sometimes fades almost to cream, then deepens again.