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THE LARGE white spot that appeared on Saturn last month is baffling
astronomers who have no explanation for why it recurs every 27 to 30 years.
Saturn orbits the Sun once every 29.4 years but scientists do not know of
a mechanism that explains the apparent synchronisation.

The spot is a giant storm 20 000 kilometres across. Frozen ammonia makes
the spot appear white against the yellow of Saturn’s surface.

According to Alan Heath, director of the Royal Astronomical Society’s
Saturn section, the society’s records reveal that similar spots appeared
on Saturn in 1876, 1903, 1933 and 1960. ‘This spot turning up on cue has
given support to the idea that the spots have a period of between 27 and
30 years,’ says Heath.

But Rita Beebe, an astronomer at the New Mexico State University in
the US, is sceptical of any causal link between Saturn’s orbit and the spots.
‘The spot in 1960 was much smaller,’ she says. ‘The last spot to compare
with the current one was the one in 1933.’ This spot was discovered by the
British comedian Will Hay, who was a keen amateur astronomer. It was named
Hay’s spot in his honour.

Beebe thinks the spot is a bubble of gas carried up from the interior
of Saturn by convection in the gaseous planet. It appears white because
it is not yet contaminated with dirty aerosols in the upper atmosphere,
she says.

‘Saturn is emitting 50 per cent more heat than it absorbs,’ Beebe said.
‘This means the interior is warm and there is flow from the inside out.
When you have convection you get bubbles. This is a big bubble.’

Observations reported on Monday (15 October) by the International Astronomical
Union indicate that the spot has now lengthened into an oval, which stretches
almost from one side of the visible disc of Saturn to the other. Three brilliant
white spots have also become visible within the spot. One appeared close
to the point where the original spot emerged.