Jupiter's shrinking red spot


Planet's 400-year-old superstorm is finally running out of steam. Helen Maynard-Casely reports.


Images of Jupiter's red spot taken by the Hubble telescope over 20 years show it has decreased in size. – NASA/ESA

Since Robert Hooke first observed it in 1664, Jupiter’s red spot has been one of the must-see sights of our solar system.

But you better hurry up and get out your telescope. Results from Hubble, our sentry telescope out in space, suggest Jupiter’s red spot is shrinking.

The spot is a gigantic storm that’s been raging across the planet for at least 400 years.

The first accurate measurements of the storm in the 1800s placed it at 41,000 km across – more than three times the diameter of Earth. But Hubble’s latest measurement of the red spot has pegged it at less than half of that, only 16,000 km in diameter.

The shape of the storm is also changing. The spot has always appeared oval, as if slightly squashed by the surrounding bands of clouds. Now, as it dwindles, it is becoming more circular.

No one can explain just why this is happening. Jupiter, the largest planet in our solar system, is a gas giant made mostly of hydrogen and helium, with atmospheric dynamics that are a mystery to us here on Earth. Even the reason for the spot’s red colour is unknown. The movement of clouds we see on Jupiter – made of ammonia, not water – is largely driven by the planet’s own thermal energy, in contrast to Earth’s atmosphere which is fuelled by energy from the Sun. Besides having our attention as one of the wonders of the solar system, studying the shrinking giant red spot could give an insight into what is happening in the mysterious gassy depths of our giant neighbour.

Helen Maynard-Casely is a physicist and science writer based in Sydney.
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