'Great Cold Spot' discovered on Jupiter

Scientists have discovered a second Great Spot on Jupiter created by the powerful energies exerted by the planet's polar aurorae, rivalling the scale of the famous Great Red Spot on the gas giant.

The 'Great Cold Spot' has been observed as a localised dark spot, up to 24,000 kilometres in longitude and 12,000 kilometres in latitude.

It is located in the gas giant's thin high-altitude thermosphere, that is around cooler than the surrounding atmosphere, which can range in temperature between 426 and 726 degrees Celsius.

"This is the first time any weather feature in Jupiter's upper atmosphere has been observed away from the planet's bright aurorae," said Tom Stallard, Associate Professor at University of Leicester in the UK.

"The Great Cold Spot is much more volatile than the slowly changing Great Red Spot, changing dramatically in shape and size over only a few days and weeks, but it has re- appeared for as long as we have data to search for it, for over 15 years.

"That suggests that it continually reforms itself, and as a result it might be as old as the aurorae that form it - perhaps many thousands of years old," said Stallard.

The Great Cold Spot is thought to be caused by the effects of the magnetic field of the planet, with the massive planet's spectacular polar aurorae driving energy into the atmosphere in the form of heat flowing around the planet.

This creates a region of cooling in the thermosphere, the boundary layer between the underlying atmosphere and the vacuum of space.

Although we can not be sure what drives this weather feature, a sustained cooling is very likely to drive a vortex similar to the Great Red Spot.

The astronomers used the CRIRES instrument on the Very Large Telescope (VLT) to observe spectral emissions of H3+, an ion of hydrogen present in large amounts in Jupiter's atmosphere, which allowed the scientists to map the mean temperature and density of the planet's atmosphere.

They then used images of H3+ emission from Jupiter's ionosphere taken by NASA's InfraRed Telescope Facility between 1995-2000 to compare.

Through combining images taken over a period of time, including over 13,000 images taken over more than 40 nights by the InfraRed Telescope Facility, the astronomers revealed the presence of the Great Cold Spot as an area of darkness amongst the hot environment of Jupiter's upper atmosphere.

"What is surprising at Jupiter is that, unlike weather systems on Earth, the Great Cold Spot has been observed at the same place across 15 years. That makes it more comparable to weather systems in Jupiter's lower atmosphere, like the Great Red Spot," Stallard added.

The research was published in the journal Geophysical Research Letters.