A "Great Cold Spot" has been discovered on Jupiter that rivals the giant planet's most distinctive feature.
The dark stain on Jupiter's raging atmosphere is up to 24,000 km across and thought to be a vast storm system driven by magnetic energy.
It is one of the most dramatic discoveries on Jupiter, the largest planet in the Solar System, since the Great Red Spot was identified in 1830.
Lead scientist Dr Tom Stallard, from the University of Leicester, said: "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."
The Great Cold Spot is thought to be caused by magnetic forces responsible for Jupiter's spectacular polar aurorae driving energy into the atmosphere.
This creates a region of cooling in the boundary layer between the underlying atmosphere and the vacuum of space.
The discovery, reported in the journal Geophysical Research Letters, was made using the Very Large Telescope (VLT) operated by the European Southern Observatory in northern Chile.
Dr Stallard added: "The detection of the Great Cold Spot was a real surprise to us, but there are indications that other features might also exist in Jupiter's upper atmosphere.
"Our next step will be to look for other features in the upper atmosphere, as well as investigating the Great Cold Spot itself in more detail."