Astronomers have discovered a massive aurorae-generated weather system, dubbed the Great Cold Spot, on Jupiter. Comparable in scale to the planet’s famous Great Red Spot, the phenomenon may have existed for thousands of years and is the first direct evidence of a sustained weather system generated by polar aurorae, opening the possibility of similar phenomena on other planets.
Observed by University of Leicester astronomers as a localised dark spot that is up to 24,000km tall and 12,000km wide, the spot is located in the planet’s thin high-altitude thermosphere. It is thought to bearound 200 Kelvin cooler than the surrounding atmosphere, which has a temperature range of between 700K (426°C) and 1000K (726°C).
“This is the first time any weather feature in Jupiter’s upper atmosphere has been observed away from the planet’s bright aurorae,” said study lead author Dr Tom Stallard, associate professor in Planetary Astronomy at the University of Leicester.
“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 phenomenon is thought to be caused by the magnetic field of the planet, with Jupiter’s polar aurorae pushing energy into the atmosphere in the form of heat flowing around the planet. This push creates a cooling region in the thermosphere; the boundary layer between the underlying atmosphere and the vacuum of space.
The Great Cold Spot was found by using 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. With the ion observed, the astronomers were able to map the mean temperature and density of the planet’s atmosphere.
The team was then able to compare its map to images of H3+ emission from Jupiter’s ionosphere taken by NASA’s InfraRed Telescope Facility from 1995 to 2000. By combining images taken over set periods of time, including over 13,000 images taken over more than 40 nights by the InfraRed Telescope Facility, the team was able to find the Cold Spot as a dark area in Jupiter’s hot 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,” added Stallard, who is funded by the Science and Technology Facilities Council.
“Observations and modelling of Earth’s upper atmosphere have shown that, on the short term, there may be changes in the temperature and density of the upper atmosphere.”
The team now hopes to use what was learnt of the Cold Spot to search for other such features that may be hidden in the gas giant’s atmosphere.
The study, which is published in Geophysical Research Letters, is available in full online.