‘Cold’ Great Spot discovered on Jupiter

A second Great Spot has been discovered on Jupiter by astronomers, rivaling the scale of the planet’s famous Great Red Spot and created by the powerful energies exerted by the great planet’s polar aurorae. Credit: Joseph DePasquale, Smithsonian Astrophysical Observatory Chandra X-ray Center.

WASHINGTON, DC— A second Great Spot has been discovered on Jupiter by astronomers, rivaling the scale of the planet’s famous Great Red Spot and created by the powerful energies exerted by the great planet’s polar aurorae.

Dubbed the ‘Great Cold Spot’, it has been observed as a localized dark spot, up to 24,000 kilometers in longitude and 12,000 kilometers in latitude, in the gas giant’s thin high-altitude thermosphere, that is around 200 degrees Kelvin cooler than the surrounding atmosphere, which can range in temperature between 700 degrees Kelvin (426 degrees Celsius) and 1000 degrees Kelvin (726 degrees Celsius).

The results are published today in Geophysical Research Letters, a journal of the American Geophysical Union.

“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 in Planetary Astronomy at the University of Leicester in the United Kingdom, and lead author of the new study.

“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,” Stallard said. “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 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 is a computerized view of the Great Cold Spot seen as it would appear from directly above by a satellite in orbit around Jupiter at a distance of 35,000 km above the planet (half the radius of Jupiter). This view shows how the Great Cold Spot changed between 1998-2000, with dramatically different shapes in this period. The original images used to construct this animation were made by the NSFCam instrument on NASA’s IRTF telescope in Hawaii. Credit: Tom Stallard. 

This creates a region of cooling in the thermosphere, the boundary layer between the underlying atmosphere and the vacuum of space. Although we can’t 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.

This image shows how the Great Cold Spot changes dramatically in shape and size on different days. Each view comes from a different day, with the spot sometimes almost disappearing. Because the feature is dynamic, changing over both daily and yearly timescales, it is likely that this feature is a weather system that is in a constant state of change. But despite this variability, it is seen again fifteen years later, showing it must reform again and again. Credit: Tom Stallard. 

Stallard, who is funded by the Science and Technology Facilities Council, added: “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.

“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 two main differences are firstly that Earth’s aurora sees dramatic changes caused by activity from the Sun, whereas Jupiter’s aurora are dominated by gases from the volcanic moon Io, which are relatively slow and steady, and secondly that the atmospheric flows generated by Earth’s aurora can drive heat quickly across the whole planet, making the upper atmosphere ring like a bell, while Jupiter’s fast spin traps this energy nearer the poles.”

Once the Great Cold Spot was detected, the researchers searched for it in past observations. Jupiter’s infrared aurora was studied in detail by the NSFCam instrument on NASA’s InfraRed Telescope Facility (IRTF) for six year between 1995-2000, in preparation for the last mission to visit Jupiter, Galileo. These extensive observations resulted in a huge set of images of Jupiter, with more than 13,000 images taken over more than 40 nights. The video shown here, taken on the 19 December 2000, shows how Jupiter’s aurora is offset from the magnetic pole, so that it rotates into and out of view. The faint emission away from the aurora hides the Great Cold Spot. Credit: Tom Stallard. 

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.

“The Juno spacecraft is currently in orbit around Jupiter and the observations of Jupiter’s aurora and upper atmosphere by the JIRAM instrument that have been released so far already provide a wealth of new information about the planet. When combined with our ongoing campaign of observations using telescopes on Earth, we hope to gain a much better understanding of this weather system in the next few years.”

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