Astronomers have been characterizing Jupiter's Great Red Spot, a brilliant, swirling storm located just south of the planet's equator, for the better part of three centuries. Now, scientists say they have found another great spot on Jupiter at higher latitudes.

The "Great Cold Spot" is defined by its temperature relative to the surrounding upper atmosphere on the planet. Temperatures in Jupiter's upper atmosphere range from about 700 to 1,000 Kelvin, and the relatively well-defined cold area is about 200 Kelvin cooler. Measuring about 24,000 kilometers in longitude and 12,000 kilometers in latitude, the cold spot is nearly as large as the more famous red spot.

So what has caused it? Planetary scientists aren't yet sure, as much of Jupiter's atmosphere remains a mystery. "The detection of a localized region of cooling within the upper atmosphere is unexpected," the authors of a new study, published in Geophysical Research Letters, reported this week. Previous observations have been limited in their spatial resolution, especially at higher latitudes, and as a result scientists had thought variations in temperature would be more smoothed out.

But as ever, the Solar System continues to surprise. The study authors posit that the cold spot is likely related to the planet's nearby powerful auroras that drive energy into the Jovian atmosphere and spur heat flows around the planet. However the relationship is not entirely clear, as the Jovian auroras are themselves a complex feature due to changes in the Solar wind, outgassing from volcanoes on the moon Io, and other processes.

In their observations of the cold spot, astronomers see it disappearing from time to time, but then it reappears with sometimes significant changes in position and morphology. "It is likely that the spot represents a weather system that has been regenerated time and again, for as long as Jupiter has had its northern magnetic field asymmetry," the authors state. "We do not know how long this is, but given the slow pace of change within the Earth's magnetic field, the Great Cold Spot has existed for thousands of years, and perhaps much longer."

Fortunately, NASA's Juno probe is presently in orbit around the polar regions of Jupiter. As it completes several dozen flybys over the next few years, Juno scientists should be able to collect ample data to complement observations made by Earth-bound telescopes and shed some light on this cold case.