Jupiter’s Great Red Spot is not only big and red. It’s also hot.

Using a telescope on Earth, astronomers peered at infrared emissions from Jupiter and found that the temperature of the upper atmosphere, 350 to 600 miles above the giant swirling storm, averages 2,500 degrees Fahrenheit.

That finding, reported on Wednesday in a paper published in the journal Nature, is the latest piece of a puzzle that has been confusing planetary scientists since 1973 when NASA’s Pioneer 10 spacecraft flew by and took the first temperature measurements of the solar system’s biggest planet.

By their calculations, scientists expected that the warmth of sunlight impinging on Jupiter should heat the upper atmosphere to a cool -100 degrees. Instead, the temperature was about 1,000 degrees.