With Hubble and Gemini observing Jupiter more frequently during the Juno mission, scientists are also able to study short-term changes and short-lived features like those in the Great Red Spot.
Images from Juno as well as previous missions to Jupiter revealed dark features within the Great Red Spot that appear, disappear and change shape over time. It was not clear from individual images whether these are caused by some mysterious dark-colored material within the high cloud layer, or if they are instead holes in the high clouds — windows into a deeper, darker layer below.
Now, with the ability to compare visible-light images from Hubble with thermal infrared images from Gemini captured within hours of each other, it is possible to answer the question. Regions that are dark in visible light are very bright in infrared, indicating that they are, in fact, holes in the cloud layer. In cloud-free regions, heat from Jupiter’s interior that is emitted in the form of infrared light — otherwise blocked by high-level clouds — is free to escape into space and therefore appears bright in Gemini images.
“It’s kind of like a jack-o-lantern,” said Wong. “You see bright infrared light coming from cloud-free areas, but where there are clouds, it’s really dark in the infrared.”
Jupiter’s Great Red Spot
The above images of Jupiter’s Great Red Spot were made using data collected by the Hubble Space Telescope and the Gemini Observatory on April 1, 2018. By combining observations captured at almost the same time from the two different observatories, astronomers were able to determine that dark features on the Great Red Spot are holes in the clouds rather than masses of dark material.
Upper left (wide view) and lower left (detail): The Hubble image of sunlight (visible wavelengths) reflecting off clouds in Jupiter’s atmosphere shows dark features within the Great Red Spot.
Upper right: A thermal infrared image of the same area from Gemini shows heat emitted as infrared energy. Cool overlying clouds appear as dark regions, but clearings in the clouds allow bright infrared emission to escape from warmer layers below.
Lower middle: An ultraviolet image from Hubble shows sunlight scattered back from the hazes over the Great Red Spot. The Great Red Spot appears red in visible light because these hazes absorb blue wavelengths. The Hubble data show that the hazes continue to absorb even at shorter ultraviolet wavelengths.
Lower right: A multiwavelength composite of Hubble and Gemini data shows visible light in blue and thermal infrared in red. The combined observations show that areas that are bright in infrared are clearings or places where there is less cloud cover blocking heat from the interior.
The Hubble and Gemini observations were made to provide a wide-view context for Juno’s 12th pass (Perijove 12).