On Dec. 11, NASA’s Juno spacecraft was performing its third close flyby of Jupiter when it captured the image below. It was about 15,300 miles away from the gas giant at the time.
The image dumps from Juno are slower than most of us expected, but the images we are getting are still impressive. Here, we see one of Jupiter’s “pearls.” A string of eight white oval storms located in the planet’s southern hemisphere. That number has fluctuated between six and nine since 1986.
Seeing this image got me thinking. Why is this storm white while the Great Red Spot is well, red?
The answer is still being debated, but a discovery in 2014 points to the storm cloud’s height. Before 2014, the leading theory pointed to reddish chemicals coming from beneath Jupiter’s swirling clouds. But data from Cassini’s Jupiter flyby in 2000 along with laboratory experiments suggest it’s a case of sunburn.
Kevin Baines (Cassini team scientist) and several JPL colleagues trained ultraviolet light on ammonia and acetylene gasses (both present on Jupiter) to simulate the sun’s effects on these materials at the extreme cloud heights seen at the Great Red Spot. The UV light created a red material, which the team compared to data gathered by Cassini’s Visible and Infrared Mapping Spectrometer (VIMS).
“Our models suggest most of the Great Red Spot is actually pretty bland in color, beneath the upper cloud layer of reddish material,” said Baines. “Under the reddish ‘sunburn’ the clouds are probably whitish or grayish.”
Baines’ theory doesn’t mesh well with the previous leading theory at all. If the red color came from chemicals beneath the visible cloud layers, it should be present at other altitudes too according to Baines.
The discovery first started as the researchers were looking into how the sun would breakdown ammonium hydrosulfide (a more complex molecule found in one of Jupiter’s main cloud layers). Their experiments were producing a green material. That’s when they started looking into ammonia (and hydrocarbons) at high altitudes. It produced the reddish material they were looking for and was the best fit for the Cassini data.
Height appears to be one of the key reasons in why certain storms on Jupiter are red. “The Great Red Spot is extremely tall,” says Baines. “It reaches much higher altitudes than clouds elsewhere on Jupiter.”
Other clouds showing a mixture of oranges, browns and reds are areas where the clouds are much thinner, according to Baines. We are getting a deeper view into the atmosphere.
The white storm clouds we see in the picture above just aren’t as high as the Great Red Spot. At least, that’s what data from Cassini’s flyby in 2000 along with Baines’ experiments suggest.
A more definitive answer for what causes the different colors could come from one of Juno’s many close flybys of the gas giant. Juno’s instruments will peer beneath the cloud tops and map the atmosphere’s composition, temperature, clouds and movement patterns at depths never before seen.
As for when we’ll see the first images that will truly blow us away? Hopefully soon. Juno’s mission is a little different than New Horizons. The camera (JunoCam) was added more for public engagement than for science gathering. It’s not seen as a main science instrument aboard the spacecraft. We’ll still see stunning images, but the data is what NASA is after.