Sky and Telescope - December 2017 - 16

p LO, THE GREAT RED SPOT JunoCam took this true-color image (top) from 13,917 km (8,648 mi) above the cloudtops on July 10th.
Contrast enhancements and white-balancing of another image from the
same pass (middle) reveal glorious details the eye might overlook.

This dramatically smaller perijove gives Juno a much different perspective than the Voyager spacecraft had. The twin craft
took weeks-long movies of Jupiter's dramatic global weather
systems, showing bands of white clouds zooming left to right
and deep red bands streaming right to left. It's interesting to
contrast these Voyager movies with the still views Juno is getting 38 years later. On a spinning spacecraft and a 2-hour poleto-pole dash, Juno's camera sees only snapshots of atmospheric
activity. But these close-ups reveal small-scale structures that
were blurred at Voyager's distance from the planet.
In fact on every scale, down to the 50-km resolution of
JunoCAM, we see structure in Jupiter's atmosphere - vortices,
thunder clouds, waves, and turbulence. In retrospect, we probably shouldn't have been so surprised, since nature across the
universe tends to get more structured when we zoom in.
Jupiter is more than 11 times wider than Earth, yet it spins
around in just under 10 hours - the shortest day of the major
planets. This rotation drives the dynamics in the shallow,
outermost layer of the mid-latitude atmosphere, where the
multiple red "belts" and white "zones" mark striped weather
systems. Convection and turbulence spur small-scale structure, and spots grow by merging together. We've seen largescale order emerging from small-scale chaos in a number of
planetary atmospheres, but Jupiter is the best example.
Why has Jupiter's rotation not smoothed out this smallscale structure? If you put milk in your coffee and give it a stir,
it quickly becomes a uniform mixture. Jupiter has been "stirring" for billions of years, so what causes these structures to
persist? The answer is similar to the ongoing activity in Earth's
atmosphere. On Earth, the water cycle feeds weather patterns,
keeping our atmosphere in a constant state of transformation.
On Jupiter, ammonia plays a similar role as water on Earth. As
a blob of air with ammonia vapor rises, it cools. Eventually the
vapor condenses, releasing energy into the surrounding gas,
making the gas rise higher in the atmosphere. Such feedback
systems persistently drive small-scale structures and prevent
the atmosphere from mixing into a great bland ball.
And then there's the Great Red Spot. On July 10, 2017,
Juno ﬂew directly over the giant storm that astronomers have
observed since at least the 19th century (S&T: Mar. 2016,
p. 18). More than 16,000 km across but less than 300 km
deep, the GRS is like a spinning New York pizza - except it's
made out of hydrogen. Voyager 2's movie of the Great Red
Spot shows the vast storm gobbling up smaller eddies and
sometimes spitting them out again. Juno's zoomed-in pictures
show turbulent structure within the GRS's twirling vortex.
To the south of the GRS, small, fresh, white clouds poke up
like monsoon thunderheads on an afternoon in the Rocky
t IN MICROWAVES The cut-out shows data from Juno's Microwave Radiometer, which detects microwaves emitted from ammonia in Jupiter's atmosphere down to 350 km. Colors correspond
to abundance, with orange being more and blue less. Scientists
had thought the ammonia would be uniformly mixed, but the data
reveal a deep circulation pattern, including an equatorial band (the
prominence-like column).

D E C E M B E R 2 0 17 * S K Y & T E L E S C O P E

TOP: N ASA / JPL- CA LTECH / SWRI / MSSS / B JÖR N JÓNSSON; MIDDLE: CREDIT: N ASA / SWRI /
MSSS / G ER A LD EICHSTÄ DT / JUSTIN COWA RT; BOT TON: N ASA / JPL- CA LTECH / SWRI

Juno Update

Sky and Telescope - December 2017

Table of Contents for the Digital Edition of Sky and Telescope - December 2017