BBC Sky at Night - UK (2021-08)

62 BBC Sky at Night Magazine August 2021

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Þ The extended

Juno mission

involves 42

additional orbits,

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of Jupiter’s moons

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‘perijove’, the point

in each orbit

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closest to Jupiter

though, as it will now be able to take a closer look at

several interesting features that the prime mission

revealed. As Juno passes over the poles, rather than

circling the equator, planetary scientists attained

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they were dominated by huge, swirling storms.

“We’ll be able to study the storms in the north in

ways never before possible,” says Bolton. “We really

don’t understand how they form, why they’re stable

or what happens over time. We’ll be able to look deep

into the atmosphere, underneath the giant vortices

and see how they compare to the large vortex storms

at lower latitudes, like the Great Red Spot.”

During the extended mission, Juno will get close

enough to use its Microwave Radiometer, which

can pierce up to 400km down through the clouds

to look at both the storms’ root structures and the

distribution of water and ammonia within them

• both important chemicals for understanding how
  the planet’s atmosphere behaves.
  “We know that Jupiter’s deep atmosphere
  changes, but how does that change happen as you
  go further north? What happens when the stripes
  that we call the zones and belts – where the winds
  go back and forth – start to change?” says Bolton.

Creating a storm map

One of the areas they’ll be paying particular

attention to is where these stripes transition into the

giant polar cyclones. Juno’s northward creeping

will allow it to create a gravitational map of these

storms. It does this by taking careful measurements

of its orbit, which can be compared to its predicted

path to see where gravity has pulled it off course. If

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and that part of the planet is denser than expected,

and vice versa.

“The zones and belts go through some kind of

transition at northern latitudes, so getting more

gravity data closer up will help investigate what

happens during that transition,” says Bolton.

Another area Juno will look into is the lightning

that jumps between Jupiter’s clouds. At lower

latitudes Juno observed this lightning occurring at

high altitudes, suggesting there must be liquid water

clouds far above where they were expected.

“It should all be frozen at the altitude we saw the

lightning,” says Bolton. “Most theories of lightning,

the kind we’re seeing, suggest it needs three phases

of something – liquid, gas and ice. There must be

some sort of liquid at the altitude of the lightning,

but it can’t be water because that would all be frozen.

We came up with the idea that the ice was being

melted by ammonia, which would act like antifreeze.”

As most of the lightning was at northern latitudes,

a closer look at the northern hemisphere will help pin

down exactly how much it is happening on Jupiter

and where it is.

Juno will also get closer to some of Jupiter’s

most dramatic features – the aurorae. Like Earth,

þ Using data from

the Juno mission,

this illustration

depicts high-

altitude electrical

storms on Jupiter

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