I don’t know where the time has gone, but it’s now been 3 years since Juno arrived
at Jupiter.
During this time, it has been collecting valuable and insightful data about the largest of our
neighbour planets.
It has recently completed Perijove 21, or its 21st polar orbit, out of a total of 35
planned orbits, which means we are now well past the halfway point of this mission.
Some of you veterans to this channel will remember the video I made about Juno at its
one year mark, but what has it discovered since then, and has it disproved some of the
assumptions we had about Jupiter from before it arrived?
I’m Alex McColgan, and you’re watching Astrum, and together we will go through everything
Juno has discovered and seen around Jupiter so far.
There was some scepticism about whether Juno would last this long, due to the intense radiation
around the planet, but Juno is currently in good health.
Its polar orbit takes it very close to the planet, only 4000km above its atmosphere,
meaning it avoids most - but not all - of Jupiter’s plasma torus, or this region of
extremely energised particles, particles which have been trapped in place by Jupiter’s
powerful magnetic field.
But thankfully, Juno quickly discovered that the radiation where it orbits was a lot weaker
than initially expected.
This means that even the camera is still operational, which was one of the first instruments expected
to go.
Juno completely surprised scientists though by also discovering another, small and less
powerful radiation belt right above the equator, which hugs the planet tightly.
So far, the mechanisms behind this radiation belt are unknown.
However, although the radiation exposure hasn’t been as bad as scientists expected, due to
the nature of Juno’s orbit, every passing Perijove takes it more and more into the main
radiation belt, meaning Juno certainly can’t last forever, and Perijove 35 is currently
when mission controllers believe the mission will be forced to end, whereupon they will
crash Juno into Jupiter to avoid any future collisions with Europa.
The charged particles in the plasma torus come particularly from the volcanic activity
of Jupiter’s closest large moon, Io, which blasts particles into orbit around Jupiter.
Just to give you an idea of how volcanically active Io is, this was New Horizon’s view
of Io as it passed by Jupiter on its way to Pluto, with the Tvashtar volcano in a full
eruption.
Juno has also had a look at Io in the infrared, the hot spots indicating where volcanic activity
is occurring.
Io ejects one ton of particles into orbit around Jupiter per second.
As Io travels through the plasma torus and interacts with Jupiter’s magnetosphere,
this causes a flux tube to exist between the planet and the moon, a flux tube being an
electric current that travels along a cylindrical tube of magnetic field lines.
It is very powerful; it can develop up to 400,000 volts and one million to five million
amps of current.
Juno was able to get very accurate readings of the flux tube during its 12th orbit, as
it passed directly through it.
No, this didn’t fry the spacecraft, as the flux tube has a large diameter and so it isn’t
concentrated enough to do damage to the craft.
Also, Juno was in and out of it in a matter of seconds.
Juno is a massive spacecraft, 20m in diameter.
And it really has to be, as it is a solar powered spacecraft, and it only gets 4% of
the Sun it would do around Earth.
This means even though these panels are huge, it can only generate just above 400 watts.
But you’ll also notice that this design paired with the fact that Juno rotates makes
it look a bit like a fidget spinner.
This isn’t just to make a pretty spinning spacecraft.
Juno was specifically designed to detect various fields and particles around Jupiter, and having
a spacecraft with a large, spinning radius helps with that.
This is particularly evident with this instrument here, the magnetometer found at the end of
one of the solar panels, tasked with mapping out Jupiter’s magnetic field.
Through Juno’s data, we now have a highly detailed map of Jupiter’s magnetic field
which is only getting more accurate with every passing orbit.
As expected, Juno confirmed that Jupiter has a dipole like magnetic field, although it
is not very aligned which the rotational axis.
What was very interesting though is that scientists discovered something called the Great Blue
Spot, a region on Jupiter where the magnetic field is very concentrated.
Comparing Juno’s magnetic field data with previous Jupiter missions, like Pioneer, Voyager
and Galileo has also revealed a first for the solar system.
Jupiter’s magnetic field structure has been found to change very gradually over time,
which is called secular variation.
Interestingly, this was most apparent around Jupiter’s Great Blue Spot.
This variation is thought to be driven by a region right at the base of Jupiter’s
atmosphere, which we’ll get to in a bit.
A combination of the powerful magnetic field and the charged particles in the plasma torus
means that Jupiter has the brightest aurora in the solar system, with a radiant power
of 100 terawatts.
Like Earth, aurora appear as bands around the north and south poles, but unlike Earth,
these aurorae are mainly visible in the ultraviolet, and are mainly produced from alternating currents,
not direct currents.
When Juno measured the power generated from the direct currents in Jupiter’s magnetosphere,
it was nowhere near enough to account for the brightness of the aurora, leading scientists
to speculate that the remainder of the power is coming from alternating currents.
At this time, it is believed that these alternating currents are produced because of the turbulence
in the magnetic field.
Especially at the north pole, the magnetic field lines are much more complex, which interferes
with a direct flow of currents.
This is evident when comparing the North and South Pole aurorae, at the North the aurora
is much more dispersed, looking more like filaments and flares, whereas at the South
Pole where the magnetic field lines are smoother, the aurora seems to be more structured and
round.
What you will also notice is this bright spot and tail in the aurora.
This is visibly where the Io flux tube meets the planet.
What is slightly less apparent though are these other spots.
These are from the other large moons in the Jovian system, Europa and Ganymede.
So, while not as powerful as Io’s flux tube, these other moons also have their own flux
tubes connecting them to the planet.
The magnetic field of Jupiter brings us nicely to one of the main science goals of Juno,
to figure out the interior of Jupiter.
Since Juno arrived, previous theories have had to be completely thrown out the window
by the data it has collected.
Previously, it was thought that there was a solid core, then a sharp cut-off line between
the core and the next layer, the metallic hydrogen layer.
The cloud layer was then only thought to be a few hundred kilometres deep at most.
But based on the Juno data, the atmosphere of Jupiter extends to 3000km down, and beneath
this is an ocean of metallic hydrogen going all the way down to the centre, and even if
there is a core, it is very fuzzy, potentially mixing up with the metallic hydrogen layer.
So actually, to call Jupiter a gas giant is a bit disingenuous, as 80-90% of its radius
is believed to be a liquid now, or technically an electrically conducting plasma, perhaps
similar in appearance to liquid mercury.
Here, the pressure is so great that the hydrogen doesn’t retain its molecular structure with
2 combined protons and electrons, and instead they separate meaning positive and negative
charges can move about, becoming an electrically conducting substance.
We say believe, as we haven’t been able to recreate metallic hydrogen in lab conditions
yet, the pressure needed is millions of times greater than the atmospheric pressure of Earth.
Although we assume this must be the case, due to Jupiter’s powerful magnetic field.
To create a magnetic field of this strength, the dynamo must originate in an electrically
conducting substance.
It can’t be a denser metal like iron in Earth’s core, because Jupiter doesn’t
have the density for that.
In fact, based on its density, we know it must be made primarily of hydrogen and smaller
amounts of helium, very similar in composition to the Sun.
Another factor for the strength of the magnetic field is due to the rapid rotation of Jupiter.
One day on Jupiter only lasts about 10 hours.
Various forces from this stir the liquid up, which generates the dynamo.
It is the rotation of the magnetic field from which we can measure a day on Jupiter, as
simply viewing Jupiter’s visible bands couldn’t give you a definitive result, and this is
why.
You’ll notice that these bands look very peculiar, moving in opposite directions from
each other and at different speeds.
But this isn’t so unusual if you also consider the invisible jet streams on Earth.
What is striking though is the colours and turbulence found in these bands, so let’s
try and understand what’s going on from examining these Juno images.
The cloud layer you are seeing here is the ammonia cloud layer.
Some are white, these represent fresh clouds likely only recently pulled up from the deeper
parts of the atmosphere.
On the other hand, while the red colours you see are also ammonia clouds, these clouds
have interacted with UV light from the Sun.
Think of it like a photochemical smog, the reddish smog you see in summer over large
cities.
The colouring substance isn’t exactly known, but simply put, the longer it is exposed to
the Sun, the redder it gets.
Interestingly though, comparing these bands to what you see at the poles, you’ll notice
it is a lot bluer here.
This could be because UV light doesn’t reach here as easily compared to the equator.
Looking closely, you’ll also notice what is known as pop-up clouds.
Initially, these were thought to maybe be water ice clouds, but they could be ammonia
clouds too.
They are potentially the precursors for thunderstorms on Jupiter.
The radio wave instrument onboard Juno does detect lightning on Jupiter; however, these
storms are interestingly more localised towards the poles than at the equator, and more towards
the North Pole than the South Pole.
The cause for this is also unknown.
Closely looking at Jupiter, you’d be hard pressed not to notice the stunning vortexes
and storms across the planet.
Juno has had the opportunity to orbit directly over the Great Red Spot, where it discovered
something very interesting.
It was known that the Great Red Spot rises high above the cloud deck, but what scientists
didn’t expect is how deeply it penetrates Jupiter’s atmosphere.
The instrument on onboard Juno designed to peer into the atmosphere has a range of 350km,
and it seems the Great Red Spot extends down even further than that.
Also interesting is that the Spot is cooler than the surrounding area up until a depth
of 80km, and beyond that it actually gets warmer than the surrounding area, this heat
perhaps driving the storm.
It has been theorised that the Great Red Spot is a permanent feature on Jupiter, but we’ve
only had about 400 years to observe it so far, a mere blink in astronomical timescales.
Looking over the poles, other possibly permanent features have been observed.
In contrast to Saturn, which has a hexagon on one pole and a single vortex on the other,
Jupiter has five vortexes around the South Pole and 8 around the north.
It’s hard to say exactly how permanent these storms are as Juno has only been there for
three years, Juno was the first time we have really been able to have a good look at Jupiter’s
poles, but they have been reasonably constant throughout that time.
Under the ammonia cloud layer is thought to be a water ice cloud layer, although this
has not yet been confirmed as this layer hasn’t actually been seen yet.
This is one of the science goals of Juno though, and it has several microwave detectors to
try and find this elusive substance.
Jupiter generates heat from within, which can be seen through an infrared camera, the
densest parts of the cloud layer blocking some of this heat from being visible.
Similarly, Jupiter also emits microwaves, which hypothesised water clouds would absorb.
So, in theory, Juno should be able to detect where the water is present in Jupiter’s
atmosphere by searching for where Jupiter’s microwaves aren’t visible, although this
data has either not been released or nothing has been found yet.
All that being said, Juno still has a while to go with this mission, and no doubt the
data it collects will be examined for years to come.
Our understanding of Jupiter is gradually increasing, and with this knowledge comes
better understanding of how our solar system formed, and also that of other solar system’s
with Jupiter sized worlds.
And who knows, maybe Juno will surprise us a few more times yet!
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