What NASA's Juno Discovered around Jupiter So Far

  

I don’t know where the time has gone, but it is now been 4 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, orits 21st polar orbit, out of a total of 35 planned orbits, which means we are now well past the halfway point of this mission. 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 4000 km 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, 20 m 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 tera watts. Like Earth, aurora appear as bands around the north and south poles, but unlike Earth, these aurora 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 aurora, 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 3000 km down, and beneath this is an ocean of metallic hydrogen going all the way down to the center, 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 could not 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 photo chemical 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 localized 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 350 km, 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 80 km, 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 has not 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!

 Thanks for reading my article and see you next time. 

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