Transcript of The Giant Planets - Outer Worlds of the Solar System

the giant planets are the dominant players of our solar system second only to our Sun and yet for as much as we know about some of these worlds there's still much left to be discovered but first let's take a quick recap of some of the differences between the terrestrials and giant planets giant planets are much farther away from the Sun than the terrestrial planets therefore they're going to get a lot less solar energy they're certainly much more massive than the terrestrial planets and they're of course much larger now the main difference though between Giants and terrestrial planets are their very nature in the sense that there are no surfaces no solid surfaces that you can walk around everything that we see in the giant planets are really just their cloud tops but those clouds continue all the way down until maybe just reaching a rocky core if there is one we're still not 100% certain yet but there are no solid surfaces that you could walk around as you can do on the terrestrial planets as we saw in a previous video the giant planets formed farther away from the Sun and they had access to both refractory and volatile materials whereas the terrestrial planets only had access to refractory materials hence there are relatively low mass and rocky composition and giant planets have shorter rotation periods the shortest rotating terrestrial planet is Earth at just 24 hours Uranus is the longest rotater at just 17 hours that because they are farther away from the Sun giant planets get much less sunlight than we receive here at Earth remember brightness falls off as one over the distance squared that means at 5.2 astronomical units Jupiter is getting just 4/100 the amount of sunlight we receive on earth Saturn at just 1/100 the sunlight Uranus at three one thousandth and neptune at just one 1000 the sunlight that we get here on earth and the differences don't just stop there the masses of the planets are much greater than we have on earth Jupiter is more massive than all of the giant in fact all of the giant plus the terrestrial planets plus their moons combined and the radius of the planets very but they're all much larger than Earth with neptune at just under four times the radius of Earth the densities of these worlds are also very low as a matter of fact Saturn's density is less than one gram per cubic centimetre in other words Saturn will float in water assuming you can find a bathtub large enough to accommodate it the surface gravities of these planets do tend to vary now at Jupiter you would weigh a little bit more than twice your weight here on earth now you notice that Jupiter's mass is 318 times more massive than Earth and yet its surface gravity is just two point four times the gravity that you would feel at the surface of Earth so why wouldn't you weigh more remember that your weight is proportional to the planet's mass and the radius you are from the planet's center so on the one hand Jupiter is a very massive planet but on the other hand it's a very large planet so if you were let's say floating around in a dirigible in the clouds of Jupiter and you were standing on a platform you'd find that you only weigh two point four times your weight on earth but if you were floating in the clouds of Saturn Uranus and Neptune you would weight pretty much what you weigh here on earth so you'd be relatively comfortable now the rotation periods of these planets are also very short Jupiter's day is just under 10 hours for example and their oblique Matiz or their axial tilts are kind of all over the place Saturn and Neptune's axes are tilted at roughly the same angle as Earth's a Jupiter has very little axial tilt to speak of and Uranus has an extremely axial tilt as if though it were essentially knocked on its side so what does this mean for the seasons on these planets well in the case of Jupiter not very much the Jupiter's axial tilt is only three degrees therefore it's getting roughly the same even amount of lighting in both its northern and southern hemispheres so there's little if any seasonal variations to speak of you could think of it as essentially springtime on Jupiter all Jupiter year-round but Uranus on the other hand is knocked over on its side and it has an extreme obliquity and because it has a very long orbital period this gives rise to some very long and very extreme seasons so in 1986 when Voyager 2 was passing by it was summer in Uranus's Northern Hemisphere winter in its southern hemisphere but by 2007 spring had come to Uranus and now we have even lighting in both of its hemispheres but by 2028 the entire northern hemisphere of Uranus will be in complete darkness as it's summer hemisphere begins it's summer so that means that Uranus's North Pole spent 42 years of continuous daylight while its South Pole spent 42 years in darkness I mean talk about the phrase winter is coming if you live on Uranus you're going to experience a very long very dark winter now because these planets have such short rotation periods their centrifugal forces kind of stretch them out into ellipsoidal shapes so they get these oblate appearances particularly Jupiter and Saturn because their rotation periods are a little bit shorter than Uranus and Neptune now remember we also categorize these planets based on their compositions so Jupiter and Saturn are called gas giants while Uranus and Neptune are referred to as ice giants so let's compare their compositions very briefly first we're going to start with the gas giants they're both basically composed of the exact same thing they have hydrogen atmospheres followed by a layer of liquid hydrogen a layer of liquid helium rains down onto a layer of highly compressed hydrogen and this hydrogen is under such tremendous pressure about a hundred million times the surface pressure on earth that the hydrogen can conduct electricity as though it were a metal so we call this layer metallic hydrogen planetary scientists believe that there may be a layer of ices of highly compressed liquid ammonia water and hydrogen ices surrounding Saturn's core but the inner cores of the planets are still very much a mystery we don't really know what lies at the very centres of these worlds at these pressures they may solidify into something or they may be a collection of rocks and ices it's still an open question now let's take a look at the ice giants Uranus and Neptune are essentially twin planets there's virtually little difference between these two worlds both have ammonia cloud layers followed by a an atmosphere of largely molecular hydrogen and then underneath that is a ocean of water methane and ammonia ices now this is not to say a layer of solid ice but rather it's an ocean of mostly liquid water and methane and ammonia and there may be some regions where it gets cold enough to become slushy and even icy in some areas whether or not they're rocky cores at the center of these planets again is an open question we're not really sure so the main difference between the gas giants and the ice giants is what they are made of gas giants are hydrogen and helium mostly and they're really very much in the same proportion of hydrogen helium as our Sun so you can think of the gas giants as kind of mini Suns that never switched on but the ice giants well the s they are mostly hydrogen and some helium but they're really dominated by water methane ammonia hydrogen all of those really highly volatile materials that require very cold temperatures to condense we mentioned before of how the giant planets are getting much less sunlight than the terrestrial planets do however they are much warmer than they should be so when you compare the theoretical temperatures of these planets in blugh to the actual temperatures which are shown in green you find that Jupiter Saturn and Neptune are considerably warmer than they should be and the reason for this is because they are all warmed by internal heat with the exception of Uranus which doesn't seem to have any internal heat to speak of so the sources of this internal heat vary from one planet to the next now Jupiter is shrinking by about two centimeters per year and that doesn't seem like a whole lot but remember Jupiter is very very big that means that there's still a lot of material that is contracting and as this stuff contracts it releases energy in other words it builds heat up and this is what's believed to be the primary driver of Jupiter's Heat likewise Saturn is releasing heat at a rate of about twice that which it receives from the Sun and this is believed to be a combination of gravitational contraction primordial heat leftover but also the fact that Saturn is still differentiating we talked about some of that helium rain for example and what we believe is going on is that there are still some heavy or rather dense materials that are still sinking into Saturn's core all this time later and releasing heat in the process now Uranus is essentially in thermal equilibrium from the Sun but Neptune on the other hand is perhaps the biggest surprise of this outer solar system it radiates 2.6 times the amount of energy it gets from the Sun and we don't know why it is a mystery waiting to be resolved now Jupiter and Saturn's magnetic fields are very powerful in fact Jupiter's magnetic field is the most powerful of any planet in the solar system and both Saturn and Jupiter's fields are more or less aligned with their rotation axes now Uranus and Neptune magnetic fields are really weird they're both tilted significantly from the rotational axes and yet stranger than that they are also knocked off axis they're knocked off the centres of the planets so they're lopsided magnetic fields and we really don't yet understand why but this tells us that there's something fundamentally different about the way the magnetic fields operates in the ice giants than they do in the gas giants in fact we could take a quick visualization of the magnetic field so if we have these yellow arrows facing the Sun and the magnetic axis represented in cyan you could see that Jupiter and Saturn's fields are pretty much Co rotational with their rotation axes kind of what you expect and really just enlarged versions of the magnetic fields that we have here on earth but when we get to Uranus and Neptune fields yeah they're just really off axis and also off center so again this is something fundamentally weird about these two worlds now Jupiter's magnetic field is the largest and strongest of any object in the solar system except for the Sun and this magnetic field is buffeted by the solar wind so it creates this massive magneto sphere that surrounds Jupiter and extends far out into deep space in fact it could easily extend out to the orbit of Saturn and probably goes even farther and it's interesting to think about because if we could somehow see Jupiter in magneto vision from Earth we would find it's enormous it would be a little bit longer than the diameter of the full moon so it's an extremely powerful magnetic field and Jupiter's magnetic field is the source of spectacular Aurora especially when viewed at ultraviolet and here's a movie of Saturn taking an ultraviolet light and if you look along the northern and southern polar regions you can see the lights caused by charged particles from the Sun funneling into the magnetic field lines and reacting with gases and Saturn's atmosphere and we've even captured Aurora on Uranus and remember Uranus's magnetic axis is off-center it's lopsided and off-center and sure enough the Aurora are occurring kind of away from the rotation axis of Uranus let's talk about the atmospheres of these worlds now Jupiter and Saturn essentially have the same compositions they're both made of primarily hydrogen and some helium and yet Jupiter has these beautiful vibrant clouds and bands whereas Saturn's are more muted they're a little less spectacular so what accounts for the difference well these atmospheres are largely composed of the same things made namely ammonia and ammonium hydrosulfide and water ice cloud layers but if you notice these players on Jupiter are kind of packed a little bit tightly together almost like one on top of one another like a kind of a cloud layer sandwich whereas on Saturn these layers are spread out over a much wider altitude range so that tells us that Saturn's atmosphere is much less dense than Jupiter's the cloud layers just don't stack up allowing us to see them very easily and not only that but the cloud layers on Saturn form at much lower altitudes so we have to see through more of that upper haze layer before we can start to see the cloud layers and Saturn so it's not that the chemical compositions are fundamentally different what these cloud layers rather are arranged a little bit differently because of the low density and because of the lower atmospheres at which they form we really don't get to see very much in the way of Saturn's cloud layers but this doesn't mean that Saturn lacks any features that are interesting I mean take a look at this this is a magnificent image of Saturn's polar region there's this hexagonal jet stream that flows around Saturn's polar caps much like the jet stream on earth does as well except it's at a much larger scale now Uranus and Neptune Tzar kind of featureless by comparison remember Uranus lacks any internal heat so there really isn't any convection and so this gives you Ernest a very clean stable atmosphere but Neptune on the other hand has internal heat so there is some convection and we do get to see some atmospheric features albeit not as dramatic as some of the ones that we see on Jupiter and even on Saturn so let's take a look at their atmospheric compositions well the planets are essentially twins of one another and they have essentially the same atmospheric composition namely methane and diphosphate cloud layers with a haze layer of largely hydrocarbon ices methane and ammonia Isis and the atmospheres and it's these hydrocarbons that give these plants their characteristic blue aqua colors essentially they absorb red light and scatter blue light everywhere now the wind and weather of Jupiter in particular is the most spectacular of any planet that you're going to see in the solar system I mean it just kind of looks like a mesmerizing kaleidoscope of for taxis and swirls and hands and winds and just all kinds of complex weather patterns this image was taken by the Juno spacecraft it's a spacecraft that's orbiting very close to Jupiter allowing us to get these really nice detailed images and when we enhance these images to really bring out different details you can just see that how complex this stuff really gets and Jupiter short rotation and heat convection give rise to these dramatic east/west zonal winds so this is all due to the rotation and convection and remember there's no surface on any of these planets for the winds to kind of lose energy against so they just keep going and going particularly on Jupiter which is rotating so rapidly in fact if you take a look at the speeds themselves as a function of latitude you can see where they are blowing the fastest which is largely around the equatorial regions and as you get up into the polar regions the wind speed variations aren't nearly as dramatic now let's scale this down and compare that to Saturn despite the fact that Saturn doesn't seem to be as active a planet as Jupiter it boasts some of the fastest winds in the solar system around Sacre toriel region by comparison Uranus's winds are very stable so because of a lack of convection there is generally a lack of wind variation they're essentially symmetrical around the poles but Neptune once again was the big surprise its winds were clocked as supersonic I mean these were just the most powerful winds detected anywhere in the solar system and keep in mind that Neptune is just getting one one thousandth the sunlight that we get here on earth so Neptune's got an energy source that's driving some of the most spectacular weather anywhere in the solar system and that weather manifests itself in storms so these storms such as Jupiter's Great Red Spot are essentially high pressure regions and that's very different than the kind of storm that we get here on earth which are mostly low pressure regions and these storms are gonna be fed by the convection and the rotation and remember there's no land and so these storms can rage for a very long time and in fact Jupiter's Great Red Spot has been visible to astronomers ever since telescopes were first used so that means that at a minimum the storm has been going for about 200 years but some estimates suggest it may have been around for as much as 500 years at least and this is the largest storm in the solar system in fact once upon a time it was three times the size of Earth but over time it shrank and now it's just only a little bit larger than Earth so the Great Red Spot is shrinking and it's getting less oval it's becoming more circular and we don't yet understand what the fate of the Great Red Spot will be will it continue to shrink until it finally dissipates or will it reach some kind of equilibrium and stabilize it's something that we are still trying to figure out now the Great Red Spot is also apparently a source of heat in other words it's kind of like a gaping region where heat from the interior of Jupiter can escape and this is why Jupiter's atmosphere is even hotter than it otherwise should so the Great Red Spot seems to be a source of heat and maybe there's a relationship between the amount of heat it's releasing and its contraction but storms like the Great Red Spot aren't just limited to Jupiter Saturn itself has been known to display its fair share of storms and in 2010 the Cassini spacecraft which was at the time orbiting Saturn noticed a storm breaking out in its northern hemisphere and the storm got larger and larger and began to spread eastward and eventually wrap itself around the entire planet so these storms can sometimes become planet engulfing phenomena in 1986 Voyager 2 flew past Uranus and didn't really see much of anything going on three years later it passed by Neptune and found some spectacular weather so for a while we thought that maybe Uranus was always going to be a featureless world but as Uranus approached its Equinix its southern hemisphere began to wake up and with it some stormy weather these are these storms that you see are about the size of the continental United States so it's possible that Uranus is just as active and just as magnificent as Neptune we just caught it at the wrong time when we flew past in 1986 now Neptune on the other hand had the fastest winds ever detected so this was a real surprise considering that there is such little sunlight reaching this planet and we even found a great dark spot in Neptune southern hemisphere however by 1994 that dark spot had already disappeared and then by 2016 a new dark spot reappeared so Neptune's still undergoing some really interesting weather changes that were still trying to understand we know a lot about Jupiter and Saturn but very little about Uranus and Neptune and it would be great to get some spacecraft out there someday if we are willing to do that but there's something really amazing about these worlds still waiting to be discovered