The Ice Giants Planets and their importance

There are two enigmatic giants in the cold periphery of our solar system, very far away, in fact Uranus is 19 times further than the earth from the Sun, and 30 times that in the case of Neptune, which also means that they take a long time to complete an orbit, a year on Neptune amounts to 165 years on earth, which also means that they are very cold, parts of their atmosphere can reach temperatures below - 200 ᵒ Celsius and given their locations it makes them the exotic worlds that they are, and example of this is their striking colour which comes from having think atmospheres rich in methane that absorbs red light but reflects the green and blue. Both of these planets are clearly very different from others in the Solar System.

And they are waiting for scientist to revealed more of their secrets, are they ready to give them up now? There is a growing interest in recent times for a for a return mission to ice giants Uranus and Neptune, however it has to be said that at the time when the space probe Voyager 2 was launched in 1977, the planets were aligned in a very special way which permitted that we could fly by Jupiter get a gravitational assist fly by Saturn, than get another gravitational bust and fly by Uranus and then reach because of that Neptune, all giant planets were visited in one fly, there have not been such an alignment since and won’t be another for many decades. 

At the time of the flyby of Voyager 2 few surprises were revealed, perhaps we were spoiled by the wonders of Jupiter and Saturn than when we saw these outer most worlds we were a bit disappointed, but today we have come to realized their importance and the reality is that there is still much we don´t know, that together with the fact that there are new and more powerful rockets in the horizons by NASA paints an interesting picture for the near future. So in August last year, NASA’s Jim Green gave engineers at the Jet Propulsion Laboratory in Pasadena, in California one year to figure out how and what it would take to go back there and put a spacecraft in orbit around Uranus or Neptune.

Uranus as a featureless disc, photographed by Voyager 2 in 1986. Credit: NASA

This is what Green, director of the Planetary Science Division at NASA said: “These worlds are an important frontier, we really don’t know much about them. This is a really exciting time for us to be able to study them.” So given what we have learned from the newly exoplanet discoveries and the future capacities of the new rocket designs it has made these distant worlds more relevant than ever.

We have basically four giant planets in our solar system, Jupiter and Saturn, Uranus and Neptune, years ago they were all regarded as the ‘Gas Giants’ but have since learned that the internal structures of Uranus and Neptune are substantially different than the interior structures of Jupiter and Saturn and hence we now call the first two ‘Ice Giants’. 

The main difference is that Jupiter and Saturn have very thick atmospheres of hydrogen, in Uranus and Neptune there are also hydrogen atmospheres too, however here they are relatively thin layer in comparison. In fact, the interior of both Uranus and Neptune have a very thick mantle of icy briny material of water, methane and the like making then fundamentally different from the gas giants planets, and so that makes three types of planets in the Solar System, the terrestrial ones, the gas giants and the ice planets. 

Interior models of the giant planets. Wikipedia

Kepler Space mission has revealed lots of new planets of which many are ice giants, making them a very important type of planets given their abundance. Mainly we have discovered that they are Earth like planets, Super Earth planets, Gas Giants, Ice Giants, and Super Gas Giant planets, and one of the most common types happens to be the ice giant planets. 

In fact, our gas giants may be one of the driving forces in crafting the Solar System as we know it today, there is a model called Nice from French astronomers whom developed a theory to explain the Solar Systems as we see it today, and which basically states that the planets did not formed where they are today, they migrated long after the dissipation of the initial protoplanetary gas disk.

A planetary disk, arte by Pat Rawlings, NASA 

As the solar system was evolving Jupiter migrated slowly inwards, Saturn migrated slowly outwards and at one point they came into a resonance, in other words for every time Saturn when around the sun Jupiter when around twice, so when that resonance happened Neptune got kicked out and was moved out and Uranus took its current location, so Uranus and Neptune switched places, causing a swirl around to all the small objects in the Outer Solar System, asteroids, comets, Kuiper belt objects throwing them into the inner solar system accounting for the late heavy bombardment period. It is also believed that perhaps water was carried to the inner Solar System this way, making the earth habitable. 

The question is, why are these planets like so? The answer could tell us about how the other planets formed. Our planets in the Solar System are composed of unique combinations of chemicals, whilst the gas giants are made almost entirely of hydrogen and helium, which is the same basic staff that made up stars, Uranus and Neptune are a chock full of methane and ammonia ice, and of course these icy giants are completely different than the inner rocky planets like our own earth. 

So why do we have this variety? The answer to this question lies in the temperature gradient in our own backyard the Solar System, it has to do with heat. To picture it we have to go back in time, to the actual birth of the Solar System itself. Before we have planets there was a huge cloud of dust and gas, known as the Solar Nebula and it was in the heart of this nebula that our own star, the sun, was born, however the creation of the star didn’t use up all of the gas and dust, the rest of the cloud continue to spin, flattening into a disk.

Theoretically computed variation of temperature across the primitive solar nebula
based on Eric Chaisson and Steve McMillan 

To put it into perspective, right at the centre of the Solar System, the Sun heats rises the temperature over 1,700 degrees Celsius but once we move away from it towards the edge of this disc, the temperature drops, getting to the incredible distance of fourteen and a half billion kilometres away the temperature drops to a chilly minus 250 degrees Celsius. And it is this temperature gradient from total hell to a to freezer which is critical to the way each planet formes, because every element and compound have their own specific properties, unique ways to respond to certain environments.

Take water for example, cool it until it’s very cold and it’ll form ice but heat it up and is going to vaporise into a gas, the critical factor here is the temperature at which any given element changes to a solid, because when matter is in a solid state rather than liquid or gas, is when is more likely to become a building block for a planet, which is why the temperature gradient is so important.

Now distance and temperature obviously are related here, close to the centre to our primordial Solar System, the temperature rises higher than a thousand degrees Celsius, in this searing heat only certain rocks and metals can remain solids and that is why Mercury, Venus, Earth and Mars are made up mostly of rocks and silicates, because most rocky materials have and extremely high melting point of 1,200 degrees Celsius, so other than very close to the sun, rock is solid throughout the Solar System, and that explains why we find rocks and metals in all planets, including the ice giants.

let us move further out into the Solar System, beyond Mars, and we reach the snow line, an imaginary line where temperature drops to zero degrees and water ice can form, the interesting things about this is that ice sticks together much more easily than rocks do, and that explains why gas giants like Jupiter and Saturn, are a bit like balloons filled with hydrogen and helium, the ice helped the cores of this giant planets to form very early and suck up vast amounts of gas from the proto-planetary disk. Is only when we get further out, much further out, double the distance from earth to Saturn that we start to see the ingredients of the ice giants to appear. By the time that we are at the orbit of Uranus, the temperature is so low that it’s not just water turning into ice complex gases like methane and ammonia are forming ice as well.

Artist impression of the snow line

Suddenly these volatile gasses are locked into solids form and they could star to coalesce making the seeds of what will become Uranus and Neptune. These complex chemicals are what make Uranus and Neptune so iconic. They gave them their striking colour and drove their extraordinary weather systems. And as more and more of the volatile ice clumped together and mixed with frozen water, it formed a staggering 60% of the ice giant’s mass, but none of these could have happened without the deep freeze of the outer Solar System, any closer to the sun and these planets simple couldn’t have formed. 

These ice giants might seem exotic to us, but as was mention, recent studies suggest that they could be the most common type of planet in the galaxy. One possible reason for that lies in one particular element within our gas giants, and that is a strange peculiarity with our ice giants Uranus and Neptune, which is that they have 60 times more carbon in them than we see in the Sun and that is very exciting because that’s more carbon that we have on Earth and it is certainly more carbon, ten or 20 times more, than we have in Jupiter or Saturn, so this is a signal which points to a clue of their formation and to why they are somewhat different.

It probably comes down to the fact that Uranus and Neptune formed very close to what we called the CO snow line or the Carbon Monoxide snow line. This is the point that carbon monoxide freezes, and that solid ice could have become a key building block for Uranus and Neptune. Scientist believe that this band of icy carbon monoxide, might also exist elsewhere, and now there is proof. In 2013, the ALMA telescope, was used for the first time to image the carbon monoxide snow line in a proto-planetary disk outside the Solar System, a place that could be forming planets, and the evidence suggests that most planetary disks would also have carbon monoxide snow lines, so in each of these disks there could vast amounts of material to form planets ice giant types of planets, so it might be that if we have all of these CO line everywhere, that then this may be the dominating process of planetary formation in other solar systems. So this is why it is so important to us to understand the formation of our own ice giants, and therefore of our own solar system.

The green is emission from a molecule revealing the location of carbon monoxide ice in a young star system.
Credit: ALMA (ESO/NAOJ/NRAO)

URANUS:

The Atmospheric composition by volume of Uranus is as follows, 82.5% Molecular hydrogen, 15.2% of Helium and 2.3% of Methane. Uranus has the coldest planetary atmosphere of our planets, with temperatures of 49K (-224.2⁰C) interestingly Neptune much further away radiates up to 2.61 times as much energy as it receives from the sun back into space. In contrast, Uranus radiates hardly any heat at all. Regarding this situation, a theory states that the planets was hit by a ‘supermassive impactor’ early on, but another explanation suggests that there is some form of barrier within the complex layers of Uranus atmosphere, preventing the core’s heat from reaching the surface.

When Voyager flew by our seventh planet, there weren’t many features to look at, in fact there could only be counted ten clouds in total, it looked featureless and kind of boring, from earth it is very difficult to resolve unless we flew by, however with the state of the art telescopes like the Keck Observatory in Hawaii or the Hubble Space Telescope the situation has improved in recent years and now we have detected more features in Uranus, which turned out to be a product of the seasons. At the time that Voyager 2 was at the planet, this, was pointed at the sun because is tilted almost 90 degrees and we could only see half of it, and also the atmosphere was dead in terms of activity. Later on in 2007 the planet is sideways to the sun and so the entire planet gets illuminated by sunlight as it spins and the atmosphere turns on, what it demonstrated now, is that in fact, Uranus is has a quite dynamic atmosphere and changes occur fast over there.

Neptune seen by Hubble. Image credit Hubble

One feature which was observed, the "Berg", named like so because the storm looked like an iceberg, disappeared in 2009, it developed after NASA's Voyager probe's flyby of the planet in 1986, this became very bright in 2004 and started to move toward the planet's equator in 2005, and then later a new storm is reminiscent of that feature was spotted in 2014 by Keck, which was even brighter than the Berg. 

The changing view of Uranus as seen by the Keck Telescope.

Uranus has an axial tilt of 97.77⁰, so it’s axis of rotation is approximately parallel to that of the plane of the Solar System, Earth has a magnetic field dipole moment pretty much centred on the centre of our planet and created by the core of our earth but when Voyager flew by Uranus it detected its magnetic field which turned out to be really strange, the planet was laying sideways and the dipole instead of been rotated with the rotation of the planet is tilted 59 ᵒ

Before the arrival of Voyager 2 no measurements of the Uranian magnetosphere had been taken, so its nature remained a mystery, scientists had expected the magnetic field of Uranus to be in line with the solar wind, because it would then align with Uranus's poles that lie in the ecliptic, but it wasn´t so, and not only that, also the centre of the dipole was offset, it was shifted from Uranus's centre towards the south rotational pole by as much as one third of the planetary radius. So this is why the idea of the massive collision was thought of, although new theories are currently studied. 

The tilt of Uranus and Neptune compare with Earth. Credit: Chaisson and Steve McMillan. 

One of the most interesting recent discoveries is that Uranus has an extremely blue ring detected by Hubble, the other only planet to have this is Saturn and it is associated with the moons Enceladus, in Uranus this is related with Mab. All giant planets have rings but are reddish in colour because they are composed of larger particles that reflect red light, and also the actual particles themselves may also be reddish, possibly from iron. 

Uranus blue ring. Credit Science Magazine.

NEPTUNE:

Regarding Neptune, well three years and seven months later than the encounter with Uranus, in 1989, the space probe Voyager 2 reached this blue world, and as it turned out it had a much more vibrant atmosphere, it also revealed a storm, nicknamed the Great Dark Spot which reminded scientists of the colossal red storm on Jupiter. Much more impressive was the winds clocked on Neptune, at a staggering speed of 2,000 kilometres per hour, making them the fastest recorded winds in the solar system, Neptune weather is characterized by extremely violent hurricanes.

The Great Dark Spot. NASA

Voyager also discovered that the magnetic axis of Neptune was also tilted 47 ⁰ and it was also offset from the centre of the planet as well, so perhaps the tilting may be something to do with ice giants and not a collision as it has been proposed for the Uranus scenario, in fact it is believed that may be the magnetic field is not generated in the core but rather on their mantle which contains the briny icy material which surrounds the core 

Neptune as see by Voyager 2 on August 1989.

Also very interesting about Neptune is its moon Triton, which is the largest natural satellite of the planet, orbiting backwards and the reason for that is that it must be a capture object from the Kuiper belt, a world that contains cryovolcanos, which may be active, without sending a probe we cannot know, however when this moon is view through an occultation of a distant star, one can measure its tenues atmosphere and detect changes in temperature, which seems to be increasing. 

Photomosaic of Triton from Voyager 2 images. Credit: NASA

Studying these planets with ground based telescopes, however improved they might be, or with Hubble Space telescope, which has a due date and will be out of service in the near future, seems that is not enough, and although that the James Webb Telescope soon to be launch will greatly better this situation, but obviously there is no substitute for sending a mission to our ice giants. NASA is already thinking about it and ESA the European Space Agency has ranked the ice giant proposal mission as high, but they don’t have access to the nuclear energy which is needed for travel so far from the sun, where solar panels will be useless, as was demonstrated with the New Horizon mission, which is powered by a Space Radioisotope Power Systems Generator or (RTG) we have to wait and see how events unfold in the near future.

Scheduled for 2018 NASA's Space Launch Rocket may help to get to outer solar system. Credit: NASA

Understanding these planets can makes understand why our solar system is so perfect for life to exist in this planet number three, our earth, also one of the big question we are trying to address is that if you look at Uranus and Neptune, they formed at roughly the same sort of temperature and they took about the same length of time to form and you’d expect them to be roughly the same, like same sort of composition, same sort of weather but the only similarities are the bluish green colour because of the methane in their atmosphere, in fact they are quite different planets. 

Curiously Neptune, despite been the furthest planet is one of the most dynamic ones in the Solar System, it has incredible strong weather patterns with clouds popping up and large cumulus systems developing and you get sheared apart by winds and jets, something that can happen on hourly basis. So Neptune never looks the same every night that we look in contrast Uranus is kind of serene and calm, despite the difference in astronomical units 20 to 30 between the two planets. Generally, we think the sun as the power for weather, driving this, so, why further away from the sun Neptune has a much more dynamic weather system than Uranus? This is a contradiction. 

When seen in infrared all giant planets they emit radiation glowing hot, however, Uranus does no emit much appreciable heat source which is another puzzle as well, may be all that energy is been locked away in the interior, and cannot get out through the various layers of its atmosphere, and it cannot convex and defuse out and we cannot see it, or the energy is not there at all, perhaps something in the distant past removed it, like a mayor collision, especially given that the planet is tilted on its side, so they are plenty of reason to go back and study them.