What NASA's New Horizons Discovered around Pluto

 

In 2015, there was a huge excitement in the space community. That is because up until then, the best image we had of the Pluto system was this. Hubble also squinted its lens at Pluto, but it is so small and distant, the best it could see was a few blobs of colour variation. But in 2015, this all changed. That is because after a nine year journey, the New Horizons space probe flew by the dwarf planet, giving us a detail and fidelity of Pluto and its moons like we had never seen before. So the question is, what did the New Horizons probe see and discover during its flyby of the Pluto system? Stick with me I will show you all the highlights from the New Horizons mission to Pluto. Pluto was the last of the traditional 9 planets to be explored. This was due to its distance from us, but also because can you believe this it wasn’t considered a very interesting celestial object. Thankfully, the New Horizon’s team pushed hard for this mission to be approved, and in 2006, New Horizons launched as part of NASA’s New Frontier’s program, for medium budget space missions. The goal of the mission was to get to Pluto as soon as possible, and as such, New Horizons was the fastest launch ever, it being a light spacecraft on the most powerful rocket of the time the Atlas V. It whizzed past the moon in only 9 hours. The Apollo missions took 10 times as long. On its way to Pluto, it used Jupiter as a gravity assist which shaved 3 years off the arrival time. It also used Jupiter as a trial run for its systems, taking some remarkable videos and images of the planet and its moons. After this successful trial, New Horizons went into hibernation mode to prevent wear and tear of its instruments. Leading up to its approach in 2015, the team turned the systems back online, and every day the spacecraft sent back images of the Pluto system. This was an incredibly exciting time for enthusiasts following the story. We began to get hints of what Pluto could possibly look like, and saw how different Pluto was from its biggest moon, Charon. Every day, the resolution got higher and higher, and more details could be made out. Yes, there were other scientific goals for the mission, but the most interesting thing to me was what it looked like. Soon there could be seen what looked to be a heart shape on the dwarf planet! On the 14th July, the New Horizons probe made its closest approach, at only 12,500 km from the surface of Pluto. However, mission controllers didn’t get a look straight away. Firstly, the probe was too busy taking a lot of photos during the flyby to send any back immediately. Once data transfer commenced, they had to deal with the slow up link speed of only 1 kbit/sec. Further to that, there was a 4.5 hour latency between the spacecraft and the Earth. But what it saw and sent back was spectacular: mountain ranges, ice plains, glaciers and an atmosphere. It also had a good look at some of Pluto’s moons. Let’s go into detail about what it actually discovered during this flyby. One of the first things observed about the Pluto is its unusual relationship with its moons. For a start, Pluto’s biggest moon, Charon, orbits very closely to Pluto, and is also very big in comparison. This means that the bary centre of the two objects, or in other words, their center of mass, is outside of the primary object. They actually both orbit around a point in space. Not only that, but both objects are tidally locked to each other. This means if you stand on one, the other won’t move from that point in the sky. This is very unusual because while some moons are tidally locked to their parent planet, the planet is not also tidally locked to the moon. Charon is very different visually from Pluto being much darker. This implies they are not from the same origin. The rest of Pluto’s moons are very small, only being a few km across. Their orbits are exceptionally circular and are all co planar with Pluto’s orbit. The geology of Pluto is very interesting. The biggest visible feature on Pluto is this giant heart shape, which wowed the world when it first came into view. It has since been named Sputnik Planitia. It is the size of Texas, and it has a strong colour contrast to the surrounding area. This is because it is a giant ice plain. In fact, during the flyby, it was confirmed that 98% of Pluto’s surface is comprised of nitrogen ice. On average, the temperature on the surface of Pluto is 229 C, which means water ice would be rigid and brittle. On the other hand, nitrogen ices at this temperature act like water ice on Earth, meaning it can flow as glaciers. This can especially be seen around the edge of the heart, glaciers flowing into the gaps around the craters and mountain ranges. The ice plains themselves have giant polygon shapes across the entire area. There also are no craters, which means it must be a relatively new feature, or a feature that is being continually renewed. It is perhaps only 10 million years old. The polygonal cells show ridges on them which are likely caused by sublimation, the process of an ice turning directly into a gas. Although it’s not known for certain, Sputnik Planitia could have formed from an impact, and ices filled the crater in from a potential subsurface liquid ocean. This filled in basin actually causes a positive gravitational anomaly. A gravitational anomaly is where the gravity at one point is different from elsewhere on the object. The ice plain is directly facing away from Charon, which would align it up with the objects’ tidal axis. Due to the short distance between Pluto and Charon, tidal effects are very strong on both objects. This could be the reason why Pluto is tidally locked to Charon and the two objects can’t look away from each other. Surrounding the ice plains are vast mountain ranges made of water ice, which, when viewed from the side on, look spectacular. Water ice is the only type of ice detected on Pluto that would be strong enough to support heights of several kilometres at this temperature. Among the mountains found on Pluto, there might also be some which are cryo volcanoes, one of the most likely candidates being Wright Mons. It is 4 kms tall, one of the highest peaks on Pluto, and a big depression is found in the center. Cryo volcanoes could be a contributing factor for Pluto’s young surface. Another obvious feature of Pluto is the dark material that seems to be sprinkled on the surface in some areas. The biggest such area is called Cthulhu Macula. It is weirdly reminiscent of a whale in shape, as can be seen in this image. The dark colour is thought to be a deposit of tholins, a kind of tar made up of hydrocarbons that have interacted with sunlight. Similar deposits can be seen on one of Saturn’s moons, Iapetus, so the process has been seen elsewhere in the solar system. The region on Pluto is much more heavily cratered than the heart, which implies the surface there is much older. Mountain ranges can be seen in the middle of Cthulhu Macula, topped with what is thought to be methane ices. Methane apparently condenses as frost at higher altitudes on Pluto. The last surface feature I will mention here is this region called Tartarus Dorsa. It is an extensive, highly distinctive set of 500 meter high mountains that resembles snakeskin or tree bark. They are thought to be Penitentes. If that is true, Pluto is the only place in our solar system other than Earth where they have been observed. Even on Earth they are very rare, but some can be found in the Atacama desert and other dry, high altitude regions. The ones on Pluto are much taller and cover a much vaster area than on Earth. We can only imagine what they look like closeup. For me, the most impressive discovery that New Horizons was able to confirm was that Pluto has an atmosphere. And not only that, but the images are incredible. Due to Pluto’s small size and weak gravity, the atmosphere appears to extend high above the surface of Pluto. Earth’s atmosphere, while being much more massive and dense compared to Pluto, hugs the planets comparatively tightly as the gravity is a lot stronger. The atmospheric pressure on Pluto is exceptionally low, however, roughly 10 microbars, or 100,000 – 1,000,000 times weaker than the surface pressure on Earth. It is theorised that the pressure could increase to as much as 18 to 280 millibars, three times the surface pressure on Mars and a quarter of the surface pressure on Earth, if the temperature was to rise and the surface ices would sublime into gases, the process which we’ve seen in the ice plains. The last time Pluto was thought to have this atmospheric density was 900,000 years ago. At this pressure and temperature, the conditions could even be right for liquid nitrogen to form on Pluto’s surface. Some evidence of this might be found here, in what appears to be a frozen over lake. At any rate, within just one year, Pluto’s atmospheric density can vary by a factor of four due to seasonal variations. That is a massive contrast compared to other solar system objects with atmospheres, which generally stay pretty consistent. The atmosphere consists of the same ices found condensed on the surface, namely nitrogen, methane, and carbon monoxide. The other fascinating discovery New Horizons made about the atmosphere is that it has up to 20 haze layers. Haze layers themselves were not unexpected, but the amount of them was. They can clearly be seen in some of these images, acting like layers of a thin kind of fog. Sunlight can be seen streaming through one such layer in this photo, the shadows from the mountains clearly seen in contrast to the sunlight shining through the haze. The layers do not appear to be level across the planet. Here you can see this haze layer high above the surface, but on this side of the image it touches the surface. On a side note, to me these are the most breath taking photos of Pluto, and I purposefully saved them until last. You can truly appreciate depth and the scale of the mountain ranges; Pluto almost seems like a toy replica due to the extreme topographical relief, but these mountains appear so high because Pluto is so small, and its gravity is not strong enough to pull them down. This gives for a varied and impressive landscape and a fitting end to this Article. 

Thanks for reading this article, and I’ll see you next time. 

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