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New Horizons finally arrived at Pluto last summer. It zipped past the beloved dwarf planet at almost 40,000 mph and came as close as 8000 miles to the planet’s surface. The flyby was a tremendous scientific success, collecting so much data that scientists are still analysing it today and making new discoveries.

Even on the first day it was clear that the mission was worth the wait. John Grunsfeld, associate administrator for NASA’s Science Mission Directorate in Washington, ecstatically described how amazing the results were. “The mission has had nine years to build expectations about what we would see during closest approach to Pluto and Charon. Today, we get the first sampling of the scientific treasure collected during those critical moments, and I can tell you it dramatically surpasses those high expectations.”

Here are our favourite discoveries so far about everyone’s favourite dwarf planet.

1. The “heart”

The first view of Pluto revealed a brown blob floating in the darkness. Once New Horizons got a bit closer by summer 2015, clearer images went viral due to a bright heart-shaped region. Already a popular planet, Pluto fans rejoiced that such an adorable feature was the first thing to be seen.

2. Pluto once had flowing liquid

Just this week NASA has announced the discovery of a giant frozen lake on Pluto. It appears to consist of liquid nitrogen and must be millions or billions of years old. Back then Pluto’s conditions were warmer and the atmosphere had a higher pressure, so liquids may have flowed on the surface. To give a sense of scale, the lake is about 20 miles at its widest.

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The bright region has been named Tombaugh Region after the discover of Pluto, Clyde Tombaugh. One might assume the heart is a single geological feature but the two “lobes” are actually different. The unifying feature is that they’re covered in nitrogen and methane ice and have no craters. This is a surprising discovery as it suggests Pluto is geologically active, wiping over any craters from impacts that occurred in the past.

Tantalisingly, principle scientist Alan Stern also mentioned other observations that suggest liquids once flowed. “In addition to this possible former lake, we also see evidence of channels that may also have carried liquids in Pluto’s past.” Hopefully we’ll get images of these channels once they’re analysed.

3. Mountain ranges and floating hills

When we get a closer look, the most prominent features on Pluto’s surface are its many mountains that can be 3.5 miles high. They’re surprisingly young being around only 100 million years old (remember the solar system itself is about 4.6 billion years old). Some of these features on Pluto are among the youngest surfaces in the entire solar system.

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This is a startling discovery as it suggests Pluto is still geologically active today. It has long been thought that small, icy worlds such as the moons of Saturn can only be geologically active because of the gravitational interactions from their giant neighbours. But Pluto is an icy rock, far from anything that could provide such interactions. Clearly there are other ways for icy planets to be geologically active and hopefully missions like New Horizons will help explain the mystery.

Of all the peaks, the most bizarre are the floating hills. On giant ice plains formed by glaciers, giant hills are seen that can be several miles across. It seems really mysterious at first but scientists think they might know what’s going on. The glaciers are made of nitrogen ice and the hills are made of water ice. Somehow the hills must break off from mountains and end up being carried away by the glaciers. Because nitrogen ice is denser than water ice, the hills float. They’re like the icebergs in Earth’s oceans but on Pluto they would appear to be moving hills. Weird.

4. Snow-capped mountains like Earth

The dark reddish area that stretches almost halfway around Pluto is called Cthulhu (seriously). Prominent in this region is a string of white features. High-resolution imaging reveals that the string of white is actually a mountain range stretching 260 miles. Being biased by our own planet, it looks as if the mountains have snow on top of them since the rest of the environment is clearly reddish brown.

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Unlike on Earth, the ice is actually frozen methane. In the image above, the first inset is a simple close-up of the mountain range. The second inset shows data from the Ralph/Multispectral Visible Imaging Camera (MVIC), which highlights in purple where methane ice is located. As you can see, both images correlate with each other. It seems that in high altitudes on Pluto methane acts like water does in Earth’s atmosphere, condensing into frost. Your next sci-fi short story can feature skiing on Pluto and be scientifically accurate.

5. Ice volcanoes

If young mountains and floating hills aren’t surprising enough, Pluto also might have ice volcanoes. Also known as cryovolcanos, they erupt things like ice, water, or methane instead of molten rock. A large feature 100 miles across has been named Wright Mons and could be an ice volcano and you can see it in the centre of this image:

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If it really is a volcano, it would be the largest yet discovered in the solar system. An even bigger one has been found nearby called Piccard Mons, which is the highest point on Pluto towering 3.5 miles above the surface. A red flag is that the reddish materials around the potential volcanoes are scattered rather than uniformly distributed. Also there are no craters on Wright Mons itself, which suggests that it may have been active relatively recently in Pluto’s past.

New Horizons is a perfect example of why space exploration is worth doing. One flyby of a tiny world has revealed mysteries that cause us to rethink some of our theories about the rest of the solar system.



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