By Sabine Stanley, Ph.D., Johns Hopkins University
The surface of Pluto is an active world with glacial flows and convective ice. There are dunes, there are floating mountains, there may be volcanoes spewing ice; there may even be a subsurface ocean. Pluto also has a very thin atmosphere, which leads to some rather intriguing phenomena.
The Heart of Pluto
The youngest and brightest large region on the surface of Pluto is also the most dynamic: the Tombaugh Regio is named after Pluto’s discoverer. It is also lovingly called ‘the heart of Pluto’ because of its shape.
The appearance of the region is due to the fact that Pluto’s surface is extremely cold. The temperature is about negative 375°F, which is only 47 Kelvin above absolute zero Kelvin. The nitrogen ice that makes up the surface in this region is less dense. Hence, the whole region appears softer and more flowing than the other regions of Pluto.
The Glacial Flow of Sputnik Planitia
There are also signs that this region is geologically active. On the northernmost part of Sputnik Planitia, the heart is bordered by rugged older terrain. You can see regions where the nitrogen ice in the heart looks like it has flowed around obstacles and into depressions.
(Image: NASA/JHUAPL/SwRI/Public domain)
This is the glacial flow that we see with water ice glaciers on Earth, but it’s nitrogen ice on Pluto. Near the boundary here we also see some features in the shape of polygons. These can be seen in other areas of Sputnik Planitia too, like in the southern region of the heart. These polygonal cells on the surface suggest that convection is going on inside the Sputnik Planitia basin.
The center of the cells is where slightly warmer ice is moving upwards to the surface. Once warmer ice reaches the surface, it spreads out and cools. Colder ice then descends back into the ground at the boundaries between the cells. The size of the polygons can help us determine the depth of the convection cells. Computer simulations of convection in solid nitrogen ice suggest that Sputnik Planitia’s convection cells are about 3 to 4 kilometers thick.
Learn more about exploring the Earth-Moon system.
The Collections of Floating Hills
At some of the boundaries of the convection cells, we also see collections of hills. The hills seem to be floating on the nitrogen ice surface! Or at least, it looks like they get carried along with the convection cells until the hills congregate at the downwellings.
One possible explanation for these floating hills is that they are eroded pieces of the water ice we find in other regions of Pluto. At Pluto’s surface temperature, water ice is less dense than nitrogen ice. So, these pieces of water ice could be carried down by the underlying nitrogen ice glaciers at the edge of Sputnik Planitia.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, Wondrium.
Mountains on the Surface of Pluto
The area immediately surrounding the heart of Pluto at Sputnik Planitia also has some interesting terrain. Near the southern and western edges, we find water ice mountains several kilometers high. There are also signs of cryovolcanoes on the southern boundary. Wright Mons is informally named after the Wright Brothers, and Piccard Mons is named for a Swiss pioneer of scientific balloons.
Both mountains are over 150 kilometers across. Wright Mons is about 4 kilometers high, half the height of Mount Everest here on Earth. Picard Mons is even higher at 6 kilometers elevation. They both have central depressions which is a characteristic sign of a volcano rather than just a regular mountain, but we haven’t caught them actively erupting, so it’s not been confirmed that they really are cryo-volcanoes.
Moving to the northern hemisphere, we approach a region called Tartarus Dorsa. Here we find a completely different type of terrain. Here, Pluto looks as though it’s covered in snakeskin. This texture is caused by a series of icy mountains that are about half a kilometer high and form this strange blade-like structure.
This is a transcript from the video series A Field Guide to the Planets. Watch it now, Wondrium.
The Blue Halo around Pluto
After New Horizons passed by Pluto, it took a picture looking back as Pluto occulted the Sun. This image shows a blue halo surrounding Pluto. The bluish color is caused by the haze particles in the atmosphere, scattering certain wavelengths of light. The bluish color in the image suggests that the haze particles are quite small, perhaps ranging from 10–100 nanometers. That tells us about the chemistry in Pluto’s atmosphere.
What causes the layering of the haze? In Pluto’s atmosphere, the density changes with height, and there is an equilibrium position for atmospheric particles. If the particles move away from that equilibrium position, then gravity will work to restore them to their equilibrium position. This can cause waves to form in the atmosphere. These waves are called atmospheric gravity waves.
These are like the waves on the Earth’s ocean that occur at the interface of the air and the water. On Pluto, these waves occur at the interface between fluids of different densities; they can concentrate the haze particles into separated layers, explaining the features seen at Pluto.
Learn more about Uranus: a water world on its side.
The Escaping Atmosphere of Pluto
Pluto’s atmosphere is also escaping! That’s because the small amount of methane in Pluto’s atmosphere is heating up the atmosphere. Methane is a greenhouse gas. The heat gives atmospheric particles added energy. This energy can make some of the particles escape the planet.
When the particles are high enough, they can also collide with high-energy particles from the solar wind. The New Horizons spacecraft was able to witness this when it got past Pluto. Pluto was between New Horizons and the Sun, so the spacecraft was able to detect atmospheric particles escaping Pluto’s exosphere. These particles were ionized by the solar wind, thus creating a giant ion tail.
Common Questions about Icy Plains and Mountains of Pluto
The surface of Pluto has icy mountains, valleys, plains, and craters. The temperature on Pluto is extremely cold, thus most plains in Pluto seem to be made of frozen nitrogen gas.
Yes, Pluto is more similar to the rocky terrestrial planets than the gas giants. Pluto’s surface is covered with nitrogen ice, methane, and carbon.
Pluto’s surface area is 1.779×107 km2, roughly equivalent to the size of Russia.
Pluto was downgraded to the status of a dwarf planet as per the IAU’s revised criteria for planets: the object must clear the neighborhood around its orbit enough to dominate the orbit.
