Pluto's Icy Plains

This week’s guess the planet image comes from Pluto. This is a close up image of the icy region of Sputnik Planitia. This image was taken by the New Horizons spacecraft, and so credit goes to NASA’s New Horizons team. The full image is 80 km wide and 700 long. NASA describes it as “trending from the northwestern shoreline of Sputnik Planum and out across its icy plains.” They also note that this image was taken just before the New Horizons probe’s closest approach to Pluto, making it among the highest resolution images we have from this body. The space craft has been observing Pluto for some time as it approached the distant world, but only had a short period during the closest approach of its flyby, when it was possible to acquire the most detailed images.

Some spacecraft are put into orbit around the planets they survey, and so can operate for years, returning thousands of images with similarly high resolutions, however this isn’t always possible. For the New Horizons probe getting into orbit wasn’t an option due to the high speed required to travel to Pluto. New Horizons set off from Earth moving faster than any previous space probe. Once out in space it becomes very difficult to slow down, as doing so involves expending a vast amount of fuel. There was no way for the New Horizons probe to carry enough fuel for it to slow down and enter the orbit of Pluto. Instead it was set up to be a “fly by” mission, intended to cram as many observations as possible into the very brief period when it was at its closest approach. 

The New Horizons team delivered some amazing images of the distant dwarf planet, which has given us our first good look at its surface and geography. One of the most striking features of Pluto is a bright heart shaped region which was entirely unknown prior to this mission. Sputnik Planitia forms one lobe of this heart shaped region, and was named after the first man made satellite. 

“Planitia” is Latin for plain, and so denotes a wide open area. The term “Planum” is also sometimes used. The dominant feature of this plain is a series of polygonal cells, which can be seen as bright hexagonal bands criss-crossing the image. These are thought to be evidence of convection in the ice that covers this region. This isn’t water ice, like that found on Earth, but rather made of nitrogen and carbon monoxide. These substances are only found as gasses on earth, but on Pluto it is cold enough for them to exist in the solid state. However they remain reasonably dynamic, moving on long timescales. 

In some ways this environment is analogous to a terrestrial ice sheet. On Earth mountains of rock are eroded by flows of slowly moving water ice, called glaciers, which build up at the heads of valleys over many years. This slow moving ice carves up the stone, creating wider valleys. In some places vast ice sheets extend across the continent, as they did across much of the northern latitudes of Earth during the last ice age. 

On Pluto the process is the same, but the materials are very different. Here the hard water ice takes the place of the rock, and the softer nitrogen ice behaves in the same ways that water ice would on Earth. Some glacier like structures have been spotted at the edge of Sputnik Planitia where the water ice mountains are carved by these flows. Away from the edges of the plain the ice is free to convect, slowly churning and creating the polygonal shapes visible in the image. The rippled and pitted texture of the ice is likely due to sublimation. The upper layer of ice turns back into gas in some places, forming irregular chains of pits and troughs.

We are still only scratching the surface of Pluto’s geography, but the new data from this mission are already shedding light on how this distant dwarf planet works. 

Image Credits: NASA/JHUAPL/SwRI

Image of Pluto

The full image of Sputnik Planitia is too long to post here, but can be seen in all its glory on this NASA page:  https://www.nasa.gov/image-feature/pluto-s-icy-plains-in-highest-resolution-views-from-new-horizons

More information about ice flow on Pluto can be found here: https://www.nasa.gov/feature/new-horizons-discovers-flowing-ices-on-pluto