Outer Worlds: The Incredible Geodiversity of Pluto and Charon

Lucy Kissick

Eighty-five years after discovery: a proud day for NASA and all the scientists and engineers involved!

When NASA made the decision in 2000 to visit the Solar System’s only unexplored planet (as it was designated at the time), they knew they were in for a long journey. Pluto and its sister world Charon form a binary system some six billion kilometres from the Sun: for comparison, the distance between Earth and the Sun is a hundred and fifty million kilometres. For NASA to reach this elusive twilight zone, they would need to create the fastest moving spacecraft ever built.

The New Horizons spacecraft features visual, topographic, thermal, and spectroscopic instruments

Flash forward six years and enter New Horizons: the grand piano-sized space probe on its way to the final frontier in Solar System exploration. Incredibly, New Horizons took just under ten years to reach the Pluto system, an average speed of 58,536 km/h! And what awaited its patient scientists on the day of arrival, July 15th 2015, defied wildest hopes and predictions: Pluto was no battered, barren, cratered rock, but geologically alive.

Pluto through the eyes of New Horizons in 2015. The geodiversity is immediately astonishing.

On Pluto, there are oceans of frozen methane and nitrogen possibly convecting under today’s conditions; there are mountains of water ice as high as Mt Kilomanjaro beached upon its shoreline; there are great glaciers pouring in from rugged crater-pocked peaks; there are wide plains blood-red with snow made of the same organic molecules life on Earth evolved from. If you stood on its surface, while you weren’t stamping your heated boots and bouncing around in the low gravity, you would see a deep burgundy sky fading to black across an arcing horizon. The Sun would be a pinprick; the crescent of Charon would gleam in the weak light.

Charon, too, astonished scientists: while more cratered than its orbit-mate (hence with fewer geological renovations to overprint these most ancient features), the moon features a great rift five times deeper than the Grand Canyon. This may have formed with the freezing and expansion of a subsurface global ocean. Such a feature seems rather common in the Solar System, with Charon joining a class of icy satellites including Jupiter’s Ganymede and Uranus’s Ariel.

Here we see a recent map of Sputnik Planum (the ‘Heart’ of Pluto, with only a fraction of the total terrains labelled. What powers such processes? We may never know until we return…

For a world smaller than our Moon and receiving 0.1% of the Sun’s light compared to Earth, the level of geodiversity at Pluto and Charon is completely mystifying. With every new mission to space it seems our expectations are each time upended and exceeded. From geysers and rivers discovered by Cassini at Saturn, to  lakes and aquifers unveiled by Curiosity on Mars, New Horizons has shown again how our Solar System continues to outdo itself.

Though this is hardly discouraging to the scientists. After all, in the words of the great science fiction writer Isaac Asimov “the most exciting phrase to hear in science, the one that heralds new discoveries, is not ‘Eureka!’ but ‘That’s funny…

Interesting links for more:

“I am a research student in the 2016 cohort, studying the effect of lake chemistry on the ancient martian atmosphere.”



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