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Pluto and Charon at last!

This week I’ve been bemusing my friends by quoting Keats’ On first looking into Chapman’s Homer at them:

MUCH have I travell’d in the realms of gold,
Yet did I never breathe its pure serene
Till I heard Chapman speak out loud and bold:
Then felt I like some watcher of the skies
When a new planet swims into his ken;
Or like stout Cortez when with eagle eyes
He star’d at the Pacific—and all his men
Look’d at each other with a wild surmise—
Silent, upon a peak in Darien.

I’ve been breathing that ‘pure serene’ to which Keats refers while experiencing Pluto and its large moon Charon being revealed at close range for the first time. Granted, Pluto is not strictly a planet, but it is a world that has now very much ‘swum into my ken’, and indeed into the ken of anyone who has been watching the news lately. NASA’s New Horizons probe swept past Pluto and its moons at 17 km per second on 14 July. Even from the few close up images yet beamed back we can say that Pluto’s landscape is amazing. Planetary scientists are now, metaphorically, standing beside Cortez, staring at the newly-revealed Pacific, and surmising very wildly indeed. Charon, Pluto’s largest moon, is quite a sight too, and I’m glad that I delayed publication of my forthcoming Very Short Introduction to Moons so that I could include it.

Pluto (2370 km across, and on the left of the opening image) and Charon (1208 km across, on the right) are icy worlds in two respects. First, their surface temperature is only 40-50 degrees above absolute zero. Second, although they are sufficiently dense that there must be rock deep inside, their outer layers are made of ice. So far as we could tell before we got there, Charon’s surface is water-ice mixed with ammonia, whereas Pluto’s reflectance spectrum showed it to be covered by frozen forms of nitrogen, methane and carbon monoxide (all of which count as ‘ice’), with water-ice presumed to be buried deeper. Pluto’s surface gravity (though only about a fifteenth the strength of the Earth’s gravity) is more than twice as strong as Charon’s, so it has been able to hang on to ices that have vaporised and escaped to space from Charon. It even has a very thin atmosphere of mostly nitrogen gas.

That much we knew already. It will take over a year for New Horizons to transmit all its data, but already we can begin to put flesh on those bare bones. Pluto beguiled us during the approach by showing us its ‘heart’, a tract of bright terrain straddling the equator that seems to be a ‘frost’ or ‘snow’ deposit, now named Tombaugh Regio to commemorate Clyde Tombaugh (1906-1997) who discovered Pluto in 1930. The onboard spectrometer showed us that it is mostly carbon monoxide ice. The first clear views of Charon confirmed it to be markedly less red than Pluto, except for an unexpected dark polar cap.

As we got more detailed images, recorded at closer range, we saw hints of giant fracture systems stretching across parts of both globes. It became apparent that although there are impact craters to be seen, they are relatively few in number. This tells us that the surface of both Pluto and Charon cannot date back billions of years to soon after the birth of the Solar System, otherwise they would bear many more impact scars.

So let’s look at the best of the few pictures that are available.


This image above shows Pluto early in the approach, on 11 July. The ‘heart’ is just beginning to rotate into view at the upper left. I include this picture because at the edge of the globe in the 4 o’clock position there is a shadowed fracture system. This had rotated out of view by the time the spacecraft drew near, but it is important evidence of global faulting, similar to that seen on Charon.


This is Charon on 13 July from a range of 466,000 km, with a detailed insert (located by the box) recorded on 14 July at a range of 79,000 km. The global view show is a north-south fracture system almost on the right-hand edge of the disc, and an east-west fracture system curving all the way across the lower part of the disc. The insert is the most detailed view we yet have. Charon had rotated since the global view, so the sunset line had advanced across the terrain. Notable features in the inset include a mountain rising from within a pit near the upper right corner, and several narrow fairly straight or gently arcuate fractures reminiscent of features on the Moon attributed to local extension of the surface where a dyke (a vertical curtain of magma) has been injected into a fracture. It can’t be molten rock on Charon, but it could be the kind of gooey fluid produced by melting a mixture of water and ammonia ice.


This is the face of Pluto seen during closest approach. ‘The Heart’ (Tombaugh Regio) occupies the lower centre of the disk. Impact craters are visible in some places, such as immediately west of Tombaugh Regio, but their overall scarcity points to a generally young age for Pluto’s surface of less than a billion years and only 100 million years in some places. Is the surface ablating away to space so that old craters are erased? Has older surface been buried and recycled during events that create new surface, such as eruption of icy lavas?


This is a high-resolution image of a 300 km wide area immediately southeast of Tombaugh Regio, oriented with north towards the upper left. The mountain peaks, which are about 3 km high, must be made of water-ice, because other ices would be too weak to stand up as such high masses. Note the absence of impact craters (suggesting young age) and the general ruggedness of the high ground (suggesting faulting and fracturing).


This image is a 300km wide region about 300km north of the previous view. This is within the bright icy plains of Tombaugh Regio. The New Horizons team have speculated that the strange patterning that dominates the carbon monoxide ice surface here indicates convection within the ice. I think that is unlikely (because the convecting layer of ice would need to be nearly as thick as the convection cells are wide) and I think we may be seeing a large-scale equivalent of the polygonal patterned ground observed in Earth’s arctic and on parts of Mars as a result of free-thaw processes. If I’m correct, the knobs or harder ice around the edges of some cells will be giant boulders rather than icy ‘bedrock’.

Finally, a similar-sized region to the west of the previous views, at the western edge of Tombaugh Regio. In the east there are peaks of water-ice projecting up through the probably carbon monoxide ice. The dark surface in the west must be much older, because there are many impact craters on it. Craters at the very edge of the bright ice region have patches of ice within them, and there are almost certainly numerous craters completely hidden in the interior of the bright ice deposit where this ice is thicker.

All images in this blog post are public domain via NASA/JHUAPL/SWRI.

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