Many of us would have been amazed by the images of Pluto sent back during the fly-past of the New Horizon mission in 2015. We were even more amazed by FAS member Kevin Pretorius’ presentation at our June meeting about the analysis of those images. As always with Kevin’s talks, there was a wealth of detail clearly explained and I have just selected some of the highlights here.
Geology of Pluto
A very striking feature of Pluto is the large basin-like feature called Sputnik Planum. This has very few craters on it, unlike the surrounding areas, suggesting it is relatively young. That in turn suggests that Pluto’s surface is being re-formed with some internal heat source driving geological processes; a very unexpected discovery! The area itself, which appears to be divided into cells, is thought to be a huge nitrogen glacier. There are flow lines across it, showing the nitrogen is in motion. Along some of these boundaries, different features can be seen , now interpreted as being icebergs, literally icebergs – ie water ice blocks which may having broken off the rugged uplands to the east, from where they rolled down onto the nitrogen ice, partially sunk into the soft ice, but remained floating on the surface to be carried by convection currents until they are deposited along the convection cell boundaries.
Hillary Montes (1600m high) and Norgay Montes (3400m high) are two mountains to the south west of Sputnik Planum. Their surface is made of frozen nitrogen, methane and carbon dioxide but these solids would not be strong enough to form mountains of these heights. The idea is that there is a bedrock of frozen water ice to provide the inner strength, overlaid with the other frozen gases. To give you a sense of scale, Norgay Montes is just higher than the highest peak in the Dolomites and Hillary Montes is about 200m higher than Ben Nevis. What a sight they must be!
The last feature I’ll describe on Pluto is the Cthulhu Regio,(and that isn’t a spelling mistake, for either of the two words) which is to the west of Sputnik Planum. A key characteristic of this is its reddish colour, a colouration found on other bodies in the Solar System, notably Titan and Triton. The colour comes from the presence of tholins, complex organic chemicals formed by the interaction of UV light with methane and nitrogen.
About Charon
Now let’s think about Charon, Pluto’s main moon, only discovered in 1978 when an odd bulge was noted in the poor images of Pluto of that era which seemed to move round Pluto. The “bulge” was Charon! Charon has no atmosphere, yet does have the characteristic colour of tholin staining. How can this be?! The answer is one of the oddest theories Kevin described. The gases needed to make the tholins are thought to come from Pluto. Pluto does have a very thin atmosphere which extends to over 100 km above its surface giving rise to a distinct haze all round the planet. Charon is though 19,000km away and the transfer may be effected by a solar wind, depositing gases on Charon when it is downstream of Pluto. Once on Charon, the same UV-driven processes as happen on Pluto, occur to deposit tholins.
Naming features
One of the delights of discovering new features is that we get to name them and the International Astronomical Union has agreed the themes suggested by the New Horizon team and the team have now put forward actual names. These are still provisional but hopefully they will all be confirmed by the IAU. You will have recognised some already mentioned for Pluto. Mountains are named after explorers and adventurers, hence Hillary Montes and Norgay Montes. I rather like the fact that the higher of the two mountains is named after Tensing Norgay, with Edmund Hillary having to settle for the smaller one. Others commemorated include Clyde Tombaugh, the person who first identified Pluto, Percival Lowell, Jan Oort and Jacques Cousteau. Another theme is about space missions giving us Soyuz, Challenger and Columbia. And if you are wondering about Cthulhu, this is an ancient deity in the writings of H.P. Lovecraft – and , no, I hadn’t heard of the author or his deity either.
On Charon, naming themes are rather more playful, featuring fictional destinations and voyagers. A large upland region is Oz Terra. There is one region with many features named after Star Trek characters : Kirk, Spock and Uhuru. Dr Who fans will be pleased to know there is even a Tardis Chasma.
And finally …
some information about two extra items carried by New Horizons. One was some of the ashes of Clyde Tombaugh, by now far beyond Pluto and still going. The other was a postage stamp. A postage stamp?! It was one issued in 1991 as part of a commemorative set about planetary exploration. The Pluto stamp just said “Not yet explored”, a comment which always rankled at NASA. It now holds the official Guinness World Record for the farthest distance travelled by a postage stamp and there is a certificate to prove it, though I imagine there could be quite a long debate about quite what is meant by “world” in this context.
(My thanks to Kevin for reviewing this article and clarifying some of the information)
Talk given by FAS member Kevin Pretorius
Post written by Katherine Rusbridge
June 2017