http://www.nasa.gov/mission_pages/newhorizons/images/

Pluto’s moon Charon (the Ferryman, remember?) has craters on it’s surface whilst Pluto does not.

Here’s Charon:

It is absolutely normal for moons like Charon (and the Earth’s moon) to have lots of craters. There were huge numbers of objects flying around the solar system after it’s formation, although across the aeons most of those objects have been acquired by either the sun, a planet or a moon - and hence the craters. In more modern times it is normally only short- or long-period comets that enter the inner system (apart from all the dust).

Comets come from the Kuiper Belt or Oort Cloud. Short-period comets (like 67P) come from the Kuiper Belt, whilst long-period comets come from the Oort Cloud. [“Short-Term”: < 200 years (67P is 6.45 years). “Long-Term”: > 200 years.

Pluto & Charon are part of the Kuiper Belt (named after Dutch-American astronomer Gerard Kuiper); it’s a doughnut-shaped ring, and extends from ~30 to 55 AU (‘AU’ == “astronomical unit” == distance of Earth from the Sun). The number of objects in the Kuiper Belt that are larger than 100 km (62 miles) is reckoned to count in the 100,000s; <1 million, but that is a lot! They are all reckoned to be icy, rather than rocky, bodies, but the value of Rosetta on 67P is to try to find what those bodies may actually be made of. Meanwhile, and make sure that you are sitting down, the number of comet-sized bodies is reckoned to be counted in the trillions.

The Oort Cloud (named after Dutch astronomer Jan Oort) is (mostly) a spherical shell, and extends from about 5,000 to 100,000 AU. You will recall that the number of icy bodies in the Kuiper belt is less than 1 million. For the Oort Cloud this is trillions. The outer Oort Cloud bodies can be (almost) as likely to be affected by nearby stars or objects within the Milky Way as by our Sun. Finally, most of the space-dust that enters the Earth (~100 metric tons each day) comes from the Oort Cloud. As space-dust is directly responsible for noctilucent clouds, that means that the Oort Cloud is directly responsible for Noctilucent Clouds here on Earth.

So far, so extraordinarily normal. Now, what about Pluto?

Geology, Geophysics and Imaging (GGI) team leader Jeff Moore, of NASA’s Ames Research Center in Moffett Field, California, said:

This is one of the youngest surfaces we’ve ever seen in the solar system

Those mountains have been dubbed the “Pluto Rockies” (they are reckoned to be 11,000 feet (3,500 meters) high--and here is the point--only about 100m years old on a planet that has probably been around for 4.56-billion-years). They are possibly still growing, and that underlines the big absence, which is the almost complete lack of craters across the entire planet. There are 2 big, big questions:-

1. What are they made of?
   (Pluto isn’t supposed to have much rock)
2. What has built them?

The best, and most astonishing, answer to the first question is “Water”. As the Philae lander discovered on 67P earlier this year, water-ice can be damned hard. Methane & nitrogen are known to feature within Pluto’s weather from earlier studies (Pluto has an atmosphere, plus it’s orbit is so elongated that it will receive much more heat at one end of the orbit compared to the other, which helps explain the nitrogen snow in Winter).

Methane- & nitrogen-ice are lousy materials for mountain-building, so these are probably the biggest snowmen & women that you are ever likely to see.

But now, how did they grow? The complete absence of craters says that the surface is being remade new all the time (just like the Earth’s) but how?

There are examples of other moons that display internal heat generation. In every case, it is a much smaller satellite close to an exceptionally large planet, and what makes mathematical sense is “tidal forces” (the effects of gravity from the large planet are sufficiently different on one side of the satellite to the other side as to cause internal heating; in extreme cases such forces can rip a rigid body into pieces (eg Shoemaker-Levy 9 & Jupiter)). However, can this explain the Pluto Rockies, as Pluto is the heavier body & both Pluto & Charon are very small? The two are very close, so there must be some tidal effects, but that seems unlikely to be enough.

Bodies can also become hot through sheer size; their own gravity causes heating at the centre. The best example is Jupiter:- it is so large that this solar system is almost a binary star. Jupiter is big enough to produce more heat than it loses, but not quite big enough to ignite into a star (that *did* happen to our Sun, of course). Now that heating has to have happened at some stage with both Pluto & Charon because both are spherical rather than lumpy rocks. However, both are also far too small to be red-hot at the centre (the temperature at the Earth’s centre is reckoned to be the same as that at the surface of the Sun) so both must have lost that early heat via convection and/or radiation.

The final possibility is through radioactive breakdown at the core of the planets. There is actually an odd connection with New Horizons (and the Pioneer + Voyager probes):-

Plutonium is possibly the most dangerous element on the planet; just one atom inside a human body can kill it, whilst just 5.3kg of weapons-grade plutonium is sufficient for a 111 kiloton (466,000kg) fissile nuclear bomb.

Plutonium--it is named after Pluto--does not occur naturally on this planet. It is produced via radiation of Uranium within Nuclear reactors. The 239-isotope is the one used for production of nuclear weapons (both fissile & fusion) but the 238 isotope pictured above is invaluable for “Radioisotope thermoelectric generators” (RTG) with a power density of 0.54 watts per gram (62 watts from 115g in the photo) (New Horizons has 10.9 kg (24 lb) of ²³⁸PuO² within it’s RTG).

An RTG is a solid-state generator that relies on a self-heating radioactive element. ²³⁸PuO² is the most popular for missions to space because of the power/weight ratios. 238-Plutonium produces such a volume of alpha particles that it will heat (and ultimately melt) the metal. The pellet photographed above was wrapped in a graphite blanket for several minutes; that was sufficient to cause the Plutonium to self-heat to incandescence. The RTK contains thermocouples which rely on a temperature gradient across the thermocouple to produce electricity with zero moving parts. The isotope has a half-life of 87.74 years, which is more than enough for the mission. New Horizons, and the Pioneer & Voyager missions before it, all contain an RTG powered by ²³⁸PuO².

²³⁸Pu is an exceptional example of the principle, but all of the rocky planets concentrate radioactive elements at their core due to the higher density of those elements. The exceptional pressure due to all the material above them increases the effect.

So, could a similar radioactive effect be responsible--even in part--for this activity? It would seem to be only correct that it should. However, according to all current theories, the rock & metal in the solar system is concentrated at the centre (just as it is in the planets themselves, and for a similar reason) whilst the outer planets have the gases & water.

In brief, at this moment no-one is sure.

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Alex Kemp