New Horizons: First Pluto journal paper discusses dwarf planet's origins

• published: 16 Oct 2015
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The first scientific paper to come out of the New Horizons flyby of Pluto in July has just been published. Appearing in Science magazine, the article is largely a collation of information already made public through media briefings and press releases. It includes the mission team’s initial impressions of Pluto and its moons: Charon, Styx, Nix, Kerberos and Hydra. And, of course, it details some of the basic numbers that can now be used to describe these far-flung objects. One key early result from the US space agency (Nasa) probe is the precise measure of Pluto's radius, of 1,187km, plus or minus 4km. "This doesn't sound particularly exciting but you have to remember estimates of Pluto's radius varied from 1,150km to 1,200km, so this will really help those people modelling how Pluto/Charon formed, their atmospheres and the material exchange between them," said team member Dr Carly Howett from the Southwest Research Institute (SwRI) in Boulder, Colorado. Ancient pile-up Pluto's mass was already known, so the larger radius gives a density that is consequently now a little lower, at 1,860kg per cubic metre. This brings it nearer to that of Charon, the main moon in the system. Its density is 1,702kg per cubic metre (its radius is 606km plus or minus 3km). The closeness in density has a series of theoretical implications, which the team duly starts to discuss in the new paper. It suggests the materials that make up the objects are not that dissimilar. Yes, different types of ices dominate their surfaces, but their bulk compositions may be quite alike. That is not a surprise if, as many scientists think, the pair were formed as a result of a collision between two primordial objects billions of years ago. However, it does put some important constraints on the smash-up. To have the bulk materials broadly divided up evenly between Pluto and Charon may indicate that the colliding objects were predominantly undifferentiated masses of ice and rock. Had either significantly gone down the path of separating heavier and lighter materials into defined layers, this would be reflected in the densities of Pluto and Charon today. And accepting the undifferentiated state of the colliders could say something about the early conditions that pertained in this distant region of the Solar System, known as the Kuiper Belt. It might suggest it was a markedly low-energy environment. Had the accretion of materials to make larger and larger objects been a more dynamic, violent process then it would have injected a lot of heat into these bodies, melting materials and prompting them to separate into layers. "If this story all hangs together then it might mean these objects formed more slowly or that they formed later than at least some models would have it. This is the kind of thinking we're doing," Prof Bill McKinnon from Washington University in St Louis, Missouri, told BBC News. Duck walk In this context, it is fascinating to note that Comet 67P is two objects stuck together. 67P is being studied by that other major space venture of the moment - the European Space Agency's Rosetta probe. The duck-shaped comet very probably formed in the Kuiper Belt before being disturbed and moved into the inner Solar System. And Rosetta scientists say some of the icy dirtball's features could only persist today if the precursor objects were joined in a low-velocity, "walking pace" collision. "All of the data about Pluto and its moons (even the little bitty moons) and 67P, and the next Kuiper Belt object we go to with New Horizons in a little over three years - all of this will feed into trying to understand how the Solar System formed," said Prof McKinnon. 'More goodies' New Horizons continues to speed beyond Pluto following its historic flyby. It has now gone more than 100 million km deeper into space, putting it some five billion km from Earth. As it recedes from us, the probe continues to send back data gathered during the flyby. "We're really in the peak time in the mission for science because we're in the thick of our highest priority downlink," explained Dr John Spencer from SwRI.