cut deeply across the hemisphere imaged by New Horizons. Serenity Chasma (E) is one of the largest canyons seen in the solar system at 1,800 kilometres long, over 50km wide and 5 km deep. According to mission scientists, they are indicative of the tectonic extension of Charon’s crust. This would suggest that Charon once had an interior ocean which froze early in its history, causing a global expansion of about 1% or about 35km of shell thickening.
Pluto’s small moons Pluto’s small moons are all brighter than Charon. Visual and spectroscopic observations during the flyby indicate that they all have water-rich surfaces and are probably of similar origin. Those which New Horizons was able to image more clearly show old, cratered surfaces. Their highly irregular shapes (Figure 19) suggest that they are the result of merged bodies, probably composed of debris left over from the collision which created the Pluto-Charon system.
Figure 19: The small moons of Pluto, shown to scale with Charon. Credit: NASA/JHUAPL/SwRI
Interactions with the solar wind As well as studying Pluto and its satellites, New Horizons was also equipped to study the environment surrounding them and especially the interaction of Pluto’s atmosphere with the solar wind. Even this far from the Sun, it was thought that this continuous stream of charged particles from the sun - mostly high energy electrons, protons and alpha particles – would be eroding away the outer layers of Pluto’s atmosphere. In the absence of a magnetic field to deflect the wind, this is what occurs at Mars. Pluto’s bow-shock region (see Figure 20), where the solar wind collides with the outer layers of the atmosphere, is, like at Mars, quite abrupt – less than about seven Pluto radii, or 8,000km. However, atmospheric loss rates are low. The escape rate of nitrogen is about 10,000 times lower than that predicted before the encounter. This
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