New Horizons' images of Pluto have been the astronomical highlight of the summer. The complexity that became apparent from the pictures of the dwarf planet is discussed in detail in the first paper published by the research team. Pluto’s geology, colors and atmosphere, Charon’s surface and the first data on Nix and Hydra – all moons of Pluto – are all discussed in the study, published in Science.
Pluto is the largest and second most massive dwarf planet in the Solar System. It belongs to the Kuiper belt, an area rich in small bodies – leftovers from the Solar System formation. It extends from the orbit of Neptune and it is 20 times wider than the asteroid belt between Mars and Jupiter.
Pluto’s geology is probably the most fascinating part of this study. According to models, a significant bombardment of small objects from the Kuiper belt has hit Pluto throughout the ages. The craters left by these impacts are present in certain areas of the dwarf planet, such as the Cthulhu Regio, named after the Lovecraftian cosmic entity.
Surprisingly, other areas of the dwarf planet don’t show any large craters. This is the case in Sputnik Planum (the western lobe of Tombaugh Regio, or Pluto’s “heart”), where no craters larger than 10 kilometers (6.2 miles) in diameter were observed. This suggests that the dwarf planet was geologically active within the last few hundred million years, with processes smoothing out its surface, and it's possible that such subterranean activity continues today. When dwarf planets form, they tend to have significant internal heat, but due to their small size the heat dissipates over time. No mechanism has so far been proposed on how Pluto’s surface has rejuvenated itself way beyond the expected timescales.
“How a small planet like Pluto can be active after 4.5 billion years is a complete mystery that is sending us back to the drawing board to rework our understanding of planetary geophysics.” Dr Alan Stern, the principal investigator of the New Horizons mission, told IFLScience.
Pluto’s reddish surface coloring has been identified to be caused by tholins, molecules that form when nitrogen and methane are ionized by UV radiation from the Sun or charged particles. These molecules were first discovered on Titan and they are common on comets and other satellites in the Solar System. Pluto’s blue-tinted atmosphere is due to the presence of volatile tholins, as well as hydrocarbons and meteorite dust.
The study also focused on the geology of Charon. There is a dark polar spot, called Mordor Macula, with a well-defined border that could potentially suggest a complex tectonic structure on Pluto’s main companion. Other peculiar structures are the network of fractures that mark Charon’s surface. The chasms are unofficially named after sci-fi spaceships. The largest are the Macross and Serenity chasms which form a 1,050-kilometer-long (650 miles) rift on the Pluto-facing side of Charon that reaches a depth of around 5 kilometers (3 miles).
The paper touches briefly on the preliminary results from two other satellites of Pluto – Nix and Hydra. They are both highly irregular bodies, with extremely reflective surfaces that suggest the presence of clean water ice. Such clear surfaces represent another mystery for the system, as darkening (like in the case of Charon’s surface) is expected.
New Horizons has also conducted several searches for other satellites beyond the known five. It didn’t find any undetected moons or rings.
A large amount of data from New Horizons has yet to be downlinked, so detailed analysis of the atmospheres and satellites will be produced in the next few months.