Nine years of waiting are over! New Horizons has been to Pluto! Well done humans!
Pluto has turned out to be slightly bigger than we thought having a diameter of 2370 km. This means Pluto is about 30km larger than Eris, making Pluto the largest known object in the Kuiper Belt by diameter (Eris is still 27% more massive than Pluto). It does not mean that it will be reclassified from dwarf planet to planet!
Pluto’s surface colour is a good match for the surface of Neptune’s giant moon Triton, both worlds have surfaces of frozen nitrogen stained to a pinkish beige tone by tholins, complex but poorly understood tarry organic compounds created by the action of UV light and cosmic rays on methane and ethane. In contrast, Charon, Pluto’s largest moon has a greyish-looking surface of water ice.
Pluto has a surprisingly geologically complex landscape. There are relatively few impact craters on Pluto and even the more battered surface of Charon has less craters than expected. To geologists this suggests that they have been resurfaced recently. Could these worlds’ seemingly youthful landscapes mean they are still geologically active?
The apparently smoother left side of the heart-shaped feature now called Tombaugh Regio after Clyde Tombaugh, discoverer of Pluto, is made of carbon monoxide ice. Two much darker regions flank Tombaugh Regio, the larger of these was referred to before the encounter as “the whale” but is now being called Cthulhu Regio after the monstrous alien deity created by HP Lovecraft. The smaller dark region is being called Krun after one of five Mandaean lords of the underworld. These darker areas may be thick deposits of tholins. Pluto also has a polar cap of methane and nitrogen ice.
Near the southern end of Tombaugh Regio lies Norgay Montes, a spectacular mountainous landscape with jagged peaks jutting up to 3.5 km into the black sky. Nitrogen ice is too soft to build mountains like these, so they must be composed of water ice. Scientists have been startled by the total absence of impact craters in the area. This suggests that the area is “young”, perhaps less than 100 million years old. This discovery has already overturned our knowledge of Pluto; we had thought that there had been no significant geological events there in possibly billions of years. Instead it seems there are still processes driving mountain building there. How Pluto has retained the energy to do this is a mystery. Any icy worlds visited previously have been a moon of a gas giant planet, on these bodies geology is driven by tidal heating but this cannot occur on Pluto or Charon.
The thin Plutonian atmosphere is fizzing into space faster than previously expected losing 500 tonnes of atmosphere every hour. The atmosphere seems crystal clear without clouds or hazes.However Pluto has a climate. According to Alan Stern over a 248 year orbit “It snows on Pluto and the snow sublimates back into the atmosphere”.
New Horizons is now hurtling away from the dwarf planet at 14 km/s, its ongoing mission to explore deep into the Kuiper Belt. There will be a lot to learn in the next few days. I’ll continue to update this blog as the facts continue to come in.
Note: the names for features on Pluto and Charon have been used during the press briefings given by mission scientists but have not yet been formally agreed by the International Astronomical Union.
(Article by Colin Johnston, Science Education Director)