Tribology on Pluto
R. David Whitby | TLT Worldwide November 2020
This dwarf planet shows signs of interacting surfaces in relative motion.
Pluto is an icy dwarf planet in the Kuiper belt, a ring of objects beyond the orbit of Neptune. When it was discovered in 1930, it was the largest Kuiper belt object and was declared to be the ninth planet from the Sun. Its status as a planet was questioned following the discovery of several objects of similar size in the Kuiper belt. This led the International Astronomical Union to formally define the term “planet” in 2006. That definition excluded Pluto, which reclassified it as a dwarf planet.
Like other Kuiper belt objects, Pluto is primarily made of ice and rock and is relatively small. It is one-sixth the mass of the Moon and one-third its volume. It is so far from the Sun that its surface temperature ranges from -226 C to -240 C. At these temperatures, it might be expected that the surface would be frozen solid.
NASA’s New Horizons spacecraft performed a flyby of Pluto on July 14, 2015, making detailed measurements and observations of Pluto and its moons. Pluto’s surface is characterized by mountains, valleys, plains and craters, with large differences in brightness, color and roughness. Colors range from charcoal black to dark orange and white. The plains are composed of more than 98% nitrogen ice, with traces of methane and carbon monoxide. The mountains are made of water ice.
In September 2016, astronomers announced that the reddish-brown cap of the north pole of Pluto’s largest moon, Charon, is composed of organic macromolecules produced from nitrogen and methane that appear to have come from Pluto’s atmosphere. Three notable surface features on Pluto are Tombaugh Regio, or the “Heart,” (a large bright area on Pluto’s side opposite Charon), Cthulhu Macula, or the “Whale” (a large dark area on the trailing hemisphere) and the “Brass Knuckles” (a series of equatorial dark areas on the leading hemisphere).
Sputnik Planitia, the western lobe of the “Heart,” is a 1,000 km-wide basin of frozen nitrogen and carbon monoxide ices, divided into polygonal cells. Unlike other parts of Pluto, Sputnik Planitia is smooth and not cratered. The New Horizons team, led by Alan Stern, interprets this surface as convection cells that carry floating blocks of water ice crust and sublimation pits toward their margins. There are obvious signs of glacial flows, both into and out of the basin. The absence of craters indicates that this part of the surface is very young in terms of the age of the Solar System, and the latest studies have given an age of between 140,000 and 270,000 years.
According to the measurements made by New Horizons, the surface pressure on Pluto is about 1 Pa (10 μbar), roughly one million to one hundred thousand times less than Earth’s atmospheric pressure. It was initially thought that, as Pluto moves away from the Sun during its highly elliptical orbit, its atmosphere should gradually freeze onto the surface. The New Horizons data and ground-based occultations show that Pluto’s atmospheric density increases, and it is likely to remain gaseous throughout Pluto’s orbit.
The New Horizons team believes that even a small increase in Pluto’s surface temperature can lead to exponential increases in its atmospheric density, from 18 hPa to as much as 280 hPa (three times that of Mars to a quarter that of the Earth). At such densities, nitrogen could flow across the surface as liquid.
Sputnik Planitia is Pluto’s most dominant geologic feature. It is believed to be an ancient impact basin that, over time, has been partly filled by a thick layer of frozen nitrogen, methane and carbon monoxide. Experiments have found that mixtures of frozen nitrogen, methane and carbon monoxide at temperatures of around -226 C can flow thixotropically, albeit very slowly. This explains why the area is very smooth, as the mixture will slowly erase any impact craters.
Two analyses have concluded that Sputnik Planitia lies almost directly opposite Charon because the basin literally dragged the crust of Pluto around to the current location. There is only a 5% chance that it formed so close to the exact anti-Charon location. Geophysicists refer to this crustal shifting as true polar wander. This is the tendency of spinning objects to reorient themselves so that locations with mass excesses end up on the equator and mass deficiencies at the poles.
Clearly, tribology is working its magic, even as far away as Pluto.
David Whitby is chief executive of Pathmaster Marketing Ltd. in Surrey, England. You can reach him at pathmaster.marketing@yahoo.co.uk.