The dwarf planet Pluto displays a wide variety of geological landforms, including those resulting from glaciological and surface-atmosphere interactions as well as impact, tectonic, possible cryovolcanic, and mass-wasting processes, says a new study in the journal Science. The study represents the first published scientific results from the close flyby of the Pluto system on July 14, 2015.
This Pluto mosaic was made from New Horizons LORRI images taken on July 14, 2015, from a distance of 49,700 miles (80,000 km). This view is projected from a point 1,118 miles (1,800 km) above Pluto’s equator, looking northeast over the dark, cratered Cthulhu Regio toward the bright, smooth expanse of icy plains called Sputnik Planum. Pluto’s North Pole is off the image to the left. This mosaic was produced with panchromatic images from the New Horizons LORRI camera, with color overlaid from the Ralph color mapper onboard New Horizons. Image credit: S.A. Stern et al, doi: 10.1126/science.aad1815.
“The New Horizons mission completes our initial reconnaissance of the Solar System, giving humanity our first look at this fascinating world and its system of moons,” said Jim Green, Director of NASA’s Planetary Science Division.
New Horizons reached a distance of 7,750 miles (12,500 km) from Pluto’s surface during its July 14 closest approach, gathering so much data it will take another year to return to Earth.
The data returned so far show a surprisingly wide variety of landforms and terrain ages on Pluto, as well as substantial albedo, color, and compositional variation.
Evidence was also found for a water ice-rich crust, geologically young surface units, tectonic extension, surface volatile ice convection, possible wind streaks, volatile transport, and glacial flow.
“Water ice is a new element that we must consider as we try to piece together Pluto’s complex surface composition,” said co-author Dr Silvia Protopapa of the University of Maryland.
According to the New Horizons team, the western lobe of Pluto’s heart-shaped region contains methane ice and carbon monoxide ice, while the dark red equatorial regions appear to contain very little volatile ice.
The red color on the surface indicates the presence of organic compounds called tholins, which are the result of energetic irradiation of methane, nitrogen and carbon monoxide mixtures.
Pluto’s atmosphere is highly extended, with trace hydrocarbons, a global haze layer, and a surface pressure near 10 microbars, which is lower than expected.
“It’s unclear whether this reflects a recent decrease in the mass of the atmosphere,” the scientists said.
“The Pluto system surprised us in many ways, most notably teaching us that small planets can remain active billions of years after their formation,” said Dr Alan Stern of the Southwest Research Institute, principal investigator for New Horizons and lead author on the study.
“We were also taught important lessons by the unexpected degree of geological complexity that both Pluto and its large moon Charon display.”
The bulk densities of Pluto and Charon were found to differ by less than 10 percent, which is consistent with bulk rock contents for the two bodies that are likewise similar.
The diverse geology and apparent recent activity raise fundamental questions about how small planetary bodies remain active many billions of years after formation. The research suggests that other large worlds in the Kuiper belt – such as Eris, Makemake, and Haumea – could also have similarly complex histories that rival those of terrestrial planets.
“Triton, likely a former Kuiper Belt planet captured by Neptune, was considered the best analog for Pluto prior to the July 14 flyby,” the researchers said.
They now believe that the geologies of Triton and Pluto are more different than similar, “but will know more as additional data are returned.”
Pluto’s large moon Charon displays extensional tectonics and extensive resurfacing, as well as possible evidence for a heterogeneous crustal composition.
Its North Pole displays puzzling dark terrain. The moon has no detectable atmosphere, the data reveal.
Although no new small satellites of Pluto were detected, New Horizons provided the first resolved images of Pluto’s moons Nix and Hydra, finding Nix is approximately 34 miles (54 km) long and Hydra is 27 miles (43 km) long.
Puzzlingly, Nix and Hydra both have reflective surfaces, suggesting relatively clean water ice.
The rotational patterns of Pluto’s moons also puzzle scientists, as the two moons do not always have the same face locked toward Pluto.
“We knew Nix and Hydra were slowly tumbling in unpredictable ways, based on ground-based findings,” said co-author Prof. Douglas Hamilton, of the University of Maryland.
“With New Horizons data, we now believe Nix and Hydra are spinning really fast and rotating in an odd way, and may be the only regular moons, meaning satellites that are near their host planets, which do not always point the same face toward their primary body.”
“The strange rotation patterns of these two moons could be due to the system’s domination by Pluto and its largest moon, Charon, which together form a binary planet.”
“It’s possible that Nix and Hydra can’t focus on locking one face toward Pluto because Charon keeps sweeping past and stirring things up.”
Prof. Hamilton added: “based on the data, we can confidently say that between Pluto and its outermost moon Hydra there are no new icy objects with a diameter larger than 0.9 miles (1.5 km), which is about 5 times smaller than Pluto’s smallest known moon Styx.”
Taken together, these results from the flyby of the Pluto system pave the way for planetary researchers’ better understanding of processes of planetary evolution.
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S.A. Stern et al. 2015. The Pluto system: Initial results from its exploration by New Horizons. Science, vol. 350, no. 6258; doi: 10.1126/science.aad1815
