Planetary researchers using data from NASA’s New Horizons mission have found evidence of snow and ice features on the dwarf planet Pluto that, until now, had only been seen on Earth. The findings appear this week in the journal Nature.
In this extended color image of Pluto taken by NASA’s New Horizons spacecraft, rounded and bizarrely textured mountains, named the Tartarus Dorsa, rise up along Pluto’s day-night terminator and show intricate but puzzling patterns of blue-gray ridges and reddish material in between. This view, roughly 330 miles (530 km) across, combines blue, red and infrared images taken by New Horizons’ Ralph/Multispectral Visual Imaging Camera on July 14, 2015. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute.
“Penitentes are snow and ice features formed by erosion that, on Earth, are characterized by bowl-shaped depressions several tens of cm across, whose edges grade into spires up to several meters tall,” said the authors, led by Dr. John Moores of York University in Toronto, Canada.
“They have been suggested as an explanation for anomalous radar data on Europa, but until now no penitentes have been identified conclusively on planetary bodies other than Earth.”
Regular ridges with spacings of 3,000 to 5,000 m (9,800-16,400 feet) and depths of about 500 m (1,640 feet) with morphologies that resemble high-altitude terrestrial penitentes were observed by NASA’s New Horizons spacecraft in Tartarus Dorsa in 2015.
“Conditions at Pluto are particularly suited to the creation of large penitentes,” Dr. Moores and co-authors explained.
“The Plutonian atmosphere is extremely stable, as shown by the presence of fogs in images and gravity waves consistent with wind speeds of a meter per second or less and as predicted by numerical models that argue for negligible horizontal surface wind speeds close to zero, never more than a few meters per second and often much less.”
“Furthermore, the surface is composed of volatile ices, primarily nitrogen and methane ice, close to their sublimation temperatures, that are retained in the Plutonian system on long timescales owing to cooling in the upper atmosphere that limits atmospheric escape.”
“Finally, low pressures observed by New Horizons imply a long mean free path for molecular diffusion, increasing the thickness of the near-surface laminar sublayer, a thickness that affects the spacing of penitentes.”
Dr. Moores and co-authors compared their model to Pluto’s ridges imaged by New Horizons.
“The ridges are much larger as compared to their meter-sized earthly counterparts,” the researchers said.
“This gargantuan size is predicted by the same theory that explains the formation of these features on Earth,” Dr. Moores added.
“In fact, we were able to match the size and separation, the direction of the ridges, as well as their age: three pieces of evidence that support our identification of these ridges as penitents.”
“The presence of penitentes on Pluto suggests that stable dynamic conditions and relatively high pressures are required to create these features and suggests that they may form elsewhere in the Solar System where the atmospheric conditions are appropriate,” the scientists concluded.
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John E. Moores et al. Penitentes as the origin of the bladed terrain of Tartarus Dorsa on Pluto. Nature, published online January 4, 2017; doi: 10.1038/nature20779
