Penitentes are ablative features observed in snow and ice that, on Earth, arecharacterized by regular cm to tens of cm spaced bowl-shaped depressions whoseedges grade into tall spires up to several meters tall (Nichols et al. 1939,Lliboutry et al. 1954, Claudin et al. 2015). While penitentes have beensuggested as an explanation for anomalous radar data on Europa (Hobley et al.2013), hitherto no penitentes have been identified conclusively on otherplanetary bodies. Regular ridges with spacing of 3000 m to 5000 m and a depthof $\sim500$ m with morphologies that resemble penitentes (Fig. 1) have beenobserved by the New Horizons spacecraft (Moore et al. 2016, Stern et al. 2015,Gladstone et al. 2016, Moore et al. 2017) in the Tartarus Dorsa (TD) region ofPluto (approximately 220-250$^\circ$ E, 0-20$^\circ$ N). Here we reportsimulations, based upon a recent model (Claudinet al. 2015) adapted toconditions on Pluto (Gladstone et al. 2016, Toigo et al. 2015), that reproduceboth the tri-modal orientation and the spacing of these features by deepeningpenitentes. These penitentes deepen by of order 1 cm per orbital cycle in thepresent era and grow only during periods of relatively high atmosphericpressure, suggesting a formation timescale of several tens of millions ofyears, consistent with cratering ages and the current atmospheric loss rate ofmethane. This time scale, in turn, implies that the penitentes formed frominitial topographic variations of no more than a few 10s of meters, consistentwith Pluto's youngest terrains.
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