  Penitentes are ablative features observed in snow and ice that, on Earth, are
characterized by regular cm to tens of cm spaced bowl-shaped depressions whose
edges grade into tall spires up to several meters tall (Nichols et al. 1939,
Lliboutry et al. 1954, Claudin et al. 2015). While penitentes have been
suggested as an explanation for anomalous radar data on Europa (Hobley et al.
2013), hitherto no penitentes have been identified conclusively on other
planetary bodies. Regular ridges with spacing of 3000 m to 5000 m and a depth
of $\sim500$ m with morphologies that resemble penitentes (Fig. 1) have been
observed 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 of
Pluto (approximately 220-250$^\circ$ E, 0-20$^\circ$ N). Here we report
simulations, based upon a recent model (Claudinet al. 2015) adapted to
conditions on Pluto (Gladstone et al. 2016, Toigo et al. 2015), that reproduce
both the tri-modal orientation and the spacing of these features by deepening
penitentes. These penitentes deepen by of order 1 cm per orbital cycle in the
present era and grow only during periods of relatively high atmospheric
pressure, suggesting a formation timescale of several tens of millions of
years, consistent with cratering ages and the current atmospheric loss rate of
methane. This time scale, in turn, implies that the penitentes formed from
initial topographic variations of no more than a few 10s of meters, consistent
with Pluto's youngest terrains.
