Controlled Irradiative Formation of Penitentes

Spike-shaped structures are produced by light-driven ablation in very different contexts. Penitentes 1–4 m high are common on Andean glaciers, where their formation changes glacier dynamics and hydrology. Laser ablation can produce cones $10–100\mu \mathrm{m}$ high with a variety of proposed applications in materials science. We report the first laboratory generation of centimeter-scale snow and ice penitentes. Systematically varying conditions allows identification of the parameters controlling the formation of ablation structures. We demonstrate that penitente initiation and coarsening require cold temperatures, so that ablation leads to sublimation. Once penitentes have formed, further growth of height can occur by melting. The penitentes initially appear as small structures (3 mm high) and grow by coarsening to 1–5 cm high. Our results are an important step towards understanding ablation morphologies.

Figure 1

Images of ablation spikes. (a) Penitentes in the Andes, approximately 2 m high [reused with permission from Javier Corripio] [4]. (b) Silicon spikes created through laser ablation [Reused with permission from Tsing-Hua Her, Richard J. Finlay, Claudia Wu, Shrenik Deliwala, and Eric Mazur, Applied Physics Letters. Copyright 1998, American Institute of Physics.] [8]. The largest spikes are approximately $15\mu \mathrm{m}$ high. (c) Laboratory penitentes approximately 5 cm high.

Figure 2

Experimental apparatus. A horizontal freezer with Plexiglas cover serves as the environmental chamber. Temperature control is achieved by passing dried compressed air through liquid nitrogen and into the chamber. Humidity control is achieved by introducing water vapor. For further details, see Ref. 29.

Figure 3

Coarsening data. (a) Measurements of penitente height (circles) and peak separation (diamonds) versus time. (b) Penitente aspect ratio (height/peak separation) versus time. After an initial coarsening phase $\sim 300\mathrm{\text{min}}$ in length, the dynamics transition from sublimation throughout the structures to melting at the troughs and coarsening ceases. In this experiment the initial particle radius was approximately $50\mu \mathrm{m}$, infrared radiation intensity approximately $300\mathrm{W}{\mathrm{m}}^{-2}$, temperature $-15°\mathrm{C}$, and relative humidity 10%.

Figure 4

Images of penitentes. (a) A snow block showing larger penitentes in the center where the light illumination is most intense. The curved shape of the surface is caused by the conical irradiation profile of the flood lamp. The snow block is 80 by 50 cm and the largest, central penitentes are 6 cm high. (b) Field of penitentes after the transition to melting in the troughs has occurred. The penitente aspect ratio increases and the tips become more rounded. The largest penitentes are 3 cm high.