Generation of photoclinometric DTMs for application to transient changes on the surface of comet 67P/Churyumov-Gerasimenko

Abstract

Context. The wide spatial and temporal coverage of 67P/Churyumov-Gerasimenko (67P) by the Rosetta mission has revealed a surface created by scattered large-scale changes and numerous small-scale changes. The many small-scale changes are of particular interest because they are unexpected and ubiquitous. As their topographic relief is often smaller than one meter, which is below the resolution of any shape models, we need higher resolution topography to analyze them properly. Aims. We describe a photoclinometry method that is able to retrieve surface elevations for a single OSIRIS image of the surface of 67P. With this method, we can provide accurate measures, along with error estimates, of the centimeter-scale topography of observed transient changes. Methods. Photoclinometry, or shape-from-shading, estimates heights by examining the light reflection of the surface as dictated by a photometric model under a specified set of viewing geometries. Assuming a standard photometric model for 67P, we can recreate the shading of a surface under specified viewing geometries. The output is a high-resolution height map that matches the original image pixel by pixel. We then provide estimates of the error in the retrieved heights and ensure that our method is valid with a series of checks. Results. We generate digital terrain models (DTMs) with a vertical resolution comparable to or smaller than the pixel scale. This allows us to accurately measure changes in the surface topography on centimeter scales. We find that most changes within the smooth terrains involve the transport or removal of material thinner than one meter.