Measuring cometary material strengths and other mechanical properties can help put constraints on the formation and evolution of these, likely primordial, bodies. I will present results from two pieces of work investigating the near surface of 67P/Churyumov-Gerasimenko from Rosetta: an observational study of overhanging cliffs from OSIRIS images, and modelling work on thermo-mechanical fracturing. These studies support a low bulk strength, primordial model of comets but with a significantly thermally modified surface layer, with greater strength, as found by the Philae lander.
Rocky exoplanets are extremely common in the galaxy, but we currently know little about most of them. What are these planets like? I will discuss how theories originally developed for Earth can shed new light on exoplanets and, conversely, how the study of exoplanets can help us better understand Earth.