Context. Breathtaking imagery recorded during the European Space Agency Rosetta mission confirmed the bilobate nature of the nucleus of comet 67P/Churyumov-Gerasimenko. The peculiar appearance of the nucleus is not unique among comets. The majority of cometary cores imaged at high resolution exhibit a similar build. Various theories have been brought forward as to how cometary nuclei attain such peculiar shapes.

Aims. We illustrate that anisotropic mass loss and local collapse of subsurface structures caused by non-uniform exposure of the nucleus to solar irradiation can transform initially spherical comet cores into bilobed cores.

Methods. We derived a mathematical framework to describe the changes in morphology resulting from non-uniform insolation during the spin-orbit evolution of a nucleus. We solved the resulting partial differential equations that govern the change in the shape of a nucleus subject to mass loss and consequent collapse of depleted subsurface structures analytically for simple insolation configurations and numerically for more realistic scenarios.

Results. The proposed mechanism is capable of explaining why a large percentage of periodic comets appear to have peanut-shaped cores and why light-curve amplitudes of comet nuclei are on average larger than those of typical main belt asteroids of the same size.