Bilobate comets—small icy bodies with two distinct lobes—are a common configuration among comets, but the factors shaping these bodies are largely unknown. Cometary nuclei, the solid centres of comets, erode by ice sublimation when they are sufficiently close to the Sun, but the importance of a comet’s internal structure on its erosion is unclear. Here we present three-dimensional analyses of images from the Rosetta mission to illuminate the process that shaped the Jupiter-family bilobate comet 67P/Churyumov–Gerasimenko over billions of years. We show that the comet’s surface and interior exhibit shear-fracture and fault networks, on spatial scales of tens to hundreds of metres. Fractures propagate up to 500 m below the surface through a mechanically homogeneous material. Through fracture network analysis and stress modelling, we show that shear deformation generates fracture networks that control mechanical surface erosion, particularly in the strongly marked neck trough of 67P/Churyumov–Gerasimenko, exposing its interior. We conclude that shear deformation shapes and structures the surface and interior of bilobate comets, particularly in the outer Solar System where water ice sublimation is negligible.

双叶彗星（具有两个不同叶的小的冰冷体）是彗星之间的常见构造，但形成这些体的因素在很大程度上尚不清楚。彗核的实心是彗星的坚固中心，当它们足够接近太阳时会被冰升华侵蚀，但是彗星内部结构对其侵蚀的重要性尚不清楚。在这里，我们对罗塞塔任务的图像进行了三维分析，以阐明形成木星家族双叶彗星67P /丘留莫夫-格拉西缅科数十亿年的过程。我们表明，彗星的表面和内部在数十至数百米的空间尺度上均表现出剪切断裂和断层网络。裂缝通过机械均质的材料传播至地表以下500 m。通过裂缝网络分析和应力建模，我们发现，剪切变形会产生控制机械表面侵蚀的断裂网络，特别是在67P / Churyumov–Gerasimenko的明显标记的颈部槽中，露出其内部。我们得出的结论是，剪切变形塑造并构造了双叶类彗星的表面和内部，尤其是在外部太阳系中，水冰的升华可以忽略不计。