Changes that the Rosetta spacecraft discovered on the surface of Comet 67P/Churyumov-Gerasimenko, including the collapse of entire cliffs, were likely driven by seasonal events, according to a new study. The results help shed light on how the comet has been shaped over time. Between December 2014 and June 2016, the Rosetta spacecraft investigated Comet 67P, mapping its surface in detail at high resolution and closely monitoring the comet's journey through the inner solar system. By analyzing surface changes over this timeframe, Mohamed Ramy El-Maarry et al. identified numerous cases of cometary-specific weathering, erosion, and various transient events. They observed the collapse of cliffs at two locations on the comet, where pre-existing fractures gave way, causing sections of material tens of meters (m) long to crumble. Sequences of images taken at different times reveal that, as the comet approached perihelion -- the point along its orbital path closest to the Sun -- steep slopes at one location retreated as fast as 5.4 m/day. Other areas revealed circular ripple-like features on the surface that reached a diameter of about 100 m in less than three months, before fading away and being replaced by a new set of ripples. The authors note that most changes - including erosion and the movement of boulders -- occurred around perihelion, suggesting that they were driven by shifting patterns of exposure to sunlight. Some changes, such as the fracturing of 67P's neck, are related to the comet's spin rate, the authors say. Combining all of these features is helping them to determine how quickly Comet 67P's surface is changing, indicating that its major landforms have been in place for numerous orbits, probably longer than it has been on its current orbital pattern.