How the Comet Crumbles: Mass Wasting Drives Outbursts on Comet 67P/Churyumov-Gerasimenko

Comets are often imagined as dirty snowballs drifting silently through the dark, but they are actually active, shifting worlds. As a comet travels closer to the Sun, the heat causes its frozen ices to turn directly into gas, a process that blows dust and rock off the surface. Sometimes, this activity is steady, like a gentle breeze. Other times, it explodes into violent outbursts. These are short, intense events where a comet can eject hundreds of tonnes of material in just minutes, creating a sudden, bright flash in the sky. For decades, scientists have watched these explosions from Earth and from spacecraft, but they have struggled to understand what actually happens on the ground when one occurs. Do these blasts tear the surface apart, or do they simply shoot out from a stable landscape? The answer matters because it tells us how these ancient, icy bodies change over time and whether they might eventually break apart completely.

A new study focused on Comet 67P/Churyumov-Gerasimenko, the icy world visited by the European Space Agency's Rosetta mission, has finally connected the dots between these explosions and the physical changes on the comet's surface. By carefully reviewing thousands of images taken by Rosetta, researchers discovered that every outburst with adequate imaging coverage was accompanied by a visible change in the landscape. This finding overturns the long-held idea that the explosion happens first and then scatters the surface. Instead, the evidence shows that the surface is already crumbling and failing before the explosion even begins. The outburst is not the cause of the damage; it is the result.

The team, led by researchers at Brown University and other institutions, re-examined the entire archive of images from the Rosetta mission. Previous studies had linked only six outbursts to surface changes, but the new analysis, using advanced tools to align images taken from different angles and distances, expanded that number to 43 confirmed cases. The researchers grouped these events into 37 distinct areas on the comet's surface. In every single one of these areas where the data was clear enough to see, the ground had changed. The changes were not random or chaotic; they all pointed to a single process: mass wasting. This is a geological term for the movement of rock and soil under the influence of gravity, similar to a landslide or a cliff collapse on Earth, but happening in the low gravity of a comet.

The study revealed that this crumbling happens in several different ways, depending on the type of ground. In some places, massive cliffs of solid ice and rock simply gave way. Large blocks, some tens of meters across, detached and tumbled down, sometimes breaking into smaller pieces as they fell. In other areas, the ground was made of loose, granular material, like a pile of sand or gravel. Here, the edges of the terrain slowly retreated, inch by inch, over weeks or months, as the loose material slid away. In a few instances, the ground collapsed inward to form new pits or made existing ones larger. Despite these different appearances, the underlying mechanism was the same: the surface material, weakened by temperature changes and the stress of the comet's journey, lost its strength and failed.

Crucially, the timing of these events tells a specific story. In the few cases where the images were frequent enough to capture the sequence of events, the researchers saw the surface fail before the explosion. For example, in one region, a large boulder shifted position and slid down a slope days before a plume of gas and dust erupted from that exact spot. In another area, a cliff edge began to retreat, and as it did, smaller plumes started to appear from the newly exposed ground. This sequence suggests a clear chain of events. As the comet heats up, the surface cracks and weakens. Eventually, a section of the ground collapses or slides away. This movement strips away the layer of dirt and rock that was covering the ice beneath. Once that protective layer is gone, the buried ice is suddenly exposed to the Sun and begins to vaporize rapidly. This rapid release of gas creates the pressure that launches the massive outburst we see from space.

The researchers argue that this process is likely the main way comets erode and change their shape over time. It is not just the explosions that reshape the comet, but the slow, steady crumbling that leads to them. The study also found that the type of gas released during an outburst does not always match the type of ground collapse. Some explosions from collapsing cliffs were rich in carbon dioxide, while others were mostly water vapor, showing that the composition of the gas depends on what specific layer of ice was exposed, not just on how the ground broke. This means that by watching an outburst, scientists can learn about the hidden layers of ice inside the comet, even if they cannot see the surface directly.

This new understanding changes how we view comets. They are not just static balls of ice that occasionally blow up; they are dynamic worlds where the surface is constantly failing and reshaping itself. The explosions are simply the most dramatic sign of this ongoing process. While the Rosetta mission provided the best data ever collected, the researchers note that the comet's surface likely changes in many smaller ways that are too subtle to create a visible explosion. These smaller failures probably happen all the time, slowly grinding down the comet's landscape. The study confirms that for Comet 67P, and likely for others, the story of its evolution is written in the dust and debris of its own crumbling surface.