The surface of a comet is dramatically shaped by the light of the Sun. The ice melts away as they orbit closer to our star, causing the comets to emit large outbursts that can look like plumes of material. These outbursts were thought to be erupting geysers, but new research claims they are actually created by avalanches.

Jordan Steckloff from the Planetary Science Institute in Arizona claims that comets can’t generate the internal pressures necessary for geysers. He argues they don’t have enough heat, even at the closest point to the Sun, so he modeled an alternative scenario where the outbursts are limited to changes on the surface.

“Rapid asymmetric brightening events of comets have been observed for decades and have long been thought to be the result of some sort of eruption of materials from deep within the interior of a comet,” Steckloff said in a statement.

"However, there is a major problem with this model. There is no internal heat source on comets to power geyser-like eruptions. Instead, these outburst plumes are the natural result of avalanches."

When the Sun hits the irregular surface of a comet, the frozen material turns into a gas, which gently leaves the surface of the comet in a weak breeze. When this happens at the bases of slopes and cliffs, material slides down and it is propelled upwards by the sublimation breeze. This, for Steckloff, is the cause of the plumes.

The model is consistent with data from the ESA mission, Rosetta, which crash-landed on comet 67P /Churyumov-Gerasimenko last month. With this approach it might soon be possible to study the plumes further and even estimate the amount of material moved and the location on the surface where the plume originated from.

"Ultimately, understanding this novel mechanism of outbursting may allow the surface processes of distant comets to be studied from Earth through ground-based observations of their outbursts," Steckloff concluded.

The findings were presented at the joint meeting of the 48th American Astronomical Society Division for Planetary Sciences and 11th European Planetary Science Congress in Pasadena, California.