SHANGHAI -- Chinese scientists have attributed the sudden movement of a boulder on the surface of a comet to a phenomenon known as the "rocket effect," which resulted from the asymmetric outburst of volatile ice inside the boulder.

The findings, published in The Astrophysical Journal, are based on data captured by the European Space Agency's (ESA) Rosetta mission during its two-year close flyby and observation of the comet 67P from 2014 to 2016.

In 2015, ESA's Rosetta spacecraft detected the boulder shifting approximately 140 meters northward near the comet's perihelion. A team led by Shi Xian from the Shanghai Astronomical Observatory of the Chinese Academy of Sciences analyzed high-resolution images and thermal data, concluding that uneven sublimation of volatile ice within the boulder generated thrust, propelling it across the surface.

The main body of a comet is its nucleus. When it is far from the sun, the comet is just the bare nucleus. As it gets closer to the sun, the ice within the nucleus sublimates, carrying dust to form a coma around the nucleus and trailing a tail behind it.

By analyzing the thermodynamic history of the boulder and its surrounding area, the research team discovered an asymmetry in the heat accumulation on the northern and southern sides of the boulder. When moving, its inner temperature on the southern side reached a peak, while the northern side remained unusually cold.

Based on these findings, the researchers proposed a novel mechanism for boulder movement: the volatile ice within the boulder undergoes intense outburst on one side due to asymmetric heating. This process creates a "rocket effect," generating a net thrust directed downward along the northern slope, which triggers the boulder's long-distance movement.

The team also mapped out a detailed timeline of activity and changes in the area where the boulder moved. They noticed that during and after the boulder's movement, there were frequent outbursts of dust nearby at night, which likely happened because the boulder's movement exposed icy layers beneath it.

As remnants of the primordial solar system, comets tell a lot about the early stage of solar system, making them crucial subjects for studying the formation and evolution of planetary systems, Shi said.

"As our research progresses, we're discovering unexpectedly diverse comet activities. Understanding the mechanisms behind these could help us unveil the mysteries of the solar system's evolution and the origins of life," Shi added.