Soru

Passage: Planetary scientists long assumed that small, distant bodies in the Kuiper Belt would be geologically inert worlds preserved in deep-freeze since the solar system's formation. However, data returned by the New Horizons probe revealed surprising resurfacing activity on Pluto, particularly within Sputnik Planitia, a vast basin filled with nitrogen-rich ice. To account for the absence of impact craters across this basin, researchers proposed a thermal convection model driven by Pluto's modest internal radiogenic heat. In this model, solid nitrogen—which becomes soft and ductile even at temperatures of 40 Kelvin—rises in warm convective plumes, spreads horizontally at the surface, and sinks along cooler margins.

To support this hypothesis, planetary geologists point to the prominent networks of polygonal troughs dividing Sputnik Planitia's ice sheets into cells several tens of kilometers wide. Computer simulations demonstrate that the spatial wavelength of these polygons closely matches the predicted aspect ratio of thermal convection cells operating within an ice layer several kilometers thick. Crucially, the presence of dark material concentrated exclusively within the bounding troughs, rather than across the cell interiors, indicates that surface ice is continuously conveyed toward the margins before descending into the subsurface. Were the surface shaped primarily by wind erosion or sublimation-driven thermal fracturing, the spatial pattern of impurities would be randomly distributed across the plains. Thus, the specific arrangement of these boundary deposits serves to validate internal thermal convection as the primary driver of Pluto's youthful surface dynamics.

Which of the following best describes the primary function of the author's reference to 'the presence of dark material concentrated exclusively within the bounding troughs' in the second paragraph?