Final answer:
The smoothness of the river rock is attributed to prolonged erosion by water and sand, similar to the erosion process that smooths mountain ranges on Earth. The absence of water and ice on the Moon results in its mountains having a naturally smoother appearance.
Step-by-step explanation:
The smooth surface of the river rock that Juan noticed is most likely the result of erosion by water and possibly sand over a long period of time. This natural process occurs as rocks are tumbled and rolled along the riverbed by the flowing water, causing their surfaces to become smoothed as rough and sharp edges are gradually worn away. The consistent action of water and sand, much like sandpaper, erodes the rock's surface, making it smooth.
This explanation is similar to how mountain ranges can become smooth over time. After mountains are formed by the upthrust of the Earth's crust, erosion plays a key role in shaping their appearance. Factors such as water and ice can wear down the sharp peaks and serrated edges of mountains, leading to smoother, rounded forms. On bodies like the Moon, where there is no water or ice, mountains remain with a smoother appearance due to the lack of these erosive processes.