Io is the moon closest to Jupiter and is considered the most geologically active object in the solar system.
This is because very extensive plains rich in sulfur and sulfur dioxide, mountain ranges (mountains with an average height of about 6 km), and many volcanic formations on its surface have been observed.
As for the volcanoes, Io at least has many eruptions more or less continuous, which has very important effects on its topography, since the crust is in a process of constant renewal. This explains why no impact craters have been observed on its surface, as the continuous eruptions and lava flows cover them shortly after they occur.
