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Why do Oberon and Umbriel have more large craters than Titania and Ariel?

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Images obtained by Voyager 2. — Uranus Two views of the southern hemisphere of Uranus, produced from images obtained by Voyager 2 on Jan. 17, 1986. In colours visible to the unaided human eye, Uranus is a bland, nearly featureless sphere (left). In a colour-enhanced view processed to bring out low-contrast details, Uranus shows the banded cloud structure common to the four giant planets (right). From the polar perspective of Voyager at the time, the bands appear concentric around the planet's rotational axis, which is pointing nearly toward the Sun. Small ring-shaped features in the right image are artifacts arising from dust in the spacecraft's camera.

The presence of more large craters on Oberon and Umbriel compared to Titania and Ariel indicates that the surfaces of Oberon and Umbriel are older.1

Large craters, typically ranging from 50 to 200 km in diameter, are believed to date back to the early history of the solar system more than four billion years ago, when large planetesimals were still present. The high density of these large craters on Oberon and Umbriel suggests that their surfaces have remained largely unchanged and were never significantly reworked by internal geological processes.12

In contrast, the fewer number of large craters on Titania and Ariel implies that their surfaces are younger, having been resurfaced or modified by internal activity at some point after the period of heavy bombardment. While these differences in surface age are clear, they do not follow an obvious pattern related to the moons' sizes or their distances from Uranus, and the specific reasons for these variations remain largely unexplained.1