Pluto, a dwarf planet in the distant Kuiper Belt, has a surface scarred by cosmic impacts yet also remarkably smooth. Pluto definitely has craters, but their presence is highly localized, revealing a complex history. The surface is a dramatic mosaic of ancient, heavily-battered terrain next to vast, young plains unblemished by bombardment. This stark variation in crater density provides scientists with a powerful tool for deciphering the geological events that shaped this icy world.
Regions of Heavy Crater Concentration
Certain areas on Pluto record the dwarf planet’s earliest history, displaying a high concentration of impact craters. The dark, elongated equatorial region known as Belton Regio (formerly Cthulhu Macula) is a prime example of this ancient terrain. The eastern portion features “alpine” terrain that is heavily cratered, indicating it has remained largely undisturbed for billions of years.
These densely-packed craters are relatively degraded, suggesting they are exceptionally old and have been slowly modified over eons by processes like the gradual sublimation of surface ices. The presence of so many impacts provides a baseline for what Pluto’s entire surface likely looked like early in its formation, before internal geological activity began. These heavily cratered regions are estimated to be around four billion years old, representing the most ancient, preserved parts of the dwarf planet’s crust.
Some of the craters in these regions show bright ‘halos’ around their rims and walls. Compositional data indicates that this bright material is a form of methane ice. This suggests that the impact process on Pluto can expose or redistribute subsurface ices, which then interact with the sparse atmosphere to settle in patterns dictated by local topography. The density of these ancient craters confirms that Pluto was not exempt from the intense bombardment experienced by all solar system bodies.
The Vast Uncratered Plains
In contrast to the ancient, crater-riddled regions, enormous areas of Pluto’s surface are devoid of recognizable impact scars. The most prominent example of this geological youth is Sputnik Planitia, the western lobe of Pluto’s “heart” feature. This immense, smooth plain, made primarily of nitrogen ice, shows no observable craters, even in high-resolution images.
The absence of craters in Sputnik Planitia indicates that a dynamic geological process is actively erasing them. The primary mechanism responsible for this continuous resurfacing is convection within the thick layer of nitrogen ice. The nitrogen ice is warm enough to flow slowly, similar to boiling water, forming polygonal cells tens of kilometers across that effectively churn the surface.
This churning motion continuously pushes older surface material downward and brings fresh, smooth ice to the top, wiping away any evidence of impacts. Other resurfacing mechanisms, such as glacial flow and sublimation of volatile ices, also contribute to keeping the plains pristine. This ongoing geological activity is unexpected for a small world in the distant Kuiper Belt. The continuously renewed surface of Sputnik Planitia points to a complex and active interior, which may even involve a subsurface liquid water ocean.
What Crater Density Reveals About Geologic Time
The stark difference in crater density across Pluto’s surface provides scientists with a method of relative age dating for various geological units. The technique, known as crater counting, is based on the principle that older surfaces accumulate more impact craters than younger surfaces. This allows for the construction of a timeline of Pluto’s surface evolution.
The heavily cratered terrains, such as portions of Belton Regio, have the highest spatial density, translating to the oldest age estimates, likely exceeding four billion years. Conversely, smooth, uncratered regions like Sputnik Planitia have extremely low, or zero, crater density. This suggests an exceptionally young surface age, estimated to be less than 10 million years old.
This wide range of crater densities implies that Pluto’s history has been complex, involving periods of intense bombardment followed by significant geological resurfacing. Crater analysis shows that roughly 27% of Pluto’s mapped high-resolution surface area consists of these young, crater-free terrains. The varying levels of crater degradation across the dwarf planet confirm a heterogeneous history, with different parts of the crust experiencing unique geological processes at different times.
The New Horizons Mission
The ability to analyze Pluto’s craters and understand their distribution stems from the observations of the New Horizons mission. Launched by NASA, the spacecraft conducted a historic flyby in July 2015, providing the first high-resolution images and scientific data of the Pluto system.
The primary instrument for crater analysis was the Long Range Reconnaissance Imager (LORRI), a high-magnification telescope and camera. LORRI captured images resolving surface features down to about 50 meters, necessary to identify and count impact craters. This data confirmed the existence of both ancient, heavily cratered regions and smooth, geologically young plains. The information gathered by New Horizons transformed Pluto into a dynamic world with a complex geological history.