File:Pluto crater counts rectangular projection 2015-11-09.jpg

English: Locations of more than 1,000 craters mapped on Pluto by NASA's New Horizons mission indicate a wide range of surface ages, which likely means that Pluto has been geologically active throughout its history.

Commentary from http://pluto.jhuapl.edu/News-Center/News-Article.php?page=20151109 follows:

One of the more surprising findings from New Horizons is the wide range of surface ages found on Pluto, from ancient to intermediate to relatively young in geological terms. Crater counts used to determine surface unit ages indicate that Pluto has ancient surface areas dating to just after the formation of the planets, about 4 billion years ago. In addition, there’s a vast area that was geologically born “yesterday,” meaning it may have formed within the past 10 million years. This area – informally named Sputnik Planum – appears on the left side of Pluto’s “heart” and is completely impact-free in all images returned to date.

Scientists wondered if Sputnik Planum’s smooth, icy plains were an oddity; did a recent geological episode form the plains long after all other geologic activity ceased?

Apparently not. New data from crater counts reveal the presence of intermediate or “middle-aged” terrains on Pluto as well. This suggests that Sputnik Planum is not an anomaly—that Pluto has been geologically active throughout much of its more than 4-billion-year history. “We’ve mapped more than a thousand craters, which vary greatly in size and appearance,” said postdoctoral researcher Kelsi Singer, of the Southwest Research Institute (SwRI) in Boulder, Colorado. “Among other things, I expect cratering studies like these to give us important new insights into how this part of the solar system formed.”

Crater counts are giving the New Horizons team insight into the structure of the Kuiper Belt itself. The dearth of smaller craters across Pluto and its large moon Charon indicate that the Kuiper Belt likely had fewer smaller objects than some models had predicted. This leads New Horizons scientists to doubt a longstanding model that all Kuiper Belt objects formed by accumulating much smaller objects of less than a mile wide. The absence of small craters on Pluto and Charon support other models theorizing that Kuiper Belt objects tens of miles across may have formed directly, at their current—or close to current—size.