The spots appear on the side of Pluto that always faces its largest moon, Charon—the face that will be invisible to New Horizons when the spacecraft makes its close flyby the morning of July 14. New Horizons principal investigator Alan Stern of the Southwest Research Institute, Boulder, Colorado, describes this image as “the last, best look that anyone will have of Pluto’s far side for decades to come.”
The spots are connected to a dark belt that circles Pluto’s equatorial region. What continues to pique the interest of scientists is their similar size and even spacing. “It’s weird that they’re spaced so regularly,” says New Horizons program scientist Curt Niebur at NASA Headquarters in Washington. Jeff Moore of NASA’s Ames Research Center, Mountain View, California, is equally intrigued. “We can’t tell whether they’re plateaus or plains, or whether they’re brightness variations on a completely smooth surface.”
The large dark areas are now estimated to be 300 miles (480 kilometers) across, an area roughly the size of the state of Missouri. In comparison with earlier images, we now see that the dark areas are more complex than they initially appeared, while the boundaries between the dark and bright terrains are irregular and sharply defined.
On the morning of July 14, New Horizons will pass about 7,800 miles (12,500 kilometers) from the face with a large heart-shaped feature that’s captured the imagination of people around the world.
New Horizons’ newest images reveal Pluto’s largest moon Charon to be a world of chasms and craters. The most pronounced chasm, which lies in the southern hemisphere, is longer and miles deeper than Earth’s Grand Canyon, according to William McKinnon, deputy lead scientist with New Horizon’s Geology and Geophysics investigation team.
“This is the first clear evidence of faulting and surface disruption on Charon,” says McKinnon, who is based at the Washington University in St. Louis. “New Horizons has transformed our view of this distant moon from a nearly featureless ball of ice to a world displaying all kinds of geologic activity.”
The most prominent crater, which lies near the south pole of Charon in an image taken July 11 and radioed to Earth today, is about 60 miles (96.5 kilometers) across. The brightness of the rays of material blasted out of the crater suggest it formed relatively recently in geologic terms, during a collision with a small body some time in the last billion million years.
The darkness of the crater’s floor is especially intriguing, says McKinnon. One explanation is that the crater has exposed a different type of icy material than the more reflective ices that lie on the surface. Another possibility is that the ice in the crater floor is the same material as its surroundings but has a larger ice grain size, which reflects less sunlight. In this scenario, the impactor that gouged the crater melted the ice in the crater floor, which then refroze into larger grains.
A mysterious dark region near Charon’s north pole stretches for 200 miles. More detailed images that New Horizons will take around the time of closest approach to the moon on July 14 may provide hints about the dark region’s origin
Chasms, craters, and a dark north polar region are revealed in this image of Pluto’s largest moon Charon taken by New Horizons on July 11, 2015. Credits: NASA/JHUAPL/SWRI