New Horizons’ final approach images show surface geology on Pluto
With just days left until its historic Pluto flyby, NASA’s New Horizons spacecraft is taking and sending back images that reveal Pluto to be a geologically complex world. Fuzzy, blurry images are being replaced by sharper ones showing distinct surface features – to the delight of mission scientists.
Photos taken on July 9 from a distance of 3.3 million miles (5.4 million km) show a large, dark area on the Charon-facing side of Pluto dubbed “the whale” with a complex gray region above its “tail”.
New Horizons program scientist Curt Niebur noted that scientists are very interested in these features, stating: “It’s a unique transition region with a lot of dynamic processes interacting, which makes it of particular scientific interest.”
The “tail” is located along Pluto’s equator. According to principal investigator Alan Stern, “Among the structures tentatively identified in this new image are what appear to be polygonal features; a complex band of terrain stretching east-northeast across the planet, approximately 1,000 miles long; and a complex region where bright terrains meet the dark terrains of the whale. After nine and a half years in flight, Pluto is well worth the wait.”
Finally close enough to reveal distinct features, New Horizons’ Long Range Reconnaissance Imager (LORRI ) instrument took its best and last image of the intriguing four dark spots on the side of Pluto that will have rotated out of sight during the July 14 encounter.
The evenly-spaced spots, each estimated to be about 300 miles (480 km) in diameter, are part of a dark belt that appears to encircle Pluto’s equatorial region. This image shows them to be more complex than they initially appeared. Additionally, the boundary areas between the bright and dark regions are well defined though irregularly shaped.
“It’s weird that they’re spaced so regularly,” Niebur commented. Jeff Moore of NASA’s Ames Research Center in Mountain View, California, and Geology, Geophysics, and Imaging (GGI) leader, added, “We can’t tell whether they’re plateaus or plains, or whether they’re brightness variations on a completely smooth surface.”
Stern emphasized that the image, taken early on July 11, is “the last, best look that anyone will have of Pluto’s far-side for decades to come.”
The mission’s GGI team is carefully studying surface features that have been recently revealed – such as impact craters. “When we combine images like this of the far side with composition and color data the spacecraft has already acquired but not yet sent to Earth, we expect to be able to read the history of this face of Pluto,” Moore said.
Pluto’s bright, heart-shaped region is on the “encounter side” of the planet.
On July 8, LORRI took a “dynamic duo” photo of Pluto and Charon, which the mission team combined with color data from Ralph to create a color image of the two worlds orbiting their common center of gravity.
While Pluto is a reddish color, Charon is gray and does not appear to have an atmosphere. Pluto’s surface contains nitrogen, methane, and carbon monoxide ices, whereas Charon’s surface is composed mostly of ammonia compounds and frozen water.
Unlike Pluto, whose interior is largely rock, Charon contains an equal amount of rock and ice.
“These two objects have been together for billions of years, in the same orbit, but they are totally different,” Stern emphasized.
Details about Charon’s surface geology are finally becoming clear. Several bright regions are believed to be impact craters.
“If we see impact craters on Charon, it will help us see what’s hidden beneath the surface,” Moore explained. “Large craters can excavate material from several miles down and reveal the composition of the interior.”
With a diameter of 750 miles (1,200 km), Charon is half the size of Pluto.
“Charon is now emerging as its own world. Its personality is beginning to really reveal itself,” said John Spencer, GGI deputy team leader.
New Horizons will take images of Pluto’s night side, its south polar region, which has faced away from the Sun for 20 years and will do so for another 80. The images will be taken by LORRI and Ralph using only “moonlight” from Charon after the spacecraft makes its closest approach.
Charon would appear seven times larger than Earth’s moon to an astronaut on the side of Pluto that faces the large moon and would look five times dimmer than the full Moon does from Earth. This is because Charon is much closer to Pluto than the Moon is to the Earth. Charon is covered by a layer of ice, making it highly reflective.
“The only way for New Horizons to observe Pluto’s elusive night region is to see it in ‘Charonshine’,” explained deputy project scientist Cathy Olkin. “It’s almost time for the big reveal, and I couldn’t be more excited.”
From the Pluto system, the Sun appears 1,000 times dimmer than it does from Earth; it would look like a very bright star and would illuminate Pluto during the day about as much as the full Moon illuminates the Earth at night.
Laurel Kornfeld is an amateur astronomer and freelance writer from Highland Park, NJ, who enjoys writing about astronomy and planetary science. She studied journalism at Douglass College, Rutgers University, and earned a Graduate Certificate of Science from Swinburne University’s Astronomy Online program. Her writings have been published online in The Atlantic, Astronomy magazine’s guest blog section, the UK Space Conference, the 2009 IAU General Assembly newspaper, The Space Reporter, and newsletters of various astronomy clubs. She is a member of the Cranford, NJ-based Amateur Astronomers, Inc. Especially interested in the outer solar system, Laurel gave a brief presentation at the 2008 Great Planet Debate held at the Johns Hopkins University Applied Physics Lab in Laurel, MD.