Sunday, July 12, 2015

New Horizons' Last Portrait of Pluto's Puzzling Spots

New Horizons' last look at Pluto's Charon-facing hemisphere reveals intriguing geologic details that are of keen interest to mission scientists. This image, taken early the morning of July 11, 2015, shows newly-resolved linear features above the equatorial region that intersect, suggestive of polygonal shapes. This image was captured when the spacecraft was 2.5 million miles (4 million kilometers) from Pluto.   Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Three billion miles from Earth and just two and a half million miles from Pluto, NASA’s New Horizons spacecraft has taken its best image of four dark spots that continue to captivate. 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. 

In addition to solving the mystery of the spots, the New Horizons Geology, Geophysics and Imaging team is interested in identifying other surface features such as impact craters, formed when smaller objects struck the dwarf planet. Moore notes, “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.”

When New Horizons makes its closest approach to Pluto in just three days, it will focus on the opposing or “encounter hemisphere” of the dwarf planet. 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.

At 7:49 AM EDT on Tuesday, July 14 New Horizons will zip past Pluto at 30,800 miles per hour (49,600 kilometers per hour), with a suite of seven science instruments busily gathering data. The mission will complete the initial reconnaissance of the solar system with the first-ever look at the icy dwarf planet.

It’s Antarctic winter on Pluto. The sun has not been visible for twenty years in this frigid south polar region; it will not shine again for another 80 years. The only source of natural light is starlight and moonlight from Pluto’s largest moon, Charon.

New Horizons mission scientists will soon obtain the first images of the night region of Pluto, using only the light from Charon, itself softly illuminated by a Sun 1,000 times dimmer than it is at Earth. The images will provide New Horizons’ only view of Pluto’s lesser-known south polar region, currently in the midst of a numbingly-long winter. The pictures will be made with the LORRI and Ralph instruments, shortly after New Horizons passes its point of closest approach to Pluto.

If you stood on the night region of Pluto at that moment of closest approach by New Horizons — looking up at a distinctly gray Charon — it would appear seven times larger in the sky than Earth’s moon. Charon, although three billion miles from the sun, is so close to Pluto and so ice-covered that it would be only five times dimmer than the full moon seen from Earth. At your feet, the icy surface — resembling a sooty snow bank — would be bathed in Charon’s faint glow. The area around you would be dim, but not so dark that you would bump into things.

On your moonlight stroll on Pluto you’d notice that your shadow, cast by Charon, is much less defined than your shadow from moonlight on Earth. A wisp of cloud might even pass in front of Charon as you look up.

If you stood on Pluto’s Charon-facing side as New Horizons speeds by, you would see Charon go through a cycle of phases during a “Pluto Day” — 6 days and 10 hours — but not the complete set of phases our moon displays to us on Earth. Seen from Pluto during that time, Charon would go from a wide crescent, to a “quarter moon,” then to gibbous (partway between quarter and full phases), and back again.

New Horizons has been traveling for nine-and-a-half-years to bring humankind its first exploration of the Pluto system. While the sunlit features of Pluto are growing sharper every day, the shadowy winter region is still cloaked in mystery—but not for long.

“The only way for New Horizons to observe Pluto’s elusive night region is to see it in ‘Charonshine,’” says Cathy Olkin, New Horizons deputy project scientist. “It’s almost time for the big reveal, and I couldn’t be more excited.”

No comments:

Post a Comment