THE moment Pluto was transformed from a fuzzy spot on the edge of the solar system to an exotic world with a spectacular landscape will be recorded by historians as July 15, 2015.
When Nasa’s New Horizons spacecraft barrelled past Pluto the day before, it became the first mission to visit the object. A day later, the probe made contact with Earth. Since then, scientists on the team have released one breathtaking image after another, revealing vast, smooth plains, towering ice mountains and an inviting blue haze of hydrocarbons.
In a report published in Science on Thursday, the researchers paint their most detailed picture yet of Pluto, from data beamed back from the distant probe more than 4.5bn miles away.
“We went from having images that were maybe three pixels across to images thousands of pixels across, so we are essentially seeing Pluto for the first time in terms of its landscapes and geological story. It’s completely new and completely spectacular,” said John Spencer, a member of the science team at the Southwest Research Institute in Boulder, Colorado. “We are seeing a surface unlike any planetary surface we’ve ever seen before.”
By sheer luck, New Horizons happened to capture images of what must be one of the most remarkable features on Pluto’s surface. The bright, smooth region named Sputnik Planum is a plain of frozen nitrogen that covers hundreds of miles. There are no craters in the region, leading scientists to think it is no more than 100m years old. From the east, nitrogen glaciers pour into the plain. On the west, what appear to be water icebergs have piled up into a rugged, mountainous heap.
“The best idea we have is that they are water icebergs which are being dislodged from the crust which we think is mostly water ice, and somehow tilted and upended and jostled around to produce these mountains,” said Spencer. On Pluto, nitrogen freezes into a soft, mushy substance in which water ice floats. “It would be like floating in blancmange,” said Spencer.
Deep in the heart of Pluto, radioactive decay produces a modicum of heat that warms the body ever so slightly. The surface temperatures vary from place to place, but rarely creep above -230 degrees Celsius.
To the east of Sputnik Planum are tall, blade-like ridges that stand several hundred metres tall. When the planet was younger, and its atmosphere thicker, strong winds may have carved out the terrain.
The atmosphere of Pluto is 100,000 times thinner than that on Earth, but when New Horizons sped past and looked back towards the sun, it captured images of a blue haze circling the world. Spencer said that frozen hydrocarbons in the atmosphere produce the effect, which reaches up to 100 miles above Pluto’s surface. “The sky would look blue if you were on the surface and looked at the sun,” said Spencer.
Beyond the exotic features of the surface and atmosphere, New Horizons measured the size of Pluto more accurately than had ever been done from observatories. Its diameter, at 1,475 miles, is smaller than thought, meaning Pluto’s density must be greater and closely match that of its moon, Charon. Just as a cosmic collision likely knocked material off the Earth to form the moon, so Charon formed when Pluto suffered a similarly catastrophic impact.
Mission scientists expect to wait a full year to get all of the data back from the flyby. To date, only half of the best stuff, according to Spencer, has been downloaded.
New Horizons is now on course for its next rendezvous in 2019, with an object called 2014MU69 in the Kuiper belt, which lies beyond the orbit of Neptune.
—By arrangement with the Guardian
Published in Dawn, October 18th, 2015