London - As Nasa's New Horizons spacecraft took its last look at Pluto from nearly 3.5 million miles away, scientists were marvelling at how “very active” the dwarf planet is in contrast with previous theories that it was an inert ball of ice and rock.
Extraordinary photographs from the mission have shown vast plains, 3 000-metre mountains, possible volcanoes, rift valleys and other features suggesting it has a molten core and shifting tectonic plates. Just like Earth. The absence of impact craters from meteorites on its plains, however, indicates they are geologically-speaking quite young, perhaps 100 million years old, and that Pluto is probably geologically active. Similar signs have been seen on its moon, Charon.
Previously it had been thought that both must be inert lumps drifting through space. But Alan Stern, principal investigator of Nasa's New Horizons mission, said that scientists had been “completely wrong about that”.
“They are very active. Pluto and Charon have been geologically active for billions of years, but we don't know what the energy is that is driving it. It's a puzzle,” he told The Sunday Times. “They have surface areas that have no craters. There must be craters unless they are young.
“We also see faultlines, scarps and other tectonic features such as rift valleys on both Pluto and Charon. It is unmistakable.”
One theory behind the movement on Pluto's surface is that there is a radioactive heat source in its rocky core, thought to make up about 60 per cent of its mass.
The rest is mostly a thick layer of ice so cold that it behaves like stone. However, the hot core could be sending geysers of warm water shooting up to the surface.
Pluto has a layer of atmosphere, made up mainly of nitrogen, which extends about 1 000 miles above its surface.
However, this is gradually being stripped away by the solar wind, creating a “plasma tail” extending up to 65 000 miles. Scientists hope to work out how quickly this is happening, with more data expected in August. So far, New Horizons has sent a gigabit of information but another 49 gigabits are expected. “What we have now is scratching the surface. The best is yet to come,” Dr Stern said.
Some of Pluto's features have already been named informally, such as the Sputnik Plain, after the first spaceship, the Norgay Mountains, after Tenzing Norgay, the first person to climb Mount Everest along with Edmund Hillary, and the Tombaugh Region after Clyde Tombaugh, who discovered Pluto in 1930.
Other names used include Meng-p'o, the Buddhist goddess of forgetfulness; Vucub-Came and Hun-Came, heroic twins and death gods of the Mayan people of Central America; and Balrog, a monster from The Lord of the Rings.
The New Horizons mission enabled scientists to accurately measure Pluto for the first time and it appears to be the largest of the dwarf planets at 1 473 miles in diameter.
New Horizons was 3.5 million miles beyond Pluto. “This will be our last look at the dark areas on Pluto's surface. Bitter sweet!” Nasa's New Horizons mission tweeted.
ICE WORLD PLUTO'S COLD SKIN AND WARM HEART
Evidence has been found by Nasa’s New Horizons space probe for a heat source below Pluto’s frozen exterior
SURFACE TEMPERATURE
-230°C
ROCK CORE
60% OF PLUTO’S MASS
Radioactive rocks in Pluto’s core could be creating heat that would cause warm water to reach the surface
Mountains, cliffs and valleys made of water ice which are as hard as rock
Pluto's plains and valleys show areas where ice appears to have been melted and cooled
The Independent