A close-up image of Pluto's surface released by Nasa on Wednesday evening
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The first detailed images of Pluto and its moon Charon, taken by the US New Horizons spacecraft, show many features that have astonished mission scientists. They include ice mountains almost as high as the Alps, rifts deeper than the Grand Canyon — and many signs of geological activity driven by mysterious internal heating.

Alan Stern, principal investigator on the $700m Nasa mission, summed up the researchers’ delight: “I don't think any one of us could have imagined it would be this kind of toy store.”

He was speaking at mission headquarters, the Johns Hopkins Applied Physics Lab in Maryland, as Nasa revealed initial findings from the spacecraft’s rendezvous with Pluto on Tuesday after a 5bn km voyage. Pluto is the outermost and last of the “classical nine” planets to be visited by a probe from Earth.

The biggest overall surprise was the extent to which geological processes such as plate tectonics and volcanism have reshaped the surface of Pluto and, to a lesser extent, Charon. They look very different to cold, dead bodies such as Earth’s moon, which bears the scars of billions of years bombardment with meteorites.

Nasa engineers have processed a detailed image of one part of Pluto covering a square about 150 miles across, near the base of a large, bright heart-shaped feature covering much of the southern hemisphere. “We have not found a single impact crater on this image,” said John Spencer, a senior New Horizons scientist. “This means it must be a very young surface.”

This active geology needs a heat source. Scientists had assumed that such activity, seen on the icy moons of Jupiter and Saturn, was explained by “tidal heating” resulting from gravitational interactions with the host planet.

"You do not need tidal heating to power geological heating on icy bodies,” said Professor Spencer. “That's a really important discovery we just made this morning.”

Mission scientists do not yet have a convincing theory for how icy objects such as Pluto and Charon, smaller than Earth’s moon and receiving negligible warmth from the sun in the frozen outer reaches of the solar system, can be heated in the absence of large gravitational interactions.

Professor Spencer put forward two possible explanations. One is that radioactive processes, an important source of heat inside large planets such as Earth, can also power activity on far smaller bodies. Another is that Pluto and Charon have somehow retained residual heat from their formation billions of years ago — perhaps in the form of a deep ocean that gradually releases energy as it freezes.

Mission scientists often had to reply “We just don’t know” in response to questions at the media briefing where the first results were released.

They pointed out that far more data and images would become available over the next few days and weeks, as New Horizons sends observations stored in its memory back to Earth at a transmission rate of just 2 kilobits per second — one thousandth the rate of a low-quality domestic broadband link. It will take 16 months to download everything.

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