Nasa has teased a new image of Charon ahead of a much-anticipated unveiling of high-definition pictures of Pluto tomorrow.

The image zeroes in on a mystery spike on the moon’s surface which the space agency describes as a ‘mountain in a moat’. It was taken from the New Horizons spacecraft on July 14th at 6:30AM ET, when it was 49,000 miles (78,850km) away from the satellite. The grey depicts a region 200 miles (320km) long, filled with craters and a mysterious depressed mountain at the bottom left-hand corner. ‘This is a feature that has geologists stunned and stumped,’ said Jeff Moore with Nasa’s Ames Research Center.

The image gives a preview of what the surface of this large moon will look like in future close-ups from Nasa’s New Horizons spacecraft. Nasa is expected to release new images of Pluto during a briefing tomorrow 1pm EDT from New Horizon’s history flyby of the dwarf planet.  So far, the probe has captured more than 1,200 images of the dwarf planet and its moons. The first image of Charon, released yesterday, revealed a large smooth region in the moon’s southern hemisphere, which suggests the moon is geologically active.

‘Charon just blew our socks off when we had our new image today,’ said Nasa’s Cathy Olkin.

‘We think that the dark colouring can perhaps be a thin veneer. You can see locations in the North Pole were perhaps a crater has dug beneath it and excavated under it.’  The image also revealed a swath of cliffs and troughs stretches about 600 miles (1,000km) from left to right, suggesting widespread fracturing of Charon’s crust, likely a result of internal processes.

On the upper right, along the moon’s curving edge, is a canyon estimated to be 4 to 6 miles (7 to 9km) deep.

Mission scientists were surprised by the apparent lack of craters on Charon, although the new image appears to show more pockmarks than first thought. South of the moon’s equator, at the bottom of this image, the terrain is lit by the slanting rays of the sun, creating shadows that make it easier to distinguish topography.

The image was compressed to reduce its file size for transmission to Earth. In high-contrast areas of the image, features as small as 3 miles (5 kilometers) across can be seen. They are now being beamed back to Earth, frame by frame. 

The first high-resolution image of Pluto was also sent by New Horizons yesterday revealing 11,000ft (3,350 metre) mountains made of ice. The remarkable image provides evidence that geological activity is still taking place on the icy world.

Scientists were shocked to see mountains as high as those in the Rockies that likely formed 100 million years ago - mere youngsters relative to the 4.56-billion-year age of the solar system. Nasa says they may still be in the process of building. 

Like the rest of Pluto, this region would presumably have been pummeled by space debris for billions of years and would have once been heavily cratered - unless recent activity had given the region a facelift, erasing those pockmarks.

‘We now have an isolated small planet that is showing activity after 4.5 billion years,’ said Professor Stern. ‘It’s going to send a lot of geophysicists back to the drawing board.’

‘This is one of the youngest surfaces we’ve ever seen in the solar system,’ added Dr Jeff Moore of New Horizons’ Geology, Geophysics and Imaging Team (GGI).  This is the first time astronomers have seen a world that is mostly composed of ice that is not orbiting a planet. Unlike the icy moons of giant planets, the dwarf planet cannot be heated by the gravitational pull of a larger planetary body. Nasa says some other process must be generating the mountainous landscape.

‘This may cause us to rethink what powers geological activity on many other icy worlds,’ says GGI deputy team leader Dr John Spencer of the Southwest Research Institute. Dr Spencer said that the team has yet to find an impact crater in any of the scans, suggesting Pluto is very compared to the solar system.