Six months after the New Horizons probe flew past Pluto, Nasa scientists are extracting ever-better images of its planetary encounter, as they process more data that have travelled five billion kilometres back to Earth from the spacecraft.

The view on the right, of an area 80km wide, shows what looks like a frozen shoreline, where the relatively soft and flat nitrogen-rich ice of Sputnik Planum (at the bottom of the picture) abuts the al-Idrisi mountains made of hard water ice.

Sputnik Planum is split into plates about 20km across, which look like rounded polygons, with darker material marking the boundaries between them. This fracturing suggests a convection process driven by warmth beneath the ice; residual radioactivity deep within the planet is the most likely heat source. Although the plates are almost level, their surface looks streaky at high resolution; wind blowing in Pluto’s thin nitrogen atmosphere, combined with sublimation (evaporation), may form this texture.

Nasa scientists believe that the al-Idrisi mountains are gigantic icebergs, up to 2,500m high, embedded in softer and more mobile material. “The mountains bordering Sputnik Planum are absolutely stunning at this resolution,” says John Spencer, a member of the New Horizons science team. “The details revealed here, particularly the crumpled ridges in the rubbly material surrounding several of the mountains, reinforce our earlier impression that the mountains are huge ice blocks that have been jostled and tumbled and somehow transported to their present locations.”

The surface of Pluto, as photographed by Nasa’s New Horizons spacecraft
The surface of Pluto, as photographed by Nasa’s New Horizons spacecraft © NASA

The image reproduced here is one of a series made with the telescopic Long Range Reconnaissance Imager (Lorri) on New Horizons, over a period of a minute on July 14, when the craft was 15 minutes from its closest approach to Pluto and 17,000km from the planet. Lorri was then taking pictures of the surface every three seconds. The resolution is 80m per pixel.

This year, Nasa will be releasing more observations and images of what I still regard as the solar system’s outermost planet (although the International Astronomical Union officially demoted it in 2006 to “dwarf planet” status).

The New Horizons probe continues to send data stored in its memory back to Earth at a transmission rate of just 2 kilobits per second — one thousandth of the rate of a poor domestic broadband link.

Photograph: NASA

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