New close-up images of Pluto from Nasa's New Horizons spacecraft have unveiled a bewildering variety of surface features - craters, ice flows, mountains, valleys and apparent dunes - that have scientists reeling because of their range and complexity.
"Pluto is showing us a diversity of landforms and complexity of processes that rival anything we've seen in the solar system," said New Horizons Principal Investigator Alan Stern, of the Southwest Research Institute (SwRI), Colorado.
"If an artist had painted this Pluto before our flyby, I probably would have called it over the top — but that's what is actually there," said Stern.
New Horizons recently began its yearlong download of new images and other data.
Images downlinked in the past few days have more than doubled the amount of Pluto's surface seen at resolutions as good as 400 meters per pixel.
They show new features as diverse as possible dunes, nitrogen ice flows that apparently oozed out of mountainous regions onto plains, and even networks of valleys that may have been carved by material flowing over Pluto's surface.
They also show large regions that display chaotically jumbled mountains reminiscent of disrupted terrains on Jupiter's icy moon Europa.
"The surface of Pluto is every bit as complex as that of Mars," said Jeff Moore, leader of the New Horizons Geology, Geophysics and Imaging (GGI) team at Nasa's Ames Research Centre in California.
"The randomly jumbled mountains might be huge blocks of hard water ice floating within a vast, denser, softer deposit of frozen nitrogen within the region informally named Sputnik Planum," said Moore.
New images also show the most heavily cratered - and thus oldest - terrain yet seen by New Horizons on Pluto next to the youngest, most crater-free icy plains. There might even be a field of dark wind-blown dunes, among other possibilities.
"Seeing dunes on Pluto - if that is what they are - would be completely wild, because Pluto's atmosphere today is so thin," said William B McKinnon, a GGI deputy lead from Washington University, St Louis.
"Either Pluto had a thicker atmosphere in the past, or some process we haven't figured out is at work. It's a head-scratcher," said McKinnon.
Images returned in the past days have also showed that Pluto's global atmospheric haze has many more layers than scientists realised, and that the haze actually creates a twilight effect that softly illuminates night-side terrain near sunset, making them visible to the cameras aboard New Horizons.
The New Horizons spacecraft is now about 5 billion kilometres from Earth, and more than 69 million kilometres beyond Pluto. The spacecraft is healthy and all systems are operating normally.