The American space agency’s New Horizons probe has returned further images of Pluto that include a view of the dwarf planet’s strange icy plains.
A region, which has been named after the Soviet Sputnik satellite, displays a flat terrain broken up into polygons.
At the edges of these 20-30km-wide features are troughs filled with dark material and even small mounds.
Scientists say it could be evidence of the surface bulging due to gentle heating coming from below.
But it could just as easily be the result of cooling and contraction as materials vaporise into the atmosphere – not unlike how mud cracks form on Earth.
Science team members say they are trying not to jump to early conclusions in their interpretations – certainly, not until they get more data down from the spacecraft.
“When I first saw the image of Sputnik plain I decided I was going to call it ‘not easy to explain terrain’,” said Jeff Moore, who leads the geology, geophysics and imaging team on New Horizons.
At a media briefing at Nasa HQ in Washington DC, the mission team also showed a first picture of Nix, one of Pluto’s smaller moons.
It is only seen in pixelated form because it is just 40km across. Nonetheless, researcher can now gauge its shape, which has never been possible before.
“Let’s set out expectations properly,” said lead scientist Alan Stern. “As little as three months ago, we didn’t have pictures of Pluto this good!”
Other measurements by the probe reveal Pluto is probably losing about 500 tonnes of atmosphere per hour. It is being stripped away by the energetic, charged particles coming off the Sun.
Pluto’s diminutive size means it does not have the gravity to hang on to the atmosphere – in the same way that a bigger world like Earth or even Mars can.
Mars, for example, loses only about one tonne per hour.
“What is the consequence of that?” pondered Fran Bagenal, a co-investigator, University of Colorado.
“If you add that loss up over the age of the Solar System, this is going to be equivalent to something on the order of 1,000-9,000ft – so that’s a substantial mountain – of nitrogen ice that’s been removed.”
This is not enough to take away all of Pluto’s atmosphere, but it is very likely to have effects at the surface where ices continue to vaporise.
New Horizons continues to monitor the dwarf planet and its five satellites – Charon, Styx, Nix, Kerberos and Hydra.
Even though it has gone more than three million km beyond the Pluto system, there is still much to learn by looking in “the rear-view mirror”.
New Horizons is attempting to study the crescent Pluto, to see if there are hazes and even clouds in its tenuous atmosphere.
It is also hunting for rings. It is possible Pluto is surrounded by concentric circles of dusty, icy particles, and these would scatter sunlight in a way that might be easier to detect from “behind” the dwarf planet.
The probe has so far sent back only a tiny amount of its stored flyby data – just 2-3%.
The vast distance to Pluto (4.7 billion km) and the modest transmitter/antenna system (12W) onboard makes for very slow bit rates – an average of one kilobit per second.
The three-axis stabilised spacecraft can boost this if it switches off its power-hungry inertial measurement unit and spins itself up to maintain a steady orientation. However, it cannot do this and take images at the same time.
Not until the probe has observed Pluto for another two full rotations will it make this change.
And given that a “Pluto day” is 6.4 Earth days, this means the command to stop imaging and spin-up is a couple of weeks away yet.
The last long-range image will be taken on 30 July, with the spin-up taking place a day later.
New Horizons will then send back all of its data – about 50 gigabits – in compressed form during September, before repeating the downlink in an uncompressed form.
“To do everything in every form for all six objects in the system will take 16 months,” said Prof Stern.
The BBC will be screening a special Sky At Night programme called Pluto Revealed on Monday 20 July, which will recap all the big moments from the New Horizons flyby.