The more we discover about Pluto, the weirder it gets. New research shows the dwarf planet is home to tall mounds that may be ice volcanoes and its atmosphere is thinner and chillier than first thought.
And its entourage of moons offers its own surprises. The largest moon, Charon, sports patches of ammonia ice and a red spot over its north pole, while the fleet of four smaller, brighter moons seem to be covered with water-ice.
The research was published in five papers in the journal Science.
Even though only eight months have passed since NASA probe New Horizons whizzed by, we've uncovered more about icy Pluto during that period than we have since it was first spotted by American astronomer Clyde Tombaugh in February 1930.
That hydrocarbon haze, we now know, is much colder and more compact than previously suspected.
The dwarf planet's lower 1,800 kilometres of atmosphere is dominated by nitrogen while methane, acetylene, ethylene and ethane feed the hazy halo.
And planetary scientists peg parts of Pluto's surface to be less than 10 million years old, meaning it has been recently geologically active, while other parts are around four billion years old, and have been subjected to erosion.
But where there are discoveries, there are new mysteries. Enigmatic features, such as two tall mounds with a depression at the top might be ice volcanoes, but scientists aren't sure.
New Horizons team member Ross Beyer, from the SETI Institute in California, says: “Wright Mons is about two miles [3.2 kilometres] tall and 90 miles [145 kilometres] wide, and Piccard Mons is even larger, about 3.5 miles [5.6 kilometres] tall and 140 miles [225 kilometres] across.
"They could be ice volcanoes, but this will take more analysis to establish.”
Despite having similar density to Pluto, Charon's surface is strikingly different.
It can be divided into two provinces, separated by an east-west "tectonic belt" of ridges and canyon which arose as ice split the moon's skin. South of the border, there are flat expanses of plains, while the north is rugged and ridged, with a reddish tinge to the polar region Mordor Macula.
This red patch on Charon is the first such feature seen on an icy moon. Scientists attribute it to tholins, sepia-coloured molecules formed when methane and nitrogen are subjected to solar ultraviolet radiation.
What about Pluto's other moons? In addition to Charon, which is about half its size, Styx, Nix, Kerberos and Hydra accompany the dwarf planet. They're elongated, range in size from 10 to 40 kilometres and are relatively bright, signifying lots of highly reflective water ice. Hydra appears bluest, so may contain the most.
They were thought to have formed at the same time as Charon in the aftermath of a large impact. But puzzlingly, New Horizons showed their surfaces to be much older than their larger counterpart.
Another stumper is that they orbit rapidly on axes tilted sideways.
“The gravitational tides from Pluto should have slowed down and re-oriented the spins of these moons by now, but they haven’t,” says the SETI Institute's Mark Showalter, who is also a member of the New Horizons team.
Collisions from outside objects may have bumped and jostled the moons into their strange rotations.
Finally, New Horizons analysed how Pluto interacts with the solar wind, a stream of particles blasting from the Sun. Even though it keeps an average distance of six billion kilometres from the Sun, instruments on the probe recorded the dwarf planet deflecting charged particles around it.
With only half New Horizons' data received so far, we can expect another slew of data in the coming months. Hopefully it will put these latest mysteries to bed.
Belinda Smith is a science and technology journalist in Melbourne, Australia.
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