On 14 July 2015, New Horizons made a historic close approach of dwarf planet Pluto.
Nine years after its launch in 2006, the spacecraft became the first robotic emissary from Earth to survey this frozen enigma, which has spent much of humanity’s existence lost in the frozen darkness of the outer Solar System.
Now, three and half years later, the probe is set to fly past its second target Ultima Thule, on New Year’s Day 2019. But what have we learned about its original target in the time since that last flyby?
In the decades following its discovery Pluto remained little more than a speck of light, even when glimpsed by the world’s largest telescopes.
In 2002-03, the Hubble Space Telescope produced the first map of its surface, which provided tantalising hints of a patchwork body.
Before the flyby there was speculation about the existence of cryovolcanism and stunning surface features, though some might have thought the suggestion was – as Patrick Moore might have said – letting the imagination run riot.
After the flyby it seemed that more imagination was needed.
Dominating the surface of Pluto is the bright, heart-shaped feature known as the Tombaugh Regio, where New Horizons has discovered evidence of some spectacular geological activity.
The western lobe is formed by the Sputnik Planitia, a vast, smooth deposit of bright carbon-monoxide ice.
It is some 1,050x800km in size, making it the largest glacier in the Solar System.
To the south we have the mountains Hillary and Norgay Montes. Norgay Montes is about 3.4km high and largely made of water-ice.
There is evidence of ice flows here, and hints of structures that resemble frozen lakes.
The views from the top of these mountains are likely to be quite spectacular.
High-resolution images of the Sputnik Planitia show it to be formed of polygon convection cells.
It is thought that nitrogen and carbon-monoxide ice is warmed by heat welling up from inside the cells, and that this ice then flows down to lower levels.
The small pits located in the ice could be the result of the sublimation of nitrogen-ice.
There are no surface craters here, and this has led scientists to conclude that this part of Pluto’s surface must be younger than 10 million years old. Clearly, Pluto is still geologically active.
Other areas of interest include ancient dark terrain like the whale-shaped Cthulhu Regio: its dark red colouration is due to the presence of complex hydrocarbons called tholins.
The cratering of this part of the surface would suggest it to be a few billion years old, certainly much older than the Sputnik Planitia.
The New Horizons data provides two possible candidates for cryovolcanism: Wright Mons and Piccard Mons.
These two features are the tallest objects on the surface of Pluto, reaching a height of 4km.
A series of dark irregular patches on the equator form the Brass Knuckles region.
The dark patches are separated by bright ice-covered mountains, which themselves contain deep canyons and valleys.
It seems that there is no dull place on the surface of Pluto!
A lively atmosphere
It had long been thought that Pluto’s atmosphere would be interesting.
Due to its rather elliptical orbit, the general consensus was that the atmosphere would freeze to the surface as Pluto moved farther from the Sun.
However, scientists now believe that Pluto may have an atmosphere for most, if not all, of its long year.
Pluto has a substantial axial tilt of about 120°, so as it orbits the Sun one pole is kept in shadow while the other remains in direct sunlight.
New Horizons has revealed that methane and nitrogen are distributed all over the surface.
This means that there is probably enough ice to sublimate and keep the atmosphere from completely condensing on the surface.
This does not mean that the atmosphere is static: indeed it is far more dynamic than we thought.
Over Pluto’s long history, changes in the axial tilt mean there may have been times when the atmosphere was much more dense than it is now.
It has been suggested that the atmosphere may even become dense enough to allow the existence of lakes of liquid nitrogen on Pluto.
After New Horizons made its closest approach, its Long Range Reconnaissance Imager began to observe the dwarf planet and it made a surprising discovery: surrounding Pluto was a notable atmospheric haze.
Unexpectedly, this haze seemed to be composed of several different layers.
It is thought to be due to the interaction of Pluto’s atmosphere with sunlight.
Although the Sun is weak from this far away, it is still sufficient to break up methane in the upper atmosphere, allowing more complex hydrocarbons to form.
These slowly fall to colder, lower altitudes, forming the haze.
The Sun’s ultraviolet rays convert them into compounds called tholins, the compound responsible for the dark colouration on Pluto’s surface.
This is a general picture however; the exact details have yet to be determined.
No doubt there is a complex interplay between the atmosphere and the surface, creating the dramatic topography we have seen.
If anything, New Horizons has revealed the atmosphere of Pluto to be just as fascinating and complex as the planet it enshrouds.
Pluto does not wander alone in space: it is accompanied by five satellites, Charon, Nix, Kerberos, Hydra and Styx.
Charon is around one-eighth the mass of Pluto, and as a result the pair are tidally locked, which means they always present the same face to each other as they move around the Sun.
Unlike our own Moon, Charon does not rise and set over the surface of Pluto, it remains fixed in the black sky.
New Horizons surveyed Charon and the results once more challenged the expectations of planetary scientists.
Instead of a dead, cratered world, the spacecraft found a surface every bit as exciting as Pluto’s.
Charon has a dark red northern polar cap, and this is probably material that has escaped from Pluto’s atmosphere.
Running along its equator is a vast canyon system nearly 1,600km in length. What could have caused this enormous fracture?
Names from science fiction are given to features here and the aptly named Vulcan Planum is, as Mr Spock would say, fascinating.
There is surprisingly little cratering on this plain, which indicates that some sort of resurfacing has taken place; the fingerprints of cryovolcanism in action.
New Horizons was also able to image the other satellites, although Nix was the only other moon close enough to show interesting surface details.
The spacecraft showed a red patch on the surface similar to the dark colouration found on Pluto and Charon.
The continuing mission
Although the Pluto flyby has long since passed, New Horizons is far from finished.
The mission has already been a spectacular success and it has transformed an object that was once just a pinprick of light on a photographic plate into a complex and diverse world.
The discovery of mountains and apparent ice floes shows that even out here, in the frozen extremities of the Solar System, geological activity is quite common.
Like the satellites of Jupiter and Saturn, Pluto and Charon remind us that we were wrong to write them off as dead, airless worlds.
No doubt in years to come the next generation of planetary scientists will use data from New Horizons to formulate new models of these distant wanderers.
In the larger picture they will help to provide a better understanding of the early Solar System.
I would imagine there will be many more surprises in store as the story of Pluto embarks on a new chapter.
Dr Paul Abel is an astronomer based at the University of Leicester. This article was originally published in BBC Sky at Night Magazine, April 2017