THE LIST of people that have summited Mount Everest is hovering around the 10,000 mark. The long-hoped-for winter ascent of K2 was finally pulled off in January. The fourteen eight-thousand metre peaks have very much been 'done' in all seasons and in myriad styles of ascent. You might say that there aren't too many virgin, iconic mountains left to climb – on Earth, that is.
But what about beyond this planet? If we had the whole solar system as our playground and neither distance nor money nor technology were an object, where could we go climbing and what sort of fun could we expect?
Before anyone objects: no, this isn't practical. There are no other worlds in our solar system with breathable air and so a spacesuit is going to be essential. They're not known for their flexibility; stretching for a hold or reaching aft for the chalk bag is going to be a struggle. But honestly that's the least of your worries. The cosmic rays are going to weaken the synthetic molecules your rope is made from and the cold, in most places, is going to make it as brittle as ice. But let's not dwell on such banalities. This isn't realistic, but it's going to be a lot of fun.
Space is hard, as the famous understatement goes. So if we are to set out climbing in the solar system, we would do well to try something at the easy end first. The Moon has the advantage of being close. At just under 400,000 kilometres away, rockets can get there in about 3 days. Plus, this is the only world except Earth where humans have at least dabbled in climbing already.
In the summer of 1969, Apollo 11 landed in a grey dusty valley to the north east of two peaks, Mons Hadley and the slightly smaller Mons Hadley delta. Apollo 15 was the first mission to use a lunar rover and so astronauts David Scott and James Irwin were able to drive a little up the sides of Hadley delta, to a height of nearly 100 metres above the valley floor. At times the going was so steep that the rover had trouble gaining traction and the astronauts couldn't lean back far enough to frame a shot of the mountain's peak.
Anyone trying to climb this mountain on foot would have a mixed time of it. The astronauts described the terrain here as being composed of loose material, like scree towards the base of mountains on Earth. That would probably make it hard going. Then again, gravity is only about 17% as strong on the Moon as it is on Earth – so swings and roundabouts.
The summit of Mons Hadley delta is about 3.5 km above the valley floor, making it significantly lower than the Moon's tallest mountain, Mons Huygens. The latter stands 5.5 km high – making it a longer climb, base to height, than Everest is. It's part of a gently curving range of peaks called the Montes Appeninus, so from the top you would be able to see a string of other mountains to the left and right. This has surely got to be must-bag peak of the Moon.
There is, of course, no chance of ascending this without oxygen. But the biggest challenge to climbing on the Moon is probably the dust. Here there is no wind and so the dust remains sharp, not rounded off as it is on Earth. It also has an electrostatic charge. This sharp, sticky dust was a nuisance for lunar explorers, who reported that it got into the joints of their spacesuits, gumming them up. Some even reported that the dust got into the lunar lander and, when breathed in, gave astronauts "space hayfever". Still, a few sneezes are a price worth paying to go climbing on the moon.
Humans may have visited a lunar mountain personally, but we probably know one mountain on Mars even better despite having never set eyes on it. NASA's Curiosity rover landed next to Aeolis Mons (better known as Mount Sharp) in 2012 and has been pottering around in its foothills ever since.
Olympus Mons may seem like a more obvious choice. If you have heard of any of Mars' mountains it would probably be this one. It is, after all, probably the highest mountain in the solar system, at more than 21km high. But it would be a boring climb – the mountain is 600 km wide, about as wide as France, which means the ascent would be less a climb, more a long, gently sloping trudge. It's difficult to grasp its scale; a climber standing on the Martian surface would be unable to see the peak's profile, while standing on the summit would be anti-climactic, giving no sense of elevation due to its slope extending beyond the horizon.
Aeolis Mons is steeper and it would be possible to follow the route Curiosity has taken up the mountain's lower slopes, known as the Murray Butes. There would be ample opportunity for bouldering here, and considering that gravity on Mars is only about 38% that of Earth, you probably wouldn't need to bother with a bouldering mat. Recent images from Curiosity suggest potential for crack climbing on the hematite-rich sedimentary rocks of the Vera Rubin Ridge, situated on the mountain's northwest slope. To cap it all, there is no water and no atmosphere to speak of on Mars, so you'd never have to worry about the wind and rain. Every sol —a Martian solar day — would be a good one for climbing.
Another reason to err away from Olympus Mons is that it may not hold the title of the solar system's highest mountain. Beyond Mars there orbits a scattered collection of more than a million asteroids. One of the largest is a rock called Vesta. Long ago, another asteroid hit Vesta smashing out a crater called Rheasilvia, which is almost as wide as the asteroid itself. The power of the blow would have liquified the rock and it rebounded afterwards like a raindrop hitting a puddle, leaving a huge, sheer-sided mountain in the middle of the crater. This peak is about 21 km high, putting it on a par with Olympus Mons. We don't have good enough data to be sure which is taller.
This is a place for some serious climbing. It would be steep – very steep. Vesta is thought to be a dry place, unlike its sister asteroid Ceres, which has dormant cryovolcanoes that spew molten ices on its surface. This means that it could well be a steep, technical climb, not unlike climbing gritstone on Earth – apart from the minimal gravity, that is. In fact, you have to wonder how easy it is to tell which way is 'up' when you're climbing on an asteroid.
Further out in the solar system things get extremely cold and there's the opportunity for some ice climbing. One great destination could be Enceladus, the sixth largest moon of Saturn. This is a delightfully strange world. It has an icy crust, beneath which is assumed to be a subsurface ocean of liquid water. There are also geysers of water that spurt ice from below the surface far out into space. Some of that material drifts off to form one of Saturn's rings while some of it snows down onto the surface of the moon, blanketing it in snow. What an epic backdrop for an afternoon's climb.
Although most of the surface looks smooth, there are features called sulci, also known as tiger stripes. These are great cracks or crevasses in the icy crust, each more than 100 kilometres long and perhaps 500 metres deep. You could lower yourself down to the bottom, don crampons, then begin the ascent. The darkness is likely to make the whole thing feel pretty scary. But on the other hand, on this moon you weigh about a tenth of what you do on Earth, so it would feel easy on your muscles. You could also keep your eyes peeled for life while you're down there, as Enceladus is thought to be one of the best places to look for life in the solar system.
Gravity is a little stronger on Iapetus, Saturn's third largest moon. But skip over to this world and you would be rewarded with another highlight: what must surely be the longest traverse in the solar system.
You are probably used to seeing planetary bodies as spheres in your mind's eye. With Iapetus, it's more helpful to think of a walnut. Its poles appear to have been squeezed inwards and a ridge runs along about a quarter of the circumference of its equator. This is the Iapetus ridge. No one is sure how it got there, but its certainly an impressive feature, with peaks along it rising to more than 20km, rivalling Olympus Mons.
Iapetus is mostly made of ices, not rock, so this is going to be more ice climbing. From here on out, there is little else. And we know almost nothing about what its individual peaks are like. So you'd be plotting your own routes, right enough – this is as virgin as territory can get.
Before the summer of 2015, we knew very little about the icy dwarf planet Pluto. It was so frigid and far away that, somehow, no one really expected it to have many interesting surface features. But what a surprise we got when NASA's New Horizons probe flew past and pictured the surface. There's a lot going on down there.
Your first stop, for the sake of posterity, should surely be Pluto's highest collection of mountains: Tenzing Montes. This range was named after the Tibetan-born mountaineer Tenzing Norgay, who completed the first summit of Everest with Edmund Hillary, so it's surely one you'll want to tell the grandchildren about. The highest peak, known as T2, stands at about 6.2 km, which is not to be sniffed at.
From the top, you'd be able to see two starkly different bits of terrain. To the West, there is Cthulhu Macula, an area covered in ancient craters and a deep red in colour. This colour is thought to come from carbon-based molecules called tholins that form a tar-like substance. To the North is Tombugh Regio, a bright whitish area covered in a pristine glacier made of frozen nitrogen.
You would also probably be able to see Wright Mons to the south. There's something special about this peak: it's a volcano, but one that spews molten ice, not rock. Perhaps if you're not too homesick, that would be a nice peak to bag while you're out here too.
We agreed that time, space and technology were no object. So perhaps it's worth heading beyond the bounds of our humble solar system. We have sent no probes to other planetary systems and our telescopes are not yet powerful enough to see any surface features of planets orbiting other stars. But we can discern these exoplanets because of the way they dim the light coming from their parent stars in a periodic fashion as they pass in front of them.
One of the most exciting places to visit would be the fourth planet orbiting TRAPPIST-1, a star about 40 lightyears from us. TRAPPIST-1e, as it is called, is one of the most Earth-like worlds we have yet found. It sits at just the right distance from its star to have liquid water and we're confident that it's rocky. We can't say whether it will have any mountains – but it's a fairly safe bet that it will at least have slopes and inclines in some form.
One wrinkle is that we think TRAPPIST-1e is tidally locked, meaning it does not spin on an axis and one side of the planet always faces the sun while the other is in permanent darkness. If this is right, then perhaps the best place to try climbing would be at the twilight region between the two faces, which would escape the extreme heat and cold of the two faces. On the minus, we also suspect that strong winds would constantly whip from the warm side to cold, making this region extremely blowy. It's not the ideal climbing spot. Mind you, if you've come this far from the place where every human in history has been born, then you've already overcome challenges far greater than climbing a blustery mountain at twilight.
Thanks to Dr Sam Royle, planetary geochemist and astrobiologist, and Steve Banham, planetologist at Imperial College London for providing expertise for this piece.