(Image: Neil Stevens)
Weary of Whistler? Find Davos dull? For a vacation with a difference, blast off with our winter sports guide to the best off-earthly snow and ice
Few things spell the holiday season like a sprinkling of snow. It may not always be the cold white stuff we鈥檙e accustomed to on Earth, but there鈥檚 plenty of opportunity for future space explorers to have winter fun in the solar system 鈥� and beyond.
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Heavy-metal snow
As the solar system鈥檚 innermost planet, Mercury is generally much too hot for a white Christmas. In some spots, however, natural freezers have harboured ice for billions of years. In a single polar crater on Mercury, daytime temperatures can soar above 250 掳C, yet patches in shadow stay cold enough to preserve water ice. There is plenty of it, too 鈥� , with even more buried beneath a thin layer of dust. You could have lunch in a crater鈥檚 sunny centre (safe from radiation in a special suit) and walk over to a frozen water deposit for an afternoon round of curling.
A little further from the sun, boiling Venus is too hot for snow as we know it. But the planet does get its share of metallic frosting. Temperatures are hot enough to vaporise minerals like pyrite, aka fool鈥檚 gold, which then falls through the atmosphere as water droplets do on Earth. Volcanoes also pump elements like tellurium into the sky. At high altitudes, this .
To reach the finest Venusian champagne powder, skiers must make for Maxwell Montes, 11 kilometres high at the peak (a little more than 2 kilometres taller than Mount Everest). Alpine enthusiasts would need heat-proof pressure suits capable of withstanding temperatures of 400 掳C and pressures 90 times those on Earth.
A much chillier environment awaits on the solar system鈥檚 largest mountain, towering 25 kilometres above the Martian plains (nearly three times as tall as Everest). Before astronomers knew Olympus Mons was a volcanic mountain, they called it 鈥淥lympic Snow鈥� because its peak shone more brightly than its surroundings. We now know these brightness differences are down to . Disappointingly, despite the high altitudes, skiing down Olympus Mons is impossible because its ice and glaciers are long gone.
For more reliable wintry conditions, you want to venture to the Martian poles, which like Earth鈥檚 are capped with ice, albeit in two forms. , covers a thick deposit of frozen water. And as the Phoenix lander spotted, , though it vaporises before reaching the ground.
It鈥檚 autumn right now in the Martian northern hemisphere 鈥� good timing for a jaunt to the ice caps, which grow larger in winter and could make great ice rinks. Thanks to Mars鈥檚 lower gravity, a figure skater could reach unprecedented heights during a .
The Europa X Games
If the outer planets in our solar system don鈥檛 have surfaces where snow and ice collect, their moons certainly do. Europa, a Jovian moon that鈥檚 slightly smaller than Earth鈥檚, has a thin oxygen atmosphere and a thick shell of ice blanketing a vast ocean of water.
鈥淚f you smoothed it out, you could go ice skating there,鈥� says Cynthia Phillips, an astronomer at the SETI Institute in Mountain View, California.
Last winter, the Hubble Space Telescope spotted evidence of water-vapour plumes erupting from vents on 贰耻谤辞辫补鈥檚 surface that interact with Jupiter鈥檚 intense magnetic fields to form auroras. These appear in ultraviolet, though, so would be invisible to most people with healthy eyes.
Some researchers argue that ice skating requires a minimal amount of melting under the skate, which helps the blade glide over the ice. If that is the case, skating may not work as well on Europa, where surface temperatures are far too cold for ice to melt at all. But there are plenty of other options.
Europa has several ice-filled craters with slopes perfect for ski jumps and half-pipes. 鈥淵ou could ski down a crater, and since the gravity is less than Earth鈥檚, you could probably do some crazy tricks,鈥� Phillips says.
鈥淟ow gravity means you can do some crazy ski tricks鈥�
贰耻谤辞辫补鈥檚 kilometre-scale ice domes would also be ideal for good downhill skiing, adds Robert Pappalardo, a NASA scientist who studies icy moons. They are rounder and smoother in appearance than the craggy mountain ranges found on Earth, he says.
Meanwhile, the cliffs on 贰耻谤辞辫补鈥檚 so-called 鈥渃haos terrain鈥� would be a big draw for ice climbers. This region consists of huge chunks of broken ice that look like icebergs. 鈥淭hey鈥檙e the size of a small city, a few kilometres across, and they have some very steep scarps,鈥� Pappalardo says.
Scuba diving isn鈥檛 quite a winter sport, but Europa would be great for that, too, he adds. The icebergs are thought to have formed over thinner ice sitting above freshwater lakes. And don鈥檛 forget the .
You鈥檇 need some way of venturing a few kilometres below the ice to get to the liquid, though, which remains in that state because of titanic tidal forces acting on the moon as a result of Jupiter鈥檚 immense gravitational pull.
Other Jovian moons could play host to winter sports, too. Ganymede and Callisto are each about half-ice, half-rock, and may host oceans deeper than those of Europa or Saturn鈥檚 moon Enceladus, Pappalardo says. There鈥檚 so much ice that some of it converts to a higher-density form, allowing it to sink. 鈥淕anymede and Callisto are likely to have an outer shell of ice, like in our freezers, then a layer of water, then high-density ices below that,鈥� he says. 鈥淪o it鈥檚 kind of an ocean sandwich, with ice above and ice below.鈥� Scuba and skating 鈥� the perfect holiday combination.
Blue snow and base jumping
Venture a few more hundred million kilometres into space and you鈥檒l find excellent wintry terrain on Saturn鈥檚 moons. Titan鈥檚 natural lakes are liquid gas, but its surface is covered in ice mountains. Liquid methane and ethane rain onto the surface, which is peppered with ice boulders and ice cobbles. Nearby, the small moon Mimas, made mostly of ice, has a massive impact crater one-third the moon鈥檚 own diameter. The crater contains a mountain rising 6 kilometres above the floor. 鈥淭hat peak might make for some good climbing, although gravity is very low, so it might be cheating,鈥� says Pappalardo. That also means you wouldn鈥檛 have to worry about skiing down 鈥� you could simply leap off the top and float down to the surface.
For trails less travelled, look to Enceladus (Image: JPL/Nasa)
Of all the bodies in our solar system, Saturn鈥檚 moon Enceladus may be the premier ski resort. In 2011, planetary scientist Paul Schenk scrutinised the plumes there and found a blanket of snow-like material falling onto the surface. The Cassini probe, which has been visiting the Saturn system since 2004, first noted the jets of water vapour, much of which flows towards Saturn to form one of its rings. But Schenk and colleagues noticed a blanket of material on the surface of Enceladus, running in two strips from north to south. The blanket is a bluish colour, and falls at a mind-numbingly slow rate of less than one-thousandth of a millimetre per year, Schenk found. Yet the snow cover is about 100 metres thick in some spots, meaning it must have been snowing for tens of millions of years 鈥� that鈥檚 some ski season.
Enceladus 鈥渟nowflakes鈥� are much smaller than typical ice crystals on Earth, about 1 or 2 micrometres across, according to Schenk. That鈥檚 finer than talcum powder, so anyone hoping to traipse through the blue snow would want to bring some extra-large snowshoes that are better at staying afloat on light, powdery snow. Or you might want to strap on some skis 鈥� the ultra-light material is the finest powder any skier could hope for.
Exoplanet extremes
Beyond our solar system, a bevy of exoplanets presents tantalising wintry targets. Most stars in the Milky Way probably have at least a planet or two, if findings from the Kepler Space Telescope and other planet hunters are to be believed. The exoplanet census is full of super-Earths, mini-Neptunes, scorched tiny rocks and gigantic Jupiters, but astronomers are only just beginning to peer at their atmospheres to find out what they are like.
In September, astronomers discovered clear skies and steamy water vapour on a planet about the size of Neptune, located 120 light years from Earth. It is thought to have a rocky core and a mantle of ice.
Plenty of other exoplanets could harbour water in its frozen and liquid forms too. At the moment, imaging the surfaces of other planets is beyond the reach of our telescopes. However, there are more ways astronomers can look out for ice and snow in other solar systems. Last summer, the massive ALMA telescope snapped the first-ever image of a . On Earth, snow forms when temperatures are cold enough to turn moisture into the white stuff; in solar systems, snow lines form in a similar fashion, where it gets cold enough around young stars. That frigid boundary marks the spot where molecules like carbon dioxide, methane and carbon monoxide freeze and become dust grains, forming the building blocks of planets.
Despite the prevalence of snow, much exoplanet research is centred on finding worlds where water is liquid, not frozen, because it is a prime ingredient in the hunt for life. 鈥淏ut then you could think about water sports,鈥� Pappalardo says 鈥� and look forward to summer.
This article appeared in print under the headline 鈥淲inter wonderworlds鈥�
