ESCAPE TO TITAN
When the Sun scorches Earth, a tiny moon in orbit around the ringed giant Saturn is our next home
Our planet may have survived for 4.5 billion years, but humanity faces some major threats. There’s always the possibility that an asteroid will wipe us out, just as one did for the dinosaurs some 65 million years ago. We could be engulfed by a gamma-ray burst or disrupted by a wandering star. There are also dangers closer to home, from volcanoes to nuclear war. Even supposing humans manage to survive all these threats, we can still say for certain that life here on Earth will eventually be no more. In around 5 billion years from now the Sun will undergo a massive change that will fundamentally alter our Solar System. It will cause the end of not only all life here on Earth, but possibly the entire planet, and we will have no choice other than to find somewhere else to live.
Astronomers have been looking at the possibilities of colonising other planets for years. Mars currently tops the list of destinations, with NASA working hard to develop the capabilities needed to send humans to the Red Planet in the 2030s. Yet some scientists are taking a much longer view. Rather than looking towards the terrestrial planets for our new home, they say humans will one day have to relocate to the outer Solar System if they want to survive. As it currently stands, sending scores of humans to live beyond the asteroid belt is out of the question. The four gas giants are utterly unsuitable for life, and the moons of the outer Solar System are well outside of the habitable zone – the region around the Sun where the atmospheric pressure is able to support liquid water, making conditions for life as we know it ‘just right’.
Yet things can and will change. The Sun is getting gradually warmer, and it will eventually become so hot that it will boil off Earth’s oceans. This will happen sooner than we think. “In around a billion years’ time, Earth will probably no longer be habitable for humans,” says Benjamin Charnay, a research associate at the French National Centre for Scientific Research from the Observatoire de Paris. “The increasing solar insulation means Earth will either evaporate all of its oceans or lose them by the atmospheric escape of hydrogen.”
But that will only be the start. Even if humans do somehow survive the mass loss of water on Earth, the next thing to be affected would be the current habitable zone. At some stage the Sun’s hydrogen supplies at its core are going to deplete, and gravity will take over. Nuclear fusion – the energy-generating process of converting hydrogen into helium – will cease, bringing an end to 10 billion years of stability. As the Sun’s core collapses, helium will fuse into carbon and the Sun will bloat up into a red giant, 256 times its original size. “The Sun will swell out to beyond the orbit of Earth,” says
Dr Christopher McKay, a planetary scientist at NASA Ames Research Center. The effects of all of this will be devastating for both Earth and the entire inner Solar System, and at this stage staying put on our planet will not be an option.
After all, the outer layers of the Sun will now be at escape velocity and peeling away. Mercury and Venus will be engulfed, and the orbits of the planets will be widening due to a weakened gravitational pull. “Even if it survives, Earth will be inside the Sun’s atmosphere,” McKay adds. If all of this sounds quite gloomy for the future of humankind, then be assured that it is. “The issue for humans would be to survive to this time,” says Charnay, strongly hinting that there is every chance that nobody will be around to see any of it.
Let’s suppose that humans do manage to get that far. Where will they be able to go when the Sun has turned a vivid red? The smart money is on a lovely home overlooking Saturn and its stunning system of rings. In the new order of the Solar System,
Titan, Saturn’s largest moon, is likely to become the number one destination for humans. It won’t be easy – the journey to Titan from Earth takes some seven years, which will excessively burden the body and mind of any astronaut – but it could be the perfect escape route that will keep humankind going for many more millions of years.
It may be hard to imagine that a moon which is ten-times further away from the Sun than Earth could possibly become a new human base, but
Titan is actually a close match for our planet. In many ways it mimics Earth’s primitive state. As
“In around a billion years’ time, Earth will probably no longer be habitable for humans”
Benjamin Charnay
such it has proved fascinating for astronomers who have been building up data about the moon since Huygens, an atmospheric entry probe, landed there in 2005 following a seven-year journey as part of the Cassini-Huygens mission. It was the first-ever landing accomplished in the outer Solar System, and it will not be the last by any means. When the day comes that space agencies are seeking to send manned flights to Titan, you can be assured that the technology needed to safely transport people 3.2 billion kilometres (2 billion miles) across the Solar System will be very much in place.
Titan is one of only three worlds in the Solar System with rocky surfaces and thick atmospheres – Venus and Earth being the others. “The thick atmosphere cuts down on radiation, so it is a very neutral environment,” says Dr Mike Malaska, a scientist at NASA. It has a gravity that is similar to that of our own natural orbital satellite, making Titan the easiest place to fly and land in the Solar System – something that should help with future colonisation. Astronauts will be able to navigate Titan wearing just warm coats as it benefits from having zero to low pressure, unlike our Moon or Mars. “The Moon and Mars both share the problem that if humans didn’t wear spacesuits they would die rapidly from depressurisation, which the movies like to show as being explosive,” says McKay. “On Titan a spacesuit is not required.”
Titan has weather, and it is the only body in the Solar System other than Earth to possess surface lakes and seas. It also has river channels, dunes and complex hydrocarbons, along with pebbles of ice that point to an existence of water in the past. Crucially, it has copious organic raw materials. “These would be great for colonialists to use for manufacturing things,” adds Malaska. “The diversity of features on the surface suggests that there might be different patches of different types of organics – kind of like the different rock outcrops here on Earth. There might also be outcrops of water ice, so water might be available after heating it.”
Indeed, astronomers say that all Titan effectively needs is warming up to make it a viable home.
“Temperature is a current problem on Titan,” says McKay, as the moon receives one-hundredth of the solar heat we get here on Earth. “At -180 degrees Celsius (-292 degrees Fahrenheit), if you visited today it would feel like plunging into freezing cold water, so humans would have to find a way of keeping very warm. We’d have to wear special spacesuits like the ones divers wear. Yet all this changes once the Sun becomes a red giant.”
When the Sun has transformed, Titan will be in the middle of a new habitable zone, which will have moved deeper into the Solar System, taking it as far as the Kuiper Belt. The frozen moons of the outer planets will become far warmer, melting ice into liquid water and allowing life a chance to
flourish. As McKay, Ralph Lorenz and Jonathan Lunine wrote in an important paper published back in 1997, Titan would respond well to being given a new lease of life, and humans could benefit greatly from it.
Quite apart from the Sun raising the moon’s temperature to -70 degrees Celsius (-94 degrees Fahrenheit), they noted that the surrounding thick, orange haze of Titan’s atmosphere would also be depleted. Since the haze currently allows the surface to be unaffected by the increase of solar radiation caused by the Sun being closer and hotter, this would enable a greenhouse effect, creating an environment that would be suitable for life. Things would begin to slot into place.
What’s more, the moon’s warmer temperature would also change the composition of the atmosphere. Currently it is made up of 95 per cent nitrogen and five per cent methane and “the air is thicker than Earth by a factor of seven,” says McKay. But 5 billion years from now “the luminosity will be large enough to affect the icy crust and liberate a water-ammonium ocean,” says Charnay. The effects will be jaw-dropping.
Dr Carrie Anderson, planetary astronomer at the Astrochemistry Laboratory at NASA Goddard Space Flight Center, says the water will melt, mix with the organics and make up amino acids, which contain carbon, nitrogen, oxygen and hydrogen – the basic elements necessary for life. “Titan has all of these,” she told an audience at the Library of Congress in the US. “It’s just waiting. It’s ready to go.”
Even so, there are still some doubts: “It would be difficult to produce an oxygen-rich atmosphere because Titan’s atmosphere and interior are very reducing,” says Charnay. “For instance, there is a lot of methane, which would react to destroy oxygen.” There would also be something of a race against time in order for life to form and flourish. Titan will have a window of ‘just’ 100 million years for life to emerge, for reasons we’ll come to in a moment. Anderson believes that this is sufficient time for life to form on Titan, however, making it a viable future home for humans. “At this moment in time the ice will melt in the mantle, and a lot of it should melt, so you should have liquid water. Then we have all those organics just sitting around on the surface just waiting for the Sun to heat them up,” she says.
It is a prospect that also excites Malaska. “When the Sun evolves into a red giant, Titan will heat
“The Sun will swell out to beyond the orbit of Earth. Even if it survives, Earth will be inside the Sun’s atmosphere”
Christopher McKay
up,” he says. “The water ice in the crust will melt, and the organics on the surface will probably react with water, each other and themselves. It’ll be a wonderfully interesting organic chemistry mess. Most of the organic ‘goo’ will be floating on the surface of the water.”
And yet there is a possibility that life already exists on Titan. Malaska says life could be based on different sets of molecules and interactions, and that microscopic alien organisms may already be swimming in seas of methane. Could this have a profound effect on our ability to colonise Titan in the event of a red giant? Would questions be raised over our chances of adapting and living alongside such alien life? Time will tell, and scientists will be working on those very answers.
“If we discover life on Titan, it would be incredibly huge,” says Malaska. “It would be a fundamentally different type of life and would take our fundamental understanding of biological processes to a new level.”
He claims that Titan has already changed how we think about geology: “Comparing and contrasting the geology of Earth and Titan is a powerful tool. With regards to geology, we talk about how lakes and rivers work and now have examples using both water and hydrocarbons, so we can understand the fundamental processes even though the materials, temperatures and gravity fields are totally different.
“Discovering life on Titan would likewise change our understanding of the fundamental processes of biology. It would extend our concept of the habitable zone where surface liquids exist to a different temperature range and set of surface conditions. It would also tell us that there may be even wilder and weirder temperature and chemical regimes that we can start to think about.”
But even if life exists, emerges or travels to
Titan, one thing is certain: it won’t be staying there forever. Any migration from Earth to Titan will always be temporary since it will start to get too close to the Sun. Once those 100 million years are up, liquid water on Titan will evaporate and the moon will suffer an incredible rise in heat.
But there is potential for a reprieve. The Sun will later contract and become a white dwarf that will burn for a billion years. This will place Titan back into the habitable zone once again. It means that any humans who escape to Titan and then suffer another setback as Titan is burned dry could – if they somehow hold out, and quite how is anybody’s guess – have another opportunity for a long existence on Saturn’s moon. As Anderson told her audience: “Maybe life has a real chance on Titan during those billion years.” For the sake of the future of humanity, we sincerely hope it does.