When we think of the golden age of space exploration, our minds turn back to those black and white images of Apollo astronauts, the beeps of Sputnik as it broadcast from space. Today, space seems something of an afterthought: only this month, the pressures of the recession forced Barack Obama to scrap the US’s planned return to the Moon. So how, in my new BBC documentary, Wonders of the Solar System, can I claim that “we are living through the greatest age of exploration our civilisation has known”?
Well, when I was born, in March 1968, we had yet to visit the giant planets of the outer solar system. Jupiter, Saturn, Uranus and Neptune were mysterious balls of gas, with moons that, as far as we knew, were lifeless balls of rock. These were distant worlds, glimpsed only through the lenses of Earth-bound telescopes. We had only just completed the first successful interplanetary mission, when Nasa’s Mariner 2 spacecraft swept over the cloud tops of Venus in December 1962.
Today, as I write, Nasa’s twin rovers, Spirit and Opportunity, are beginning their sixth year of exploring the Martian surface. The highly successful Cassini mission, a joint project between the US and EU, is returning ever more beautiful images from the orbit of Saturn, leading to a host of scientific discoveries. Nasa’s Horizons spacecraft is en route to the outer reaches of the solar system: it is already a billion miles from the Sun, and will wake from its state of hibernation when it arrives at Pluto in 2015, before continuing outwards into the mysterious Kuiper belt of frozen “ice dwarves” that stretches out towards interstellar space. And perhaps most wonderful of all, Voyager 1, the most distant man-made object, is still working 23 years after its launch, sending back faint signals, which take more than 15 hours to make the 10-billion-mile journey home, as it searches for the edge of the wind from the Sun, which would mark the edge of the solar system.
And while these technological achievements are impressive, the recent discoveries from these far-flung worlds have the potential to have an impact way beyond the subsection of our society that has a passing interest in astronomy.
For me, the most startling news from our explorers has been the strong evidence for the presence of liquids on the moons of Jupiter and Saturn. Before we visited these gas giants, most scientists expected their moons to be somewhat like our own: beautiful, perhaps, but ultimately dead. It was known from telescopic observations that Saturn’s largest moon, Titan, had a dense atmosphere, but at a distance of 750 million miles from the Sun, this world would surely be a frozen wasteland.
This has turned out to be emphatically not the case. On Christmas Day 2004, the European Huygens probe detached from Cassini and began its slow descent to the surface of Titan. Just over two weeks later, it landed on what appeared to be a dry riverbed, populated with smooth, eroded stones. With the aid of photographs and radar data from orbit, Cassini and Huygens discovered that there is liquid on the surface of Titan: clouds from which giant droplets of rain float through the moon’s dense atmosphere, feeding streams and rivers that flow into seas far larger than Lake Superior here on Earth.
This surface looks so familiar to our eyes; a landscape sculpted by flowing liquid, just as the rain, rivers, and seas sculpt the surface of our planet. Yet Titan’s surface temperature is -180C, far too cold for liquid water to exist. The clouds and seas of Titan are made of liquid methane, a flammable gas here on Earth. It turns out that Titan has just the right atmospheric temperature and pressure to permit methane to exist as a gas, a liquid and a solid, leading to this wondrous landscape on a world three quarters of a billion miles from home.
Liquid methane is one thing, but liquid water would be quite another. Enter Jupiter’s frozen moon, Europa. Pictures of its surface, and measurements of its magnetic field from Voyagers 1 and 2, and the Galileo probe, which orbited Jupiter until 2003, have confirmed that Europa contains an ocean of liquid water beneath an icy crust perhaps 10 miles thick. The ocean is thought to surround the moon, and be up to 100 miles deep. This means that it is likely that there is more liquid water on Europa than on all the oceans, rivers and glaciers of Earth put together.
This has potentially profound implications, because we know from our own planet that wherever there is water, there is life – even in the darkest caves and on the deep, ocean floor, where there is no sunlight and crushing pressure. If the laws of physics and chemistry are universal, applying from our own solar system to the most distant stars, why should the laws of biology be any different? If the conditions are right, then life may well evolve and flourish.
And it’s not just Europa. There is good evidence that two more of Jupiter’s large moons, Ganymede and Callisto, may also have large sub-surface oceans. Not to be outdone, Cassini has recently discovered fountains of ice on the surface of Saturn’s tiny moon Enceladus, erupting hundreds of miles into space. These ice-geysers are responsible for creating one of Saturn’s faint but beautiful rings – but they also suggest the existence of sub-surface lakes of liquid water. And closer to home, the mineral gypsum has been found in abundant quantities on Mars, which indicates that there must have been large areas of standing water in the past, which could still be present in pockets beneath the surface today.
These discoveries all point to a solar system that is potentially more habitable – at least for microbes – than we could have possibly imagined. And in my view, this rapidly unfolding picture of a diverse, beautiful, alien yet familiar family of worlds orbiting our star should have positive social consequences.
For we now know two things: first, that we may well be on the verge of discovering that we are not alone in the Universe; that life may not be unique to Earth. And second, that the solar system is fragile. The worlds we have visited almost certainly lack the conditions for complex, multi-cellar life to evolve, and are incapable of hosting a civilization that can build space probes and explore its cosmic surroundings. All the evidence suggests that Earth-like planets may be very rare.
That rarity does not confer insignificance, but immense value. To realise this, though, we have had to journey outwards to the stars. These are the gifts awaiting the explorer: perspective, humility and purpose, and ultimately a renewed sense of the value of home.
* Brian Cox is professor of physics at Manchester University, and one of the country’s best-known communicators of science. 'Wonders of the Solar System' airs on BBC Two at 9pm on Sunday.
The seven wonders of the solar system
source www.telegraph.co.uk
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