Extraterrestrial rafting: Hunting off-world sea life
http://www.newscientist.com/article/mg2 ... tml?page=1
09 November 2009 by Stephen Battersby
Magazine issue 2733.
Sniffing out life on Titan (Image: NASA/JPL/University of Arizona/SPL)
3 more images
IF LIFE is to be found beyond our home planet, then our closest encounters with it may come in the dark abyss of some extraterrestrial sea. For Earth is certainly not the only ocean-girdled world in our solar system. As many as five moons of Jupiter and Saturn are now thought to hide seas beneath their icy crusts.
To find out more about these worlds and their hidden oceans, two ambitious voyages are now taking shape. About a decade from now, if all goes to plan, the first mission will send a pair of probes to explore Jupiter's satellites. They will concentrate on giant Ganymede and pale Europa, gauging the depths of the oceans that almost certainly lie within them.
A few years later, an even more audacious mission will head towards Saturn to sniff the polar sea spray of its snow-white moon Enceladus. It will also visit Titan, which has perhaps the most astonishing extraterrestrial landscape in our solar system. To explore this giant moon, the spacecraft will send out two seemingly antique contraptions: a hot-air balloon to fly over the deserts and mountains, and a boat that will float on a sea of liquid hydrocarbons.
This plan for ocean exploration was announced in February, when the science chiefs of NASA and the European Space Agency decided to press ahead with the planning stages of both missions. Jupiter is the destination that tops the schedule, probably because the Europa Jupiter System Mission relies on well-tested space technology. The plan is for EJSM to lift off in early 2020, in two pieces. NASA's contribution, the Jupiter Europa Orbiter (JEO), and ESA's Jupiter Ganymede Orbiter (JGO) will be launched within a month of each other and plot parallel courses for Jupiter, arriving after six years. They will then engage in a complex dance, visiting various moons before each probe homes in on its prime target.
JEO has the tougher task. It will have to spend a long time in the inner reaches of Jupiter's radiation belts, where it will come under intense bombardment by high-energy electrons that would quickly disable an ordinary spacecraft. Though JEO will be built using electronics hardened against radiation, it will have to be clad in aluminium armour to survive in this hostile region.
When it finally goes into orbit around Europa, JEO's instruments will explore not just the moon's surface, but its depths too. The first hints that this moon's crust hides a liquid water ocean came from Voyagers 1 and 2, which saw a flat landscape criss-crossed with cracks when they flew by in 1979. This was confirmed in the 1990s by NASA's Galileo spacecraft. Galileo also found that Europa distorts Jupiter's magnetic field, which could be accounted for by an electrically conducting layer below the moon's ice. Most planetary scientists see this as compelling evidence for a subsurface sea of salt water.
JEO will map the magnetic field of Europa in even finer detail, and also measure the shape of its gravitational field. Putting the two together will give us further insights into the moon's structure - especially the thickness of its ice crust and the depth of its ocean.
The orbiter will also be armed with ice-penetrating radar. If the crust is only a few kilometres thick, the radar might even be able to peer right through to the ocean beneath. In any case, JEO should reveal much new detail about the inner workings of the crust itself, says Bob Pappalardo, one of the NASA team at the Jet Propulsion Laboratory (JPL) in Pasadena, California, which is developing the mission. "We want to get at the plumbing of Europa."
Could anything be living down there in the Stygian waters? If so, it is not going to be getting its nourishment from sunlight in the way that most life on Earth ultimately does. Instead, astrobiologists suggest it could feed off hydrogen sulphide or methane spewed out by thermal vents on the sea floor, as some creatures do in our oceans. To digest this chemical soup, Europan life forms would also need a supply of oxidising chemicals. It is hard to see where these could come from, other than from the moon's surface, where the intense radiation around Jupiter splits water molecules into hydrogen and oxygen. While most of the hydrogen escapes, the oxygen can latch onto molecules on the surface. But can it get to where ocean-dwelling life can make use of it?
That's where the plumbing comes in. If the crust is thin, there may be places where the ice melts, so water could gurgle along cracks between the ocean and the surface. If it is many kilometres thick, as most geologists suspect, the plumbing may be more sluggish. This process could still carry a breath of oxygen into the depths as warmer blobs of ice rise and old crust cycles down to replace it.
There is even a chance that JEO could find direct evidence of life. Jupiter's gravity might crack open Europa's crust, creating vents that blow out plumes from the ocean beneath. JEO could sample them and analyse the molecules they contain using its on-board mass spectrometer. "If we find hydrocarbons, that will be exciting," says Brad Dalton, a planetary scientist at JPL. "If we find peptide chains or even proteins, that is going to have serious repercussions."
Complex molecules like these would be big news, but there is no reason to think that life on Europa should have Earth-like biochemistry, so Dalton and his colleagues must cast their net wide. "The trick is to design a system that can acquire the broadest and most detailed information possible, so it will be sensitive not only to what you expect, but to the things you are not expecting," he says.
There is no reason to think that life on Europa should have Earth-like biochemistry
The second spacecraft of the pair, Europe's JGO, could confirm the existence of a much larger ocean when it goes into orbit around Ganymede, the biggest moon in the solar system. If there is liquid water under the crust it would be the largest ocean outside of Earth: at more than 80 million square kilometres, it would be about the size of the Atlantic.
Deep and dead
As a habitat for extraterrestrial life, Ganymede is a far less promising prospect than Europa, however. Its magnetic field, measured by the Galileo probe, indicates that there is probably a layer of water about 150 kilometres down, isolated by an ancient, static crust of ice above and a thicker ice layer below. It is hard to imagine how a Ganymedian life form would find anything to eat.
JGO will also swoop past Callisto, the outermost of Jupiter's large moons. According to theoretical models, even cold Callisto is likely to carry an ocean deep under its ancient crater-pocked carapace of ice - although, as on Ganymede, the chances of anything living there appears slim.
Having both spacecraft in the system at once will allow for some unique observations, says Pappalardo. Comparing the outer moons with Europa should give extra insights into how moons form around gas-giant planets. Such planets are common around other stars, and many are in much closer, warmer orbits than Jupiter and Saturn. Some will have moons with surface seas, offering promising sites for life.
Even in our solar system, not all seas are locked away under an icy layer. The second expedition to explore alien oceans aims to send a craft to visit Saturn's moons, notably Titan. Here in the outer solar system it's far too cold for surface seas to be composed of water. Instead, Titan is believed to have seas of liquid hydrocarbons, mainly methane and ethane, which would form gases on Earth. The Titan Saturn System Mission, planned by NASA and ESA for blast-off in the mid-2020s, will not only sample an alien sea, but will even go boating on it.
While looping round Saturn, TSSM will also visit tiny Enceladus (pictured). This moon, which is believed to have a rocky core wrapped in a thick ice coat, stunned planetary scientists when they found a giant plume of water and ice blasting out from its south pole. "This icy moon is spitting its guts out," says Athena Coustenis, head of the European half of the TSSM mission. Small, cold moons like this are not supposed to be volcanic, and it remains unclear whether the plume originates in a sea of liquid water, or from warm caverns in the crust. The latest results from the Cassini probe provide conflicting indications on how much salt the plume contains, but a high concentration would suggest that it originated in an ocean.
The surface of Titan - TSSM's ultimate destination - remained hidden until the Cassini mission parachuted the Huygens probe onto its surface in 2005. We now know that Titan presents a bewildering landscape of desert dunes, mountains of water ice, and methane rivers and lakes. With a panoply of cameras and other instruments, TSSM will find out more about the giant moon's make-up, including whether it has a watery ocean under its icy surface.
Titan also provides a unique opportunity to splashdown and sail on an alien sea: it is the only world we know, besides our own, with seas on its surface. So in the most audacious part of the mission, TSSM will launch a pair of anachronistic explorers; a hot-air balloon to peer down on the deserts and mountains (see "Balloon for an alien moon"), and a small raft to bob about in the sea of liquid methane and ethane.
Titan provides a unique opportunity to splashdown and sail on an alien sea
The raft will splash down in Kraken Mare, near Titan's north pole, which at more than 1000 kilometres across is the moon's largest body of liquid. During the 4 hours its batteries are expected to last, the craft will scan the surface and sample the frigid fluid - its temperature is around -180 °C - to analyse the carbon-based chemicals known to be present. These molecules are generated not by life, as far as we know, but by ultraviolet light from the sun driving reactions in the atmosphere. From there they are thought to fall to the surface, perhaps caught in methane rainstorms. Titan's surface might hold more organic material even than Earth.
The big question is how much headway these molecules have made along the road to life. The raft will carry a gas chromatograph mass spectrometer to identify complex chemicals. It may seem improbable that life would be able to progress at such low temperatures but, if it has, the lander should catch its scent.
One of the chemical characteristics of life, on Earth at least, is its ability to distinguish between mirror-image molecules, generally preferring left-handed to right-handed structures. The lander's instruments will be able to tell if one handedness predominates. Terrestrial life also leaves it mark on the mix of isotopes in the molecules it makes: plants on Earth prefer carbon-12 to the heavier carbon-13, for example. If similar processes are at work on Titan, the lander should find them.
It all sounds splendid, but this expedition remains far off. The decision to prioritise the Jupiter mission means our return to Saturn will be delayed. At best, it will launch in 2024 or 2025, says Jonathan Lunine of JPL, who leads NASA's involvement in TSSM. The journey will then take a full nine years. So, fingers crossed, by 2035 or thereabouts, instruments built on Earth could be soaring in a balloon over Titan, and riding the ocean waves on our maiden voyage on an alien sea.
Balloon for an alien moon
"If you want to put a balloon anywhere in the solar system, Titan is the place," says Athena Coustenis of the Paris-Meudon Observatory in France, who heads the European half of the Titan Saturn System Mission (TSSM). "It is much easier than Venus or Mars. Actually it is easier than Earth."
Titan's gravity is one-tenth that of Earth's, and the atmosphere is denser. The weather is calm, too. Coustenis doesn't expect storms to hit the balloon. Perhaps the worst that will happen is a rare shower of methane rain, which might coat the balloon with dark particles of the organic gunk that is created in the upper atmosphere.
The craft will rely on hot gas for buoyancy. As it is parachuted into Titan's atmosphere, a device containing a highly radioactive isotope of plutonium and capable of generating almost 2 kilowatts of heat will warm up the gases in the envelope above (see diagram). This is a nuclear-powered hot-air balloon.
Borne by the gentle east-west winds recently mapped by Cassini, the balloon should then start drifting undisturbed at an altitude of about 10 kilometres. It should take about six months to travel 16,000 kilometres and circumnavigate Titan.
The craft should get a fine view of the moon's eerily Earth-like landscape. The equator is dominated by a range of water-ice mountains named Xanadu and a series of great deserts with parallel-combed dunes composed of dry, gritty particles of organic material that has rained from the sky.
Sampling this stuff is not in the plan at the moment, but it's not out of the question, says Jonathan Lunine, who leads NASA's involvement in TSSM. "We might look at being able to drop a package on a tether, or land the balloon to collect material."
The balloon might not even be entirely at the mercy of the winds: it could be fitted with a motor-driven propeller. Lunine has suggested exploiting the possibility that the winds may blow in different directions at different altitudes. By changing altitude to catch wind blowing the right way, it might be able to tack back on itself, he suggests.
If it is lottery-winner lucky, the balloon may even spot a watery sea on the surface of the moon. From time to time, a comet or asteroid must hit Titan, smashing its crust and melting some of the ice. Lunine calculates that these pools could persist for more than 100,000 years for the largest impacts. And with all those complex organic molecules lying around, who knows what kind of primordial soup could be brewing there...
Stephen Battersby is a writer based in London
