杏吧原创

WHEN NASA鈥檚 Spirit rover touched down on the vast floor of a Martian crater in January 2004, the space agency really did have something to celebrate. For the first time in history, a robot was viewing an alien world in the same exquisite detail that a person would enjoy. Since then, the panoramic cameras on Spirit and its twin, Opportunity, have gazed around the Red Planet like human eyes with 20/20 vision.

So it might sound churlish to point out that the rovers are both as deaf as a post. Like the vast majority of spacecraft sent to other worlds, neither carries a microphone. They are oblivious to the soundscape of Mars. They can鈥檛 hear the buzz of grit whirling around in dust devils, the rumbles of Martian thunder, or even the noise of their own wheels crunching across the dusty plains.

In fact, the sounds of the entire solar system are woefully unexplored. 鈥淚 compare it to the stage that space imaging was at 30 or 40 years ago 鈥� people didn鈥檛 think they needed to fly cameras,鈥� says Greg Delory, a physicist at the University of California, Berkeley. 鈥淭he concept for early space probes was that we needed detailed physics measurements of things like temperature and pressure. What鈥檚 the use of a picture?鈥�

Compared with cameras, acoustic sensors are usually smaller, lighter, cheaper and less power-hungry. However, their simplicity may have had the adverse effect of underselling them as instruments able to deliver subtle and rich scientific information, says Tim Leighton, a physicist at the University of Southampton in the UK.

鈥淲e can do an awful lot more with acoustics than simply hear a rumble, shrug our shoulders and say 鈥榯hat sounds like thunder鈥�,鈥� Leighton says. 鈥淎 rumble might tell you the thickness of an ice sheet, or the temperature across an entire ocean. Sound travels through these things over vast distances towards you and carries a lot of extraordinary information.鈥�

Acoustics is not the glamorous side of science, certainly not for space scientists, Leighton admits, but he adds that 鈥渟ince NASA has set its sights on sending people to the moon and Mars, acoustics could be a very . Think of the information and cues a deaf person on Earth loses. It鈥檚 not just in speech and communication, but in diagnosing whether a drill is working properly or whether the gravel beneath our shoes is slipping.鈥�

The European Space Agency鈥檚 Huygens probe, which landed on Saturn鈥檚 giant moon Titan in 2005, carried acoustic instruments that worked amazingly well, according to John Zarnecki of the Open University in Milton Keynes, UK, principal investigator for the probe鈥檚 surface-science instruments. 鈥淲e鈥檝e shown that it鈥檚 feasible. These are elegant, simple, low-resource instruments which really can do a great job,鈥� he says.

Science apart, the sounds of other planets and moons could give us all a fantastic connection with these worlds 鈥� an awesome sensory feast. 鈥淲e shouldn鈥檛 underestimate its value for bringing the public into the experience,鈥� says Delory. 鈥淭hey鈥檙e the taxpayers, they鈥檙e the people that we鈥檙e doing this for.鈥�

Imagine listening to babbling brooks of methane on Titan, or hearing deep thunder booming through Venus鈥檚 dense, noxious atmosphere. Or what about the creaking and groaning of ice on Jupiter鈥檚 satellite Europa, which could reveal whether the moon鈥檚 frigid exterior hides a warm ocean, hospitable to life? The solar system鈥檚 surround-sound has many delights in store, so listen up.

Mars

Five NASA landers 鈥� the twin Vikings, Mars Pathfinder, Spirit and Opportunity 鈥� have beamed back spectacular images of the Martian landscape. None of them has recorded any sound, says Delory, but that鈥檚 not to say other missions haven鈥檛 tried.

Delory was part of a team funded by the California-based Planetary Society that sent a microphone to Mars aboard the Mars Polar Lander in 1999. Alas, NASA lost contact with the lander 鈥� it probably crashed when the engines that slowed its descent switched off too early 鈥� but had the mission worked, the Mars Microphone team hoped to hear a whole gamut of sounds, such as wind and soil whirling around in dust devils. 鈥淵ou can watch of the dust devils on Mars, but imagine having a soundtrack to that. It would be so exciting,鈥� says Delory.

The microphone would also have recorded noises from the lander itself, such as the scratching and scraping of its robotic arm digging into the ground. It might also have heard the snaps, crackles and pops of electrical discharge created by the swirling dust devils. 鈥淚t could be that there are tiny electrical discharges happening everywhere in a dust column, but it鈥檚 also possible that you get big-scale events like the lightning we have on Earth 鈥� we just don鈥檛 know,鈥� says Delory.

NASA鈥檚 Phoenix lander, which was due to launch last week, will carry a microphone to Mars. But it will only record noise during the parachute descent, a bit like a black-box recorder. 鈥淚t provides a measure of the vibrational environment experienced by the entry, descent and landing system, which cannot be collected in any other way,鈥� says Mike Malin of Malin Space Science Systems in San Diego, California, a NASA contractor for the Phoenix lander. 鈥淭he microphone has significant public appeal.鈥�

Delory hopes the Mars Microphone will fly on a future mission, and says the project has already spawned many attempts to understand how well sound travels on Mars. Last year Amanda Hanford from Pennsylvania State University and her colleagues showed in that because Mars鈥檚 atmosphere is much thinner than the Earth鈥檚 鈥� the average distance between molecules is about 120 times greater 鈥� sound waves attenuate very quickly.

While a typical scream on Earth might still be audible a kilometre away, it would only travel about 16 metres on Mars, Hanford says. The speed of sound in Mars鈥檚 predominantly carbon dioxide atmosphere is also slower, so if you could stand on Mars and talk, it would have the opposite effect to inhaling helium and sounding like Donald Duck: while helium gas has a higher speed of sound than air, which makes your voice sound high, Mars鈥檚 low sound speed would lower the pitch of your voice.

Venus

In the glory days of the space race, the favourite destination for the Soviet Union was Venus, often called Earth鈥檚 evil twin. Sulphuric acid rains down through a CO₂ atmosphere with a crushing surface pressure 90 times greater than Earth鈥檚.

In 1982, two Soviet Venera landers carried microphones and recorded the sound of the breeze on Venus. Comparisons with wind tunnels suggested there were gentle winds of about 2 kilometres per hour. The landers also heard deep rumbles that might have been thunder, but could have been turbulent airflow.

The question of whether lightning occurs on Venus is unresolved. Soviet probes spotted faint flashes, possibly lightning, on the planet鈥檚 night side, while a telescope in Arizona also saw flashes in 1993. However, none of these sightings has been confirmed.

Sending a microphone to Venus could resolve this once and for all, by matching any flashes to rumbles of thunder. 鈥淚f you get those two together, you鈥檙e in business,鈥� says Delory. He hopes a microphone might one day fly through the clouds of Venus on a balloon mission; tests with a hot sulphuric acid mist showed that even in Venus鈥檚 hellish sky, the microphone could work for several hours.

So how would the thunder sound on Venus? Not necessarily like thunder on Earth, says Andi Petculescu from the University of Louisiana in Lafayette, who has modelled sound propagation on the planet with Richard Lueptow from Northwestern University in Evanston, Illinois. Only low-frequency sounds carry well in the crushing CO₂ atmosphere, Petculescu told a June meeting of the Acoustical Society of America in Salt Lake City, Utah.

鈥淭he high frequencies are muffled considerably on Venus,鈥� says Petculescu, who has created an that can morph a given noise into how it would sound after travelling a kilometre through the Venusian atmosphere. They used it to record a of the guitar intro from Deep Purple鈥檚 Smoke on the Water, giving it a much deeper, thumping bass.

Fun and games apart, knowing how the Venusian atmosphere distorts noise will be crucial to interpreting sound recordings from future probes. A distant rumble of thunder, say, might be distorted beyond obvious recognition. That鈥檚 a problem Petculescu has yet to get around to. 鈥淧erhaps a better choice would have been thunder rather than Smoke on the Water,鈥� he says, 鈥渂ut I couldn鈥檛 resist.鈥�

The moon

There is only one place in the solar system where we have made a concerted effort to listen in. Between 1969 and 1972, astronauts on four Apollo missions left seismometers on the moon to record sound waves rattling through the lunar crust. This network of sensors beamed back data on moonquakes until 1977, when it was switched off to save money.

Analysis of the measurements has continued for decades, revealing different kinds of shakes and shudders. Some originate more than 700 kilometres down and occur monthly, signalling the stresses and strains on the moon due to the tidal forces of Earth鈥檚 gravity. The network also picked up meteorite impacts, as well as quakes triggered as heat from the sun swelled the moon鈥檚 crust.

These quakes were fairly mild, but the network also recorded about 28 major quakes just 20 or 30 kilometres below the surface. Seven registered at above 5 on the Richter scale 鈥� vibrations that on Earth would be powerful enough to move heavy furniture and crack walls. The dry, rigid structure of moon rock means these strong quakes last for more than 10 minutes. On Earth they would usually fade in under a minute. So what causes them? 鈥淲e really don鈥檛 know and, to my mind, that鈥檚 a problem,鈥� says Clive Neal at the University of Notre Dame in Indiana, whose team analysed the lunar data.

NASA intends to establish a manned lunar base on the moon after 2020, and the quakes could spell trouble. A magnitude-5 quake could easily rupture the airtight seal of a moon base and cause a catastrophic loss of pressure. So Neal鈥檚 team is developing a proposal to deploy a new network of three to six seismometers around the moon. 鈥淚t would probe the hazards to potential lunar habitats,鈥� says Neal, so that lunar base engineers could plan ahead, choosing a safe site and selecting sufficiently tolerant building materials. 鈥淚t would also give us information about the composition and physical state of the lunar core and lower mantle, of which we really know nothing, except from models.鈥� Neal hopes the mission will launch around 2014 and run for six to 10 years.

Titan

When the European Space Agency鈥檚 Huygens probe landed on Saturn鈥檚 moon Titan in January 2005, it marked the first time in more than two decades that we had heard the sound of another world. The probe, which hitched a ride to Saturn with NASA鈥檚 Cassini probe, carried a basic microphone that . 鈥淚t was mostly just the wind blowing through the rigging,鈥� says Zarnecki.

Huygens also carried a sonar instrument that emitted high-pitched squeals at a frequency of 14.7 kilohertz and listened for any echoes. It was designed to measure the depth of any lake or ocean in the event that the probe splashed down into one. As it happened, Huygens crunched down onto a plain littered with icy pebbles, but Zarnecki says the instrument returned five or six echoes from Titan鈥檚 surface during the last 100 metres of the descent. That allowed the team to clock the probe鈥檚 landing speed at 4.67 metres per second and accurately reconstruct the probe鈥檚 entire trajectory.

鈥淵ou need to know where the instruments took their measurements, and that requires a reconstruction of the whole trajectory from top to bottom,鈥� says Zarnecki. 鈥淭hat was very important for the science.鈥�

The sonar also revealed the lie of the land over an area of about 1000 square metres around the landing site, confirming that the camera鈥檚 restricted view was typical. Camera and sonar agreed that the landscape featured gentle undulations of about a metre or so. 鈥淚t showed the surface was not perfectly flat like a mirror; at the same time it didn鈥檛 have canyons or cliffs, sharp edges or big boulders,鈥� says Zarnecki.

There are also hints that the sonar heard echoes from dense methane rain clouds, although the scientists are still analysing the data. Models have suggested that 鈥渞aindrops鈥� on Titan might be much larger than on Earth 鈥� up to about a centimetre wide 鈥� and would fall much more slowly, since the surface gravity is about one-seventh of Earth鈥檚.

A third acoustic instrument on Huygens measured the speed of sound during the last 11 kilometres of its descent by emitting high-frequency sound pulses and clocking their arrival at a receiver. These measurements revealed the methane content rising from about 2 per cent at an altitude of 10 kilometres to about 3.5 per cent near the surface, as the speed of sound in a gas is inversely proportional to the mean molecular weight.

鈥淥ne of the most exciting things about Titan is that its atmosphere is a factory for complex hydrocarbons,鈥� says Zarnecki. 鈥淭here鈥檚 all this photochemistry going on as methane is broken down by radiation and then chemistry builds more complex hydrocarbons. But to model it all, you need to know what鈥檚 there in the first place.鈥�

Zarnecki says that next time around he would target a lake. A radar instrument on the Cassini orbiter has found good evidence for dark hydrocarbon lakes near the polar regions of Titan, some as large as the Caspian Sea. In case of just such a landing with Huygens, Leighton simulated in advance the bubbly noise that a microphone would hear if the probe splashed down in a methane lake on Titan, or if it landed close to a methane 鈥渨aterfall鈥�, so that scientists would know what they were hearing. 鈥淏ubbles are the most potent naturally occurring sound source 鈥� they鈥檙e like little bells and gongs,鈥� he says.

When a moving liquid squeezes a bubble, the gas inside exerts a restoring force, making the bubble pulsate like a vibrating bell. This radiates sound. 鈥淓ach bubble knows its characteristic note just as each piano wire knows its characteristic note,鈥� says Leighton. 鈥淚t鈥檚 like having a chorus of little bubbles each 鈥榩linking鈥� at its own pitch: low notes for big bubbles, high notes for smaller ones.鈥� The sound of a methanefall or splashdown is the sum of this chorus, with other splashing sounds on top.

On Titan, the chemistry and temperature would make the sound of bubbles different to that on Earth. On one hand, liquid methane is denser than water, which would deepen a bubble鈥檚 tone. 鈥淚t鈥檚 like taking a piano string and wrapping some metal around it, which makes it more massive and deepens its characteristic note,鈥� says Leighton. On the other hand, the restoring force of the gas would be higher, effectively tightening the piano string and raising its pitch. The combined effect, says Leighton, is that a or the sound of a probe鈥檚 would sound more metallic, more 鈥渞obotic鈥�, than a or on Earth.

The sound speed at Titan鈥檚 surface is only about 60 per cent of that on Earth because the temperature is a frigid 90 kelvin (-183 掳C). However the atmospheric pressure is 50 per cent higher, so sound carries farther on Titan than on Earth across the whole audio range. To illustrate this, Petculescu has also created a version of the Venus filter to simulate Smoke on the Water in the . Future environmental noise authorities on Titan, be warned 鈥� it sounds even louder and more annoying there than on Earth.

Europa

If there is one world where scientists are most keen to do some eavesdropping, it鈥檚 probably Europa, Jupiter鈥檚 third-largest moon. Right now, Europa is the hot favourite site for extraterrestrial life in the solar system, after NASA鈥檚 Galileo orbiter fuelled speculation that it has a vast, warm sea under its icy surface.

In 2000, instruments on Galileo revealed that Jupiter鈥檚 magnetic field induces a magnetic field inside Europa, suggesting it must have an electrically conducting layer under the surface 鈥� most likely a salty liquid water ocean. Underneath the icy outer shell, which is perhaps 10 to 20 kilometres thick, there may lie an ocean, 100 to 200 kilometres deep, containing more water than all of Earth鈥檚 oceans put together.

Although no Europa missions are on the cards, NASA has toyed with the idea of sending a lander to the moon to drill through the ice and sample the water. However, Nick Makris, an ocean scientist at the Massachusetts Institute of Technology, argues that it would be far simpler to send one or more acoustic sensors to Europa鈥檚 surface to sit and listen.

Tidal forces from Jupiter constantly squeeze and stretch the moon during its 3.5-day orbit, altering its radius by up to 30 metres. 杏吧原创s calculate that this should constantly open up cracks in the ice, which would propagate at about 1 metre per second and arc for hundreds of kilometres. Probes haven鈥檛 seen this in action, but it would explain why the moon is criss-crossed with lines.

Makris and his team estimate that cracks form every 30 seconds or so in the most highly stressed regions. They should typically penetrate about 100 metres and radiate low-frequency sound waves of about 0.1 to 100 hertz. Even with a single sensor, Makris says it should be possible to determine the location of a crack by comparing the arrival times of compressional and shear waves, which travel at different speeds, and to listen for later echoes from subsurface boundaries to figure out the ice and ocean depths.

鈥淪ound propagates very well at low frequencies under water and in the ground,鈥� says Makris. 鈥淥n Earth, you can track vocalising whales over hundreds if not thousands of kilometres using listening stations.鈥� Because the speed of sound in an ocean depends on its temperature, scientists might be able to estimate how warm Europa鈥檚 ocean is, and whether it could host life. Depending on the thickness of the ice, who knows, could we even hear sounds of life?

That鈥檚 a long-shot, Makris thinks. Even if complex life existed there, the ice itself may drown out any subtle biological sound. 鈥淚f you listen to recorded from the Arctic on Earth, they sound like creaks, moans and groans. It鈥檚 a little like a haunted house,鈥� he says. 鈥淏ut they鈥檙e not biological, they鈥檙e the ice itself.鈥�

Until a probe lands on Europa, it remains anyone鈥檚 guess what鈥檚 really going on there. With NASA focusing on getting people to the moon and Mars, a Europa lander mission is unlikely to happen in the next decade or two. NASA cancelled an orbiter and lander mission called the Jupiter Icy Moon Orbiter (JIMO) in 2005. Even so, Makris, who worked on the JIMO plans, is confident that a lander will go there eventually. 鈥淧lanetary science is a lot like cathedral building 鈥� projects pass from one generation to the next,鈥� he says. 鈥淚t鈥檚 a matter of when, not if.鈥�

鈥�and beyond

So where else in the solar system would have an interesting soundtrack? Makris backs Jupiter鈥檚 moon Io, the most volcanically active world known. 鈥淚f I could just put my ear down on the ground anywhere, I think it would be on Io 鈥� it would be really cool just to hear the rumble of the volcanoes.鈥�

Then there are Jupiter鈥檚 moons Callisto and Ganymede, and Saturn鈥檚 Enceladus, where acoustic sensors might uncover underground water. Microphones on Neptune鈥檚 moon, Triton, could eavesdrop on geysers spluttering and whooshing into life in the sunlight, raising dusty plumes of liquid nitrogen or methane up to 8 kilometres high.

The sharp-eyed cameras of robots on Mars may still be flavour of the month with NASA but, one day, electronic ears might really steal their thunder.