INSIDE a featureless concrete building tucked away on the sprawling site of NASA’s Ames Research Center south of San Francisco, Ralph Lorenz is making weird waves. He is summoning up lines of tiny breakers in a small tank, a Californian beach in miniature, except that his waves are aliens – replicas of slow-moving giants that march across a hydrocarbon sea on a freezing world over a billion kilometres away.
Lorenz is a scientist at the Lunar and Planetary Lab at the University of Arizona in Tucson. In a wind tunnel inside a huge vacuum chamber, his team is trying to recreate the strange swell expected of lakes or seas on some of our nearest celestial neighbours. What they discover could give insights into how these worlds have evolved, and explain a variety of puzzling geological features. The experiments might even help pin down the most promising spots to search for life. Alternatively, Lorenz might simply discover the best surf in the solar system.
So how do you go about recreating an alien beach-break here on Earth? Fortunately the physics of wave generation is pretty much the same the universe over, says Lorenz. Basically, wind moving across a sea or lake creates friction which rubs the surface up into peaks. The stronger the wind, the higher a wave becomes and the more wind it can “capture” to drive its growth. At the same time, however, the forces of gravity and surface tension tug the other way, trying to drag the wave downwards. Whether a sea is covered by gentle ripples or roaring giants depends on which of these forces wins out.
Advertisement
To understand the intricacies of the process here on Earth, researchers build mathematical models that rely heavily on empirical relationships between wind and waves developed from studies of the oceans. Applying these models to oceans on other planets is fraught with difficulty, since slight changes in conditions such as atmospheric density or pressure might make an enormous difference to the results. The only solution, Lorenz believes, is to build your own extraterrestrial-surf simulator.
Lorenz is making his other-worldly waves in the Mars Surface Wind Tunnel (MARSWIT) at Ames. It consists of a wave tank inside a wind tunnel, inside a vacuum chamber. The wind tunnel is 13 metres long and 1.3 metres wide with glass windows and a powerful fan at one end that can create winds of up to 12 metres per second (see Graphic). The atmosphere within it can be adjusted to match that of any number of planets – the chamber can be filled with normal air, nitrogen or carbon dioxide, and powerful pumps can reduce the pressure to about one-thousandth that of Earth’s atmosphere.
Researchers normally use MARSWIT to simulate the way winds move dust about on the surface of Mars. To create his alien ocean, Lorenz replaces the fine dust in the tunnel with a tray 1 metre long, 5 centimetres deep and filled with water or liquids such as kerosene, depending on the planetary conditions he wants to simulate.
Conventional wave machines use mechanical paddles to generate swell, but with MARSWIT Lorenz can create waves the natural way – with moving gases. He switches on the tunnel’s fan and measures the height of the waves by bouncing pulses of light and high-frequency sound off the surface of the liquid. He then measures the time it takes for the pulses to reach sensors mounted above the tank. From this the distance between the sensors and the surface of the liquid can be calculated, and that gives the height of the waves. But how do you prevent backwash – waves that reflect off the far end of the tank and interfere with those coming the other way? Simple: drape a towel over the end of the tank to absorb the incoming waves.
For his initial experiments last year, Lorenz teamed up with Erin Kraal from the University of California, Santa Cruz, and researchers from Arizona State University at Phoenix. Their first stop? Mars.
The dusty, dry surface of Mars may seem an unlikely spot to search for surf. Admittedly there are endless stretches of perfect red, sandy beaches. It’s just that there doesn’t seem to be so much as a small pond to go with them.
Yet many researchers think Mars was not always quite like this. From orbit, NASA’s Mars Odyssey probe has photographed features that resemble huge dried-up lake beds. Around 4 billion years ago maybe 50 per cent of the planet was under water, and it could have been a kilometre deep in places, says Timothy Parker, who directs the regional planetary image facility at NASA’s Jet Propulsion Laboratory in Pasadena, California. The water probably disappeared about 2 billion years ago.
Some of the strongest evidence for these ancient oceans comes from NASA’s rover Opportunity, which landed in the Meridiani Planum region near a huge canyon called Valles Marineris in January. The rover found jarosite, an iron-rich sulphate mineral that has water locked up in its structure and usually forms when a large body of water evaporates. Layering in the rocks also indicates that sediments were deposited from flowing water.
Besides these deposits, large channels in an area of Mars called Chryse Planitia, between the southern uplands and the northern plains, are typical of huge volumes of flowing water. It seems that the Red Planet was once a blue planet, but the question is: were there waves?
Observations made by Parker and his co-workers since the mid-1980s suggest that these oceans did indeed have waves. Landforms in the northern plains of Mars resemble shorelines formed by wave erosion around dried-up lakes on Earth. These Martian “shorelines” are marked by multiple, inward-facing terraces that lie along the same contour for large distances and which Parker believes were almost certainly cut by waves.
However Lorenz and his team are not so sure. Late last year when they created waves at a range of atmospheric pressures, the waves became smaller and smaller as the pressure dropped – and below 400 millibars (less than half the Earth’s atmospheric pressure) waves all but disappeared. Atmospheric pressure on ancient Mars would probably have been a good deal lower than this, meaning that the planet’s oceans would most likely have been almost flat calm, says Lorenz.
That’s not the end of the story, however. Wind speeds on present-day Mars reach up to 45 metres per second – about three times as fast as in Lorenz’s simulation. And no one knows how strong the Martian winds might have been 2 billion years ago. There’s another problem too – gravity. Without sending the wind tunnel into space or spinning the entire facility in a giant centrifuge, it cannot simulate the low gravity of Mars or other planets. All Lorenz can do is add a factor to allow for these differences and scale the waves accordingly.
Lorenz is quick to stress that his results are only preliminary. Nonetheless he reckons that wave generation on ancient Mars was inefficient. While the planet might have had oceans, those strange terraces were most probably cut by the wind or formed by volcanic activity. It probably wasn’t a surfer’s paradise. So where else in the solar system might waxed boards and Hawaiian shirts come in useful?
Titan, Saturn’s largest moon, could turn out to be a far better bet. Donald Campbell and co-workers from Cornell University at Ithaca, New York, found that just 2 per cent of the radio waves they beamed at the moon using the Arecibo Radio Telescope in Puerto Rico were reflected back. Such a poorly reflecting surface would be characteristic of liquid rather than ice, Campbell believes.
To confirm this, Campbell and his team studied the spectrum of the reflected signals and measured how the polarisation of the signal was changed by its interaction with Titan’s surface. They found that the polarisation of the reflected radio waves differed from the polarisation of the signal they had beamed out, suggesting that the surface of Titan is covered with a relatively smooth, reflective layer. They also noticed that most of the reflected radio spectra show evidence of a specular glint – a bright reflection similar to the glittering appearance of sunlight when it bounces off smooth stretches of Earth’s oceans. These mirror-like reflections occur as a peak in the middle of the spectra at the same wavelength as the transmitted signal. However, the peak itself is broadened due to reflection from sloping features on the surface and a Doppler shift that is created as Titan rotates. On a liquid sea these slopes must be waves. Campbell realised he could use the shape of the specular reflection to estimate the angle the sides of these waves make to the horizontal. Their measurements suggest Titan’s seas have gently sloping waves with sides of up to 4 degrees from horizontal – much shallower than waves on Earth.
Clearly Titan has surf. Yet the moon’s low temperature, around −177 °C, rules out seas of liquid water. Because Titan’s atmosphere is particularly rich in organic compounds, Campbell suggests that 75 per cent of the moon’s surface is covered by liquid ethane.
A team of UK-based researchers go further. Nadeem Ghafoor of Surrey Satellite Technology in Guildford and John Zarnecki of the Open University in Milton Keynes, Buckinghamshire, along with colleagues from the Southampton Oceanography Centre in Hampshire, are preparing for when the European Space Agency’s Huygens probe parachutes onto Titan early next year. Their computer models suggest that with Titan’s weak gravity, its waves may be seven times as large as those on Earth. “Big-wave riding could take on a whole new meaning on Titan,” says Ghafoor.
Lorenz and his team have more modest expectations. Although preliminary experiments simulating Titan’s waves using kerosene indicated that the low density of the hydrocarbon ocean would indeed create large waves, he still has to factor atmospheric pressure and wind speed into his calculations. He expects the winds on Titan to be weak, so the waves shouldn’t be huge. “They will be slow moving,” he says, “but I’m not sure about them being giants.”
It looks like the most extreme swell in the solar system washes back and forth on a giant planet without any ocean – Jupiter. Little is known about Jupiter, but the planet’s core seems to be surrounded by a dense layer of hydrogen under pressures so high that, near the core, it acts like a liquid metal.
Although giant storms regularly disturb this atmosphere, 10 years ago an event occurred which created waves of such violence that they could be spotted from Earth. In July 1994, the remains of comet Shoemaker-Levy 9 smashed into Jupiter and exploded. They left a scar in the atmosphere larger than Earth and created a tsunami that raced around the planet’s liquid metal “seas” at phenomenal speed.
Recreating these waves in the lab could prove a bit too challenging for Lorenz. Surf lovers, too, shouldn’t get their hopes up. On Earth, surfers float in the sea for hours waiting for the next “big one” to come along. But events like Shoemaker-Levy 9 are far rarer. In the metal seas of Jupiter, surfers would need the patience of Job – the next big smash might not happen for several million years.