POWERFUL giants stalk the surface of Mars. Ten kilometres tall and up to a kilometre across, they prowl the planet from pole to pole, throwing up clouds of dust and spraying out lightning bolts as they pass. They even command the Martian weather, creating vast storms that can plunge the planet into darkness for months on end.
These monsters are dust devils, swirling tubes of rust-coloured dust and sand, whipped up by Martian winds into something resembling huge tornadoes. No surprise then, that the devils of Mars are giving NASA鈥檚 mission planners a serious headache. The swirling winds could wreak havoc on anyone or anything that gets in their way鈥攊mmobilising astronauts, blocking communications and frying any electrical equipment they touch. If people are to set foot on the Red Planet, they must be prepared to meet the devils鈥� awesome powers.
The first sign of these huge dust-laden clouds came from photographs taken by the Viking landers in the 1970s. But the dust devils moved so fast that the long exposure time used by the cameras meant the images lacked detail.
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Then in 1999, NASA鈥檚 Mars Global Surveyor went into orbit around the Red Planet. Its cameras beamed back the first clear pictures of these twisters and the tangle of tracks they left in the dusty Martian landscape. Along with evidence from NASA鈥檚 Mars Pathfinder, which landed on the Red Planet in 1997, researchers soon realised that these Martian monsters are anything but rare (快猫短视频, 18 September 1999, p 17). Some of the latest photos taken by Mars Global Surveyor show more than half a dozen devils within a few kilometres of each other.
Although they might look something like the tornadoes we have on Earth, Martian twisters form in a very different way. On Earth, tornadoes are born when strong winds start to circulate inside massive storms. But the Martian atmosphere is about 100 times thinner than the Earth鈥檚 and lacks the weather systems necessary to trigger tornadoes.
On Mars, there aren鈥檛 enough clouds to block out the Sun, so bright light falling on the surface can create a large temperature difference between the ground and the atmosphere just above it. In one spot close to the ground, for example, Pathfinder recorded a temperature of 16 掳C, while just a metre above the temperature had dropped to 鈭�7掳C. This kind of large temperature variation can heat up the atmosphere, triggering strong convection currents. Whenever these currents grow rapidly and meet features in the landscape such as hills or valleys, the thin atmosphere could start to spin in a tight vortex.
Similar effects occur on Earth, where warm air currents above a hot surface such as a road in a desert can trigger small dust devils tens of metres tall. However the unique conditions on Mars combine to create something far more threatening.
Since the Red Planet鈥檚 atmosphere is extremely thin, once convection currents start there is little friction between gas molecules to slow their motion. The wind this creates might be able to reach up to 100 kilometres per hour. Also, since Mars has no liquid water in its soil, there鈥檚 no surface tension to bind particles of dust together. 鈥淢artian soil is basically fluff, like talcum powder,鈥� says Peter Smith, a planetary scientist at the University of Arizona in Tucson. Even the smallest devil could rapidly scoop up many kilograms of the stuff. And as Martian gravity is just one-third that of Earth, gas molecules and dust particles whirling in the atmosphere feel less of a downward pull鈥攊n other words, what goes up, stays up. The dust devils of Mars can grow as tall as the Himalayas, carry up to a tonne of dust and last for days at a stretch. 鈥淭hey could be as strong as a mild tornado on Earth,鈥� says Nilton Renno, Smith鈥檚 fellow dust devil devotee at Tucson.
Everyone agrees Martian dust devils exist, but no one knows what they鈥檙e capable of. The chances are that anything caught in a dust devil would be in big trouble. The fine dust they carry could get into the joints of a lander鈥檚 robotic arm or an astronaut鈥檚 spacesuit and immobilise it. Grit carried by the strong winds could abrade the surface of a probe鈥檚 solar cells or camera lenses and even the faceplate of an astronaut鈥檚 helmet, making it impossible to see out. High velocity sand particles would damage everything, warns Matthew Balme, a planetary scientist at Arizona State University. 鈥淔or landers or humans, that could be a problem.鈥�
But without getting inside a Martian dust devil, it鈥檚 difficult to tell how serious these dangers are. So the most practical way to learn something about their powers is to create artificial devils in the lab (see 鈥淲e have lift-off鈥�) or to study the Martian monster鈥檚 smaller cousins in the wild, here on Earth.
Last summer, with a grant from NASA, Smith and his colleagues built mobile instrument packages complete with sensors, memory chips and computer controllers, loaded them on a truck and spent a week driving around in the Arizona desert. They headed for the Santa Cruz flats near Eloy, 80 kilometres north of the Mexican border.
The team looked for spots where dry and irrigated fields lie side by side. The soil of the dry fields would be hot and powdery, while the irrigated land would be cooler鈥攖he ideal combination of conditions to spawn devils. 鈥淣ilton is a glider pilot and knows where the dust devils are,鈥� Smith says. Once they spotted one, they鈥檇 drive ahead to the point where they thought it would cross the road. 鈥淭hen we鈥檇 get our instruments ready in the few seconds before the dust devil hit. After a while, we got good enough to get one devil out of two,鈥� says Smith.
The instrument packages they dropped in the devil鈥檚 path were designed to measure temperature, humidity, air pressure, the intensity of electric and magnetic fields, and the wind speed. The vehicle was also equipped with a lidar system鈥攁 laser-based radar. The lidar scans the surrounding area with a laser beam made up of short pulses of infrared light. When the light hits the dust and debris spinning around inside a devil, it is scattered in all directions, and some is picked up by a sensitive detector mounted on top of the laser like a telescopic sight on a gun. By measuring how long the scattered pulses take to reach the detector, the researchers could create an image of the devil that reveals its density and speed.
The team found they didn鈥檛 always need the lidar to warn of an approaching devil because they also whip up an electromagnetic maelstrom on the airwaves. Just as raindrops whirling around inside a thunderstorm become charged by rubbing against one another, so particles of dust zipping around inside a devil create and trap substantial amounts of static electricity. The smallest dust particles tend to collect negative charges and float up towards the top of the column while the larger, positively charged particles drift down. The air between these opposite charges usually acts like an insulator, but whenever these charges exceed the breakdown voltage of the air, a devil crackles with discharges that resemble miniature bolts of lightning. Each discharge releases a burst of electromagnetic energy, which Smith and his team could pick up on their radios.
The strength of the electric fields inside the dust devils on Earth gave the researchers quite a shock鈥攐ne of the beasts even destroyed their voltmeter. 鈥淲e really didn鈥檛 think we鈥檇 break the meter,鈥� says Smith, 鈥渋t鈥檚 designed to measure lightning.鈥� Even fairly small dust devils are able to create fields of up to 10 kilovolts per metre. This is not as much as the largest lightning storm, which can accumulate up to 3 megavolts per metre, but it鈥檚 more than enough to short out electrical equipment and give anyone hit by the discharge quite a jolt.
One of the few people to have any idea what it feels like to be caught inside a dust devil is Wesley Ward, chief scientist in the US Geological Survey鈥檚 astrogeology programme. In May 2000, he donned a spacesuit and walked into a man-made dust devil for a television documentary about climates on other planets.
To create a Martian dust devil in miniature, a special effects company trekked into the Californian desert where they hoisted four large fans 3 metres above a deck with a hole in its centre. The crew arranged a dozen small fans around the hole to blow air into a large vortex. They also brought several hundred kilograms of clay dust, added several bags of crushed red bricks to mimic the distinctive Martian hue, and spread it a few centimetres deep on the floor of the deck.
When the crew turned on the large fans, a column of air was sucked up through the hole in the deck. The small fans made this column spin, creating a red tornado about 2 metres across and almost 4 metres tall.
Then Ward walked slowly towards it. It was a strange experience, he recalls. 鈥淚t moved towards me. We don鈥檛 know whether the attraction was electrical, or related to pressure, but it was weird.鈥� Visibility was slashed, and standing in the centre he felt a slight torque, 鈥渕uch as if someone were trying to steer me by placing a few fingers on my shoulder鈥�.
The first person to notice any electrical effects was the film crew鈥檚 sound engineer, who picked up the clicks of electrical discharges in his headphones. Then Ward began to notice it too. 鈥淚 could feel a slight charge in my fingertips,鈥� he recalls. 鈥淚 don鈥檛 want to use an obvious word like 鈥榮hocking鈥�, but as I held my arms lower in the column I felt a bit of a jolt.鈥�
Since a dust devil鈥檚 charge varies with its size, and also possibly with the kind of material it is carrying, it鈥檚 likely that Martian dust devils could pack quite a punch, says Bill Farrell, an expert in planetary lightning at NASA鈥檚 Goddard Space Flight Center in Maryland. 鈥淚f you have a lot of charge in a storm, you鈥檇 have arc discharges like St Elmo鈥檚 fire鈥濃�攁 luminous electrical discharge that appears on ships or aircraft during a storm. 鈥淭hat could affect your computers and radios and stop you communicating.鈥�
And there鈥檚 an added complication on Mars, he warns. On Earth, we can prevent the build-up of static charge by ensuring electrical equipment and people working with it are properly earthed, so that any charge they accumulate leaks away into the ground. But on Mars we don鈥檛 know whether that鈥檚 possible, because there鈥檚 no water in the soil. An astronaut walking around and getting hit by a dust devil or two could pick up a few thousand volts of static charge. 鈥淲ho knows what would happen when the astronaut comes back and touches the door of the spacecraft,鈥� says Farrell.
Two weeks ago Smith and Renno took their team back to Eloy. Their goal is to create a classification system based on a dust devil鈥檚 lidar profile that predicts the havoc it will cause to anything in its path. 鈥淲e鈥檇 like to be able to read density, pressure, wind speed, and electrical activity and say, 鈥楾his is a class 1 dust devil, there鈥檚 nothing to worry about鈥� or 鈥楾his is a class 5 dust devil, it鈥檒l be here in six minutes, and big trouble is headed your way鈥�,鈥� says Smith.
Whatever they learn should stand them in good stead when they send a package of sensors to Mars鈥攈opefully as part of a NASA mission in 2009, but possibly as early as 2007 if they can piggyback on a European Space Agency mission to the planet.
The eventual aim, Smith says, is to build a small lidar system that would rise above a lander on a short mast. The system would spin slowly, sweeping the landscape to provide an early warning of incoming dust devils. If it spots one, the computer could measure its range and tell which way and how fast it鈥檚 moving. If necessary, he says, astronauts could take cover and robotic landers would automatically close covers designed to protect delicate equipment.
These giants may yet turn out to be gentle, says Smith, but until we know for sure you wouldn鈥檛 want to get trampled by one. 鈥淚 wouldn鈥檛 want to be the first astronaut to go there and have people telling me: 鈥榃e don鈥檛 think they鈥檙e a problem but we really don鈥檛 know鈥�. When the first one is bearing down on you, you鈥檙e the experiment.鈥�
We have lift-off
Since February, engineers at NASA鈥檚 Ames Research Center in California have been creating their own dust devils inside a vacuum chamber 20 metres tall and 15 metres wide, that was once used to test Atlas rockets. Here, the engineers suspend a metal cylinder with a fan at one end above a table covered with sand and dust. Then they pump air out of the chamber and turn on the fan to create a mini-devil. We鈥檙e trying to find out if the dust devils are responsible for the large amount of dust in the Martian atmosphere, says Matthew Balme, a researcher based at Arizona State University in Phoenix.
Researchers suspect that dust in the Martian atmosphere could act like a greenhouse, trapping heat and creating strong convection currents that lift even more dust off the surface. Martian dust devils might even cause the huge dust storms that periodically sweep the planet. Last year, for instance, one dust storm cloaked most of the planet鈥檚 surface from August until October.
So far, the researchers have found that their home-grown dust devils prefer to pick up particles about 100 micrometres across, about the size of sand grains. But dust particles on Mars are much tinier, says Balme, around two micrometres across. It may be that these small particles are clumping together, making it easier for the Martian winds to lift them.