SHAFAQNA (Shia International News Association) — THE flying-saucer-shaped probe hurtles through space, firing thrusters and flinging off weights to point its heat shield forwards for the scorching trip through Mars's atmosphere. Once it has slowed, the heat shield drops away and, 10 kilometres above the surface, a parachute billows out. More weights are jettisoned, pointing the craft's radar-tipped belly towards the fast-approaching ground.
The probe cuts loose its parachute and upper shell, then fires thrusters towards the ground, slowing its speed further to 3 kilometres per hour. Hovering 20 metres above the Martian surface, the probe starts to unspool an SUV-sized rover from its belly. Dangling on cables like a giant spider, the payload is gently lowered by a "sky crane" to the surface. Curiosity has landed.
So begins a slick animation of NASA's newest rover, which is due to blast off on 25 November. The manoeuvres are so precise in their choreography and timing that their successful implementation seems improbable - especially given that Mars is the Bermuda Triangle of the solar system, dooming to failure two-thirds of the missions that attempt to visit it. "Everything has to behave according to plan," admits mission leader John Grotzinger of the California Institute of Technology in Pasadena.
The stakes could barely be higher. The $2.5 billion rover will be the most ambitious and expensive mission ever sent to Mars, carrying state-of-the-art tools that will reveal whether its landing site was ever habitable and search for signs of life preserved in its rocks. The first rover to be powered by the radioactive decay of plutonium rather than sunlight, Curiosity will be able to work around-the-clock and through the Martian winter.
"It's going to be a huge step forward," says Steve Squyres at Cornell University in Ithaca, New York, lead scientist for the Mars Exploration Rovers Spirit and Opportunity, which landed on different sides of the planet in 2004.
But first Curiosity has to touch down safely. "Anytime you land on Mars, it's a slightly scary thing," says Squyres. Both Spirit and Opportunity were swaddled in airbags and bounced to a landing, but Curiosity weighs five times as much and would simply punch through airbags as if they weren't there. "We pushed the airbag technology about as far as you could push it," says Squyres. "The sky crane system is a good engineering solution to the problem."
Developing the gargantuan mission, which is also known as Mars Science Laboratory, has not always been smooth. Engineering troubles - including problems with motor-driven gears called actuators - forced a two-year launch delay and added millions of dollars to its already outsized budget. "This has been a trial," admits Jack Mustard of Brown University in Providence, Rhode Island. "But if NASA can pull this off, you've got a demonstrated landing system for large masses, which will be important for the next phase of landing on Mars - bringing samples back."
The ballet of moves that precedes the sky crane's deployment is also an advance. Tilting the probe by jettisoning weights gives it much greater aerodynamic control, enabling the target landing site to be an ellipse just 20 kilometres long - one-seventh of that needed for Spirit and Opportunity. "When you have a larger ellipse, you rule out all the most interesting places," says Grotzinger. That's because larger footprints are more likely to include steep slopes or fields of boulders - terrain too dangerous to risk landing on. "This is the first time in the history of the exploration of Mars where we have been unencumbered by engineering constraints to really debate landing site options," he says.
After years of consideration, the winning site finally emerged in July: the 150-kilometre-wide Gale crater near the Martian equator, whose floor plunges 5 kilometres below the surrounding surface. "It's a pretty deep hole in the ground," says Ralph Milliken at the University of Notre Dame in Indiana. Bizarrely, a mound of rocks within it rises up about as high as the crater walls.
How did such a giant structure form? Geologists are divided, although they are certain water was involved in some way. That's because rocks at the bottom of the mountain are made of layers of clays and sulphate salts, both of which need water to form. Orbiting spacecraft have dated the formation of these rocks to about 3.5 billion years ago.
"If any place had a lake on Mars, Gale would," says David Blake of NASA's Ames Research Center in Moffett Field, California. Rain, snow or rising groundwater might have pooled in the crater, and the clays and sulphates might have been left behind when the water evaporated.
With Curiosity, NASA is going beyond its previous aim to "follow the water". Barely a week seems to go by without instruments on the Mars Reconnaissance Orbiter and Mars Express probe beaming back evidence for past or even present water. No one doubts any more that liquid water once featured on the Red Planet.
"We've kind of beaten that horse to death," says Milliken. "We know there was plenty of water during certain parts of Mars's history," he adds, referring to large water-carved channels and valleys thought to have formed in the planet's early history. "Now it's a matter of trying to understand which of those environments had water for the longest time, and was it the right pH? We're asking much more difficult - and also much more informed - questions."
Curiosity will be able to provide better answers than any previous mission. Spirit and Opportunity could only detect specific elements in rocks, such as iron, by measuring the spectrum of light they reflect. Curiosity will do this sort of elemental chemistry too, but in a more Jedi-like fashion: zapping rocks with a laser from up to 7 metres away and studying the spectrum of light emitted by the ionised rock vapour. If it spots elements of interest, it will approach and use a drill on its robotic arm to collect rock samples from as deep as 5 centimetres.
Next it will perform a feat that has so far only been done on Earth - identifying the specific minerals, such as iron sulphate or magnesium sulphate, in rock samples. The pulverised rock will be placed into an instrument called CheMin, which will shoot X-rays at it and study the resulting diffraction patterns. Just like a fingerprint, each of the 7000 or so minerals on Earth has a unique X-ray diffraction signature. "CheMin will tell you all the minerals that are present and how much of each mineral there is," says Blake, lead scientist for the instrument. That provides big clues about the temperature, pressure, acidity and other conditions in which the minerals formed, he says.
So CheMin could reveal whether Gale crater was just right for life. But did it actually host it? Detecting unambiguous signs of life is fraught with difficulty (see "Why isn't NASA hunting for life?"). So Curiosity will do its best to answer the question by looking for organic molecules. These complex carbon-containing molecules do not necessarily signal life - they float in the harsh environment of interstellar space, for example - but they do form the building blocks of life as we know it. Instruments on-board the rover will be able to detect organic molecules at concentrations of just 40 parts per billion. "It's not a direct sign of life," says Grotzinger, "but if it had been there, we might see organic compounds preserved."
Finding them won't be easy, as other missions have shown. Organic molecules should be raining down on Mars regularly on meteorites, yet none were found by NASA's twin Viking landers in 1976 or by its Phoenix probe in 2008. That may be because it's very easy to destroy them. Ancient organic matter trapped in rock might have been destroyed by water flowing through the rock, which would have split apart the organic molecules and produced carbon dioxide gas. If the rocks were below the surface, the heat of the planet may also have destroyed anything organic, and if they were on the surface, cosmic rays or oxidising chemicals like hydrogen peroxide could have severed the molecular ties. "Detecting organic compounds is more than a needle in a haystack," says Grotzinger.
Curiosity has a better chance of success, in part because of its landing site. On Earth, organic molecules tend to be trapped and preserved in fine particles. That makes landing next to clays at the base of Gale crater's enormous mountain so promising.
Shortly after it has landed, the rover will begin to climb the mound. As it ascends, it will also encounter other water-related features where life may have found a toehold, including channels that might once have held water. The diversity of these features was a draw to landing at Gale, says Grotzinger: "We get to study not just one but several potentially habitable environments."
Finding any type of organic molecule would count as a huge success. Yet the rover's instruments could give us more telling hints of life, including a toolkit called SAM, which stands for sample analysis at Mars. Of most interest to researchers studying the possibility of carbon-based life on Mars is that SAM can measure the relative abundance of the isotopes carbon-12 and carbon-13. Life on Earth prefers to use the lighter isotope, so "if you measure more light carbon, it's at least consistent with the hypothesis that biology could have been involved", says Grotzinger. However, he cautions that the natural isotopic abundances might be different on Mars than on Earth, which would complicate the analysis.
In addition, SAM will investigate the chirality, or handedness, of organic molecules to look for hints that they came from life. Many molecules come in left and right-handed versions, and non-biological processes tend to create these in equal numbers. "In the case of life, you don't have the same ratio," says Michel Cabane, a SAM instrument leader at the Pierre and Marie Curie University in Paris, France. "If there is not the same quantity of left and right molecules, we can begin to postulate a biological source."
For any microbes living in Gale crater, August 2012 will be unlike any other month. Their tranquil lives will be shattered by the appearance of a flying saucer from another planet and a curious six-wheeled plutonium-powered beast trundling across the landscape. It will truly be an alien sight.
Why isn't NASA hunting for life?
Even the most ardent fans of the Red Planet must occasionally wish for more than just hints of water popping up in ever-new places. So why not send a robot to hunt directly for little green men?
One word: Viking. NASA's Viking landers did just that in 1976, laying out a tasty solution of nutrients to attract any microbes that might be living in a soil sample, like cookies left on a plate for Santa. The nutrients were laced with radioactive carbon, so if the solution was digested, a radiation monitor above the sample would detect the resulting gas.
Intriguingly, radioactive carbon was detected, but then another experiment found no evidence of organic compounds in the soil - there were no alien bodies. "They were hoping to find signs of life but the results came back basically negative - there is no life as we know it," says Ralph Milliken at the University of Notre Dame in Indiana. The US did not send another mission to Mars for 20 years.
The $2.5 billion Curiosity rover will hunt for organic molecules and isotopic hints of life, but NASA is still shying away from the L word. "NASA cannot say to taxpayers that they put $2.5 to $3 billion to search for life, and then say, 'We have found no life - thank you, bye bye'," says Michel Cabane, leader of one of Curiosity's organics-sniffing instruments, who is based at the Pierre and Marie Curie University in Paris, France.
"If you project the message that you are hunting for life, even though it is very important to many of us, and you return with a null or ambiguous answer, people would be disappointed," says Jack Mustard of Brown University in Providence, Rhode Island, who is a former chair of NASA's advisory panel on Mars.
In any case, he and others say the problem may simply be too hard to solve. "If I posed the question 'prove that life existed in Earth's past' to you, it would be tough," Mustard says. "Geologists would say, we'll go find a fossil. But bodies are not always preserved on Earth."
He points out that Curiosity and other missions that touch down on the planet are only exploring a limited region for a limited time. Bethany Ehlmann at the California Institute of Technology in Pasadena agrees. "Think locally, not globally - that's a slight perversion of what the environmental movement thinks we should do here on Earth," she says.
Curiosity's landing site may once have been a lake, but other intriguing sites suggest life might have found a refuge in hydrothermal springs below the surface. "Environments during the first billion years of Mars history varied substantially," she says. "Now that we know there's this diversity out there, it becomes harder to say that the evidence says 'Mars did not have life'."—www.shafaqna.com/english
Source: New Scientist