快猫短视频

Editorial: 鈥�We need a piece of Mars to continue search for life鈥�

REPORTERS and camera crew made sure they got to the conference centre in San Francisco early on 3 December 2012. They were expecting historic news from Mars. Speculation was rife: what exactly had NASA鈥檚 newest and biggest rover found? Curiosity鈥檚 project scientist John Grotzinger had two weeks earlier. He was explaining how the rover had analysed its first scoops of Martian soil from a pile of windswept sand, then added: 鈥淭his data is going to be one for the history books. It鈥檚 looking really good.鈥�

That could only mean one thing, right? Signs of life! Now Grotzinger and others faced a packed pressroom at the to announce鈥� almost nothing. Yes, Curiosity had technically detected organic molecules, a prerequisite for life on the Red Planet. But no, the science team emphasised, this was not significant 鈥� the organics were probably from Earth and this was just a successful first test that showed Curiosity鈥檚 instruments were working properly. To those who had devoted nearly a decade of work to this project, this was exciting news. But it was hardly the earth-shaking headline that the reporters had been expecting.

Fast forward several weeks, however, and Curiosity鈥檚 first soil sample turns out to be a big deal after all. It changes our ideas about the Martian surface. And it adds yet another twist in the decades-long quest to find signs of past, or even present, biological activity on Mars.

Curiosity is more than just a camera on wheels. It carries by far the most sophisticated suite of scientific instruments ever sent to another world. As well as a wide variety of imaging instruments that can take pictures at many different wavelengths and at ranges from the microscopic to telescopic, it can sample air and has a scoop and a drill for obtaining samples of soil and rock. These can then be analysed in cutting-edge on-board chemical laboratories to determine the mineralogy, chemistry, molecular compounds and even the isotopic ratios of elemental constituents.

Of all these instruments, perhaps the most advanced is a cluster of devices called SAM, short for . It takes rock samples and vaporises, burns or mixes them with liquid compounds in order to detect some organic molecules that otherwise could not be identified.

Organic molecules are indeed what SAM found when it baked its first few samples of soil close to Curiosity鈥檚 landing site. The team soon realised, however, that these molecules were not from Mars. They almost certainly came from the reaction of a chemical containing carbon and hydrogen called MTBSTFA that was brought along from Earth. SAM carries nine phials of the stuff to mix with Martian soil in order to identify much more complex organic materials, such as amino acids.

One of the phials had sprung a leak. 鈥淭his wasn鈥檛 a case of some subtle bit of cleaning fluid or some residual chemical,鈥� says , a planetary scientist at NASA鈥檚 Ames Research Center in Moffett Field, California, and a member of the SAM team. 鈥淭his was like a big spill in the pantry.鈥�

Viking controversy

That might sound like bad news, but the results have brought an unexpected benefit. They have revealed clear-cut evidence for perchlorates, a highly reactive chlorine compound, in the Martian soil.

To understand why perchlorates are so important, we need to turn the clock back to one of the most controversial chapters in Martian exploration history. In 1976, NASA鈥檚 twin , touched down at two locations on Mars (see diagram) and began a series of experiments. Each lander carried three different life-detection tests and a device called a gas chromatograph mass spectrometer for detecting different kinds of molecules in the soil. When the spectrometers analysed heated soil samples, they sometimes detected bursts of oxygen and carbon dioxide. This suggested the presence of organic molecules in the Martian soil, which were being broken down on heating.

At the time, though, the results were interpreted as the result of terrestrial contamination 鈥� perhaps from solvents used to clean the equipment. Not everyone accepted that view: the oxygen and CO₂ appeared only when Mars soil was present in the test chamber and not during control runs with the chamber empty. But the contaminant explanation stuck.

Two researchers who have challenged that view are McKay and his colleague Rafael Navarro-Gonzalez of the National Autonomous University of Mexico. In a paper published in 2010, they argued that the , and could be down to highly reactive soil chemistry 鈥� specifically perchlorates acting on organic compounds in the soil.

Their view was based on experiments they carried out on soil samples collected over the past decade from the arid, lifeless Atacama desert in Chile, probably the closest we can get to Martian-like soil here on Earth. When McKay and Navarro-Gonzalez added magnesium perchlorate to soil samples and heated them, they found that nearly all of the organic compounds present in the soil reacted to form water, CO₂ and chloride compounds. So the presence of perchlorates on Mars could explain the Viking results.

What鈥檚 more, they could up the stakes. The apparent absence of organics has always been the primary argument against one of Viking鈥檚 contentious life-detection experiments, known as the labelled release test. It involved taking a pinch of Martian soil and adding a nutrient solution containing the kinds of chemicals that terrestrial organisms would consume. The carbon in these nutrients was radioactive carbon-14, so if any microbial life devoured the nutrients the CO₂ they exhaled would be radioactive too. Sure enough, the labelled release experiments on both Vikings detected radioactive CO₂. And the reaction appeared to stop when the soil was heated to temperatures that would kill any microbes.

Although these results met all the criteria that the Viking science team had agreed as positive signs of living organisms before the mission, the apparent lack of organic material in the soil was baffling. If life was present in the soil, then there should be an abundance of organic compounds left over from the decay of billions of dead microbes. But the lack of organics 鈥� or at least the dismissal of the organic results 鈥� was taken to mean that life could not have been present, despite the apparently positive results from the labelled release test.

McKay and Navarro-Gonzalez鈥檚 conclusions about Viking have been harshly criticised, even ridiculed, by some other researchers. However, further backing for their idea comes from NASA鈥檚 Phoenix lander, which found perchlorates in the soil at its landing site near the Martian north pole. Even so, most scientists thought that perchlorates were unlikely to be widespread, and so there would be little chance of them turning up where Curiosity and Vikings 1 and 2 landed.

Now Curiosity has proved them wrong thanks to the spill of organic chemicals in the SAM instrument. Without it, McKay says that the amount of perchlorate in the soil would be too small to detect. The sheer size of the leak meant that the perchlorates reacted strongly with the organic compounds. 鈥淲e saw chlorine compounds up the wazoo,鈥� says McKay. 鈥淭he presence of the organics was a huge benefit.鈥�

Why this perchlorate compound should be so widespread on Mars remains a mystery. 鈥淚t was unexpected, and it鈥檚 unexplained,鈥� says McKay. 鈥淥n Earth, it exists, but it鈥檚 rare, even in the Atacama desert.鈥�

McKay and Navarro-Gonzalez go on to speculate that organic compounds are widespread on Mars, too. That鈥檚 not the same as saying there are signs of life on the Red Planet. Each year more than 1000 tonnes of organic material lands on Mars in the form of meteorites carrying compounds rich in carbon and hydrogen. What is not clear is whether it remains intact or is destroyed by ultraviolet radiation, cosmic rays and chemical reactions in the soil.

Having found perchlorates, finding organics on Mars could force a reassessment of those old Viking tests. In particular, we would have to take seriously the idea that the Vikings really did find signs of life. Any such interpretation, however, will remain controversial until new results are in.

Seeking organics on Mars is one of the SAM instrument鈥檚 most important goals. Unfortunately the spillage has heavily contaminated its soil analysis equipment. But McKay remains optimistic about its future.

Early days

First, there are ongoing efforts to figure out ways of cleaning up the mess. A combination of pumping gas through SAM鈥檚 test chambers and heating them will probably do the trick. And if that doesn鈥檛 work, chances are the team will be able to do just fine by simply compensating for it in their data. 鈥淚t will increase the 鈥榥oise鈥� level a bit,鈥� says McKay, 鈥渂ut if the level of organics in the soil is as high as we predict, we should see that. It鈥檚 not a fatal problem. We just have to correct for it.鈥�

Bear in mind, too, that Curiosity鈥檚 mission has barely begun. Its potential for returning stunning new findings from the Red Planet remains mostly untapped. Steve Squyres leads the science team studying the findings from the previous generation of Martian rovers, . 鈥淭he first really significant discovery we made from Spirit came 800 days into what had been planned as our 90-day mission,鈥� he points out. Curiosity鈥檚 mission is planned for at least two years, and in all likelihood will last much longer than that. 鈥淚t鈥檚 probably going to outlive many of us on the team,鈥� Squyres says.

鈥淐uriosity鈥檚 mission has barely begun. Its potential for returning stunning new findings from the Red Planet remains untapped鈥�

As the rover works its way across Gale crater toward the towering slopes of its primary destination, Aeolis Mons, known as Mount Sharp, it will traverse areas where deposits of clay minerals have already been clearly detected from orbit. Such minerals could be the most interesting target for SAM鈥檚 analysis, because clays usually form in very water-rich environments, which may have been conducive to the development of living organisms in the early eras of Martian climate.

What鈥檚 more, clays are particularly well suited to preserving organic compounds over aeons of time, as their layers can protect organic matter. If there are significant amounts of organic material buried in the soils of Mars, Curiosity stands a very good chance of finding them 鈥� though its arrival at Mount Sharp could be many months away.

McKay suggests that on the wish list of scientists interested in whether there is life on other worlds, there are a series of increasingly exciting possibilities for what Curiosity may be able to detect. Any organic molecules at all would be attention grabbing, but some could be positively mesmerising.

For instance, among the clays there might be compounds of special biological significance, such as amino acids 鈥� among the basic building blocks of life. Some complex biologically interesting molecules like amino acids are hard to detect in the kind of gas chromatograph equipment in SAM鈥檚 lab. That鈥檚 where the eight remaining phials of the spilled chemical MTBSTFA come into play. (If you really want to know, its full name is .) By deliberately adding that to the soil, instead of spilling it inadvertently, it would bind with amino acids and make it possible to detect them.

Vital signs

Finding amino acids on Mars would still not be evidence of living organisms: they have already been found in some meteorites. Even so, discovering them or other biologically significant molecules would go a long way toward showing that, at the very least, all of the necessary ingredients for the formation of life must have been present.

But there鈥檚 one more possible finding, just within the capabilities of Curiosity鈥檚 instruments, that could be the most exciting of all. Living organisms have one characteristic behaviour that very clearly distinguishes their metabolic activity from any known purely chemical process. The molecules of many organic compounds and amino acids are chiral, meaning that they are not identical to their mirror image. Living organisms generally process one of the forms, but not the other.

In principle, says McKay, Curiosity has one instrument that should be able to discriminate between different chiral forms of such molecules 鈥� though that capability has not been proven in practice. If it can, we have a long wait ahead. The results will take weeks, if not months, to analyse. 鈥淭he key thing is to recognise the complexity of this mission, and the slow but steady pace that is going to take place,鈥� says Squyres. 鈥淚t鈥檚 an exciting mission, but it unfolds in an excruciatingly slow way.鈥�

Such results could be well worth waiting for. If Curiosity were to find a collection of amino acids with one prevailing chiral form, that would be a very strong indicator that they were the products of living, breathing organisms at some time in Mars鈥檚 past. It would be powerful evidence, for the first time, of life on another world. Now, that really would be one for the history books.

This article appeared in print under the headline 鈥淪pills and thrills鈥�