It's Life… Isn't

Home , Christopher McKay, Signs Of LIfe Detector

It’s life… isn’t it? Scientists find it hard enough to pin down evidence of early life on our own planet. How on Earth do we plan to determine whether life exists elsewhere? John Whitfield finds out.

e can, it’s fair to say, be confident their research agendas towards looking for As a control,the landers did the same tests on that there’s life on Earth — but life,says chemist Richard Mathies of the Uni- a heat-sterilized sample of martian soil. ASA/SPL Wproving it is a different matter. In versity of California,Berkeley.“Exobiology is Remarkably, every test gave a positive N December 1990, the Galileo spacecraft becoming more prominent,” he says. NASA result. The soil samples released oxygen and pointed its sensors back at Earth before and the European Space Agency (ESA) are compounds made from ingredients in the setting off for Jupiter. The probe reported moving from rock and mineral experiments nutrient solution, and carbon in the experi- an atmosphere abundant in oxygen and to biological ones, he explains. In 2009, both ment seemed to be incorporated into unusually rich in methane. It also detected a agencies will send landers to Mars that, organic molecules. Unfortunately, most of mysterious pigment that was unlikely to be unlike the Spirit and Opportunity rovers the controls — except the experiments of mineral origin: something earthlings call now on the planet, will be designed to look designed to detect nutrient uptake — also chlorophyll. Yet the astrophysicist Carl for the chemistry of life. gave positive results. At the same time, two Sagan and his colleagues were still cautious Astrobiologists are currently working out instruments designed to analyse the planet’s in their conclusions based on these results. what to look for, designing the instruments chemistry, a gas chromatograph and a mass “Together, these are strongly suggestive of that could detect it, and using the most spectrometer, failed to detect any organic life on Earth,”they wrote1. inhospitable, Mars-like environments on compounds on the surface of Mars2. Galileo, admittedly, was not designed to Earth as training grounds. Even so, these detect life. And it did have a rather distant missions are highly unlikely to give a defini- Hit and miss view of our planet, with only hints from the tive answer. For that, we will have to wait at In sum, these experiments gave us some atmosphere to work out what was going on at least another decade, when planned mis- clues about the chemical environment on the surface. But some dedicated, close-up sions will bring back martian rocks for study Mars, but failed to confirm or refute the examinations have proven no more conclu- in terrestrial laboratories. existence of life. “The experiments all sive. Efforts to detect faint traces of life in The first experiments to look for life on turned out to have some element of ambi- Earth’s oldest rocks, some 4 billion years old, Mars were the two Viking landers of 1976. guity, but that’s utterly understandable,” and in martian rocks that have fallen to Each lander scooped up a spoonful of soil says chemist John Kerridge of the Univer- Earth,have likewise produced results that are from the planet’s surface, moistened it with sity of California, San Diego. Most both ambiguous and disputed; claims of pure, sterile water, incubated it, and watched researchers conclude that inorganic chemi- errors in procedure and interpretation fly for evidence of chemical processes similar to cal reactions, perhaps powered by ultravio- back and forth with regularity. those found in terrestrial microbes — such as let light, must have produced the chemicals Nevertheless, space agencies are shifting the uptake of nutrients or exchange of gases. detected3.A few still argue that the positive

3.5-billion-year-old rocks in Australia are fossilized bacteria or artefacts produced in hot springs has been controversial for several years9,10.Those astrobiologists not actively involved in early-Earth research keep an eye on its developments, hoping that it can pro- vide a blueprint for their field. “The issue of CHMANN/MAGNUM PHOTOS CHMANN/MAGNUM

whether these microfossils have a biological ZA origin or not is so contentious; it really shows P. the need to attack the problem from as many angles as possible,” says Jorge Vago, study scientist on ESA’s 2009 ExoMars project. Both morphological and chemical signals are needed to confirm a claim. But both have their problems. “We’ve not yet identified a smoking gun for life,” says geologist Jack Farmer of Arizona State University,Tempe. Spot the difference: the Atacama Desert (above) is very similar to the martian landscape (left), A rocky road which makes it an ideal place to test equipment The most recent attempt to look for evidence for probing Mars for signs of life. of life on Mars, the ill-fated Beagle 2 lander that presumably crashed on the planet last moon Europa,or Saturn’s moon Titan. Christmas, was equipped to dig into the ASA/R. RESSMEYER/CORBIS Astrobiologists still face the problem high- martian soil, to escape the surface environ- N lighted by Viking: how do you ensure that ment that destroys organic compounds, and ‘evidence’ of life isn’t something produced by study the isotopic ratio of different elements. non-living processes? Such concerns have On Earth, living things preferentially incor- been at the heart of the debate on ALH84001,a porate the lighter form of carbon. The ratios 4.5-billion-year-old martian meteorite found of different isotopes of sulphur, iron, nickel in Antarctica in 1984. A team led by David and chromium have also been proposed as McKay of NASA’s Johnson Space Center in signs of biological activity. Houston, Texas, argued in 1996 that several But, again, it is not clear how geology and features of the rock might have been produced chemistry influence isotope ratios, particu- by living organisms4.These were organic larly in rocks that may be billions of years old. compounds, minerals similar to those pro- Some believe that the carbon in Greenland’s duced by Earth bacteria, and structures ancient rocks has a volcanic, rather than a resembling fossil bacteria. biological origin11,12.Geological processes But in the years following the publica- that transform carbonate minerals into results should be taken as evidence for liv- tion,these claims came under sustained crit- organic matter seem to produce compounds ing microbes. icism. Some features, such as the proposed that, like biological remains, are enriched Ambiguity wasn’t the study’s only prob- microfossils and mineral structures, could with the lighter form of carbon, says geo- lem. Even if Viking had delivered a definite have been produced by non- chemist Mark van Zuilen of ‘no’ to life, this could simply have been living chemistry5,6.Others, such “We’ve not yet the Petrographic and Geochemi- because its instruments were too crude. as the organic chemicals present identified a smoking cal Research Centre in Nancy, “There could have been 10 million bacteria in the rock,may have been added gun for life.” France.“I don’t think that carbon per gram of martian soil,and Viking wouldn’t to it after it arrived on Earth7. — Jack Farmer isotope ratios are that definitive have seen them,” says Andrew Steele of the Few researchers now believe that an indicator of life,”he says. NASA Astrobiology Institute at the Carnegie ALH84001 is good evidence for the presence “We probably don’t know all the ways in Institution of Washington. of past life on Mars. which isotope fractions can come about,” Facing such problems, interest in looking Yet even those who disagree with the ini- adds geologist John Parnell of the University for life on Mars waned after Viking, says tial claims made for ALH84001 are glad of the of Aberdeen, UK.“There may be things going astrobiologist Christopher McKay of the effect the study had on the field.“Exobiology on that we haven’t thought of yet.” Parnell is NASA Ames Research Center in Moffett came out of the closet with that paper,” says leading a UK project to identify biological Field,California.“The experiments were very Mathies.“Suddenly it was OK to be working molecules on the early Earth and Mars. His hard to follow up on,” he says. But there has on this stuff.”Christopher McKay and his col- favoured biomarker is a class of organic mol- been a boom in instrumentation since then leagues also helped to define the categories of ecules called hopanes, which are produced — today’s detectors can spot a single cell, or evidence — organic chemicals, minerals and from the breakdown of simple cell walls.They molecules at concentrations of parts per tril- fossils — necessary to build a convincing case are very long-lived — on Earth they persist lion. And our understanding of the tenacity for martian life, says Steele. “Even if the for billions of years — and we know of no of life in extreme environments has grown. answers were wrong, the approach was good inorganic process that can produce them. On Earth, evidence for bacterial life has been — they raised the bar by several metres.” Mathies and his colleagues, on the other found everywhere from deep oceans devoid Similar, and sometimes acrimonious, hand, believe that a good starting point is of light, to hot springs at temperatures above debates swirl around the evidence for the old- to look for amino acids.“If you look at life on boiling point. By the mid-1990s, biologists est life on Earth. Whether or not an outcrop Earth,50% of the mass of biological materials were once more on a mission to determine of rock in Greenland contains evidence of life is amino acids,” he points out. These mol- whether life could exist elsewhere in the Solar from 3.8 billion years ago is hotly contested8. ecules can be made by inorganic processes System — particularly on Mars, Jupiter’s And the question of whether structures in as well as organic ones, and they have been

while, are both working in the Atacama Desert in Chile.“It’s the only place on Earth where there’s no life at the surface at all,”says McKay. He spent June looking for the residues of organic compounds on desert rocks,and digging holes to see if bacteria live

L. UNIV. PEASE/STOCKHOLM underground there. The researchers work in V. special suits to avoid swamping any microbes in the soil with their own bacteria. Another collaboration, between NASA and the Center for Astrobiology in Madrid, Spain, is looking for life, and testing the technology needed to drill and sample on Mars, in the acidic, mineral-rich Rio Tinto river in southern Spain.The acid is made by bacteria, which metabolize sulphur and produce sul- phuric acid. Carol Stoker of NASA Ames, the project’s leader, was delighted when the Opportunity rover landed on a plain of sul- phate minerals on Mars that likewise seem to be the remains of an acidic sea.“It was a slam dunk,” she says. “The chemistry of the martian surface is exactly like what Rio Tinto is producing now.” ASA/SPL N The researchers plan to found on comets and meteorites, which Animal, vegetable or test an automated drilling might confuse matters. But it is important mineral? Possible signs of platform next spring, not to be too ambitious at first, says Mathies: ancient life in rocks in and also have an instru- after going to the expense of getting to Mars, Greenland (above) and ment, called SOLID you want to give yourself a reasonable chance on the martian meteorite (signs-of-life detector), of finding something. “You want to start ALH84001 (inset) that they hope to get general and get more specific,”he says. remain controversial. on one of the 2009 And there may be more telling evidence missions. available from amino acids — on Earth, all It will also carry an anti- So, despite the occa- biological amino acids have a certain geome- body microarray chip that sional false start and try, whereas those not made by organisms are can detect extant life — blind alley, the astrobiolo- evenly split between two mirror-image con- or contamination by microbes gist’s tool kit and knowledge formations. In space, a bias either way could carried from Earth13. base is becoming ever broader. Most be a good indicator of biology at work. NASA’s effort will be slightly different. researchers are now confident that,if there is Amino-acid detectors are also more sensitive “The MSL is very much a chemical lab: it or has been life elsewhere in the Solar Sys- than isotope measurements, says Mathies. won’t directly look for life, but maybe for the tem,we have the ability to detect it.But to do His team has proposed an instrument for residues or precursors of life,” says Firouz so, we will still have to be lucky enough to NASA’s 2009 Mars Science Laboratory Naderi, head of the Mars programme at the study the right rock in the right place with (MSL) project that could pick this up. agency’s Jet Propulsion Laboratory in the right techniques — and get it safely back Other biomolecules that might be worth Pasadena, California. The MSL is a scouting to Earth.“For a biosignature to be believed, looking for in space include nucleic acids and party for the proposed 2013 Mars sample- it has to be returned for a sample,” says sugars.Most researchers favour a search strat- return mission and a 2016 astrobiology field Kerridge. “Reproducibility convinces hard- egy based on the chemistry of terrestrial life, laboratory; it may even hoard rocks for these nosed scientists.” Even then, don’t bet on but going slightly broader. For example, life landers to collect. everyone being convinced. ■ elsewhere might use different nucleotides or John Whitfield is a science writer based in London. amino acids from life here, or favour a differ- Out on the edge 1. Sagan, C., Thompson, W. R., Carlson, R., Gurnett, D. & Hord, ent geometry or bias of isotopes. But struc- To test all these sensitive instruments, C. Nature 365, 715–721 (1993). ture and patterns in any of these areas would researchers are currently digging through 2. Biemann, K. & Lavoie, J. M. J. Geophys. Res. B 84, 8385–8390 (1979). all be pointers to life.“Complexity is the most the dirt of Earth’s harshest ecosystems. 3. Plumb, R. C., Tantayanon, R., Libby, M. & Xu, W. W. Nature interesting biomarker,”says Steele. Parnell and his colleagues have been look- 338, 633–635 (1989). Of course, no mission would limit itself ing for organic chemicals in the Haughton 4. McKay, D. S. et al. Science 273, 924–930 (1996). to pursuing one particular line of investiga- impact crater, the relic of a past meteor 5. Bradley, J. P., Harvey, R. P. & McSween, H. Y. Jr Nature 390, 454 (1997). tion. Future Mars landers will brandish a impact in the Canadian Arctic, where NASA 6. Barber, D. J. & Scott, E. R. D. Proc. Natl Acad. Sci. USA 99, range of different tests, aided by lab-on-a- tests much of its Mars science and technol- 6556–6561 (2002). chip technology that has made instruments ogy. Steele’s team also spent last summer in 7. Jull, A. J. T., Courtney, C., Jeffrey, D. A.& Beck,J.W.Science 279, much smaller, as well as more sensitive. The the Arctic — this time in Svalbard, where 366–369 (1998). 8. Dalton, R. Nature 429, 688 (2004). ExoMars lander,for example,will carry cam- the rocks share some geological features 9. Brasier, M. D. et al. Nature 416, 76–81 (2002). eras and microscopes, subject rocks to spec- with ALH84001 — testing their life-spot- 10.Schopf, J. W., Kudryavtsev, A. B., Agresti, D. G., Wdowiak, T. J. & troscopic and chromatographic analysis to ting equipment on the local microbes. His Czaja, A. D. Nature 416, 73–76 (2002). gauge their chemical composition and, if team hopes to get a life-detection-chip on 11.Fedo, C. M. & Whitehouse, M. J. Science 296, 1448–1452 (2002). 12.Van Zuilen, M. A., Lepland, A. & Arrhenius, G. Nature 418, they look promising, grind them up to look the ExoMars lander. 627–630 (2002). for amino acids and other organic chemicals. Christopher McKay and Mathies, mean- 13.Clarke, T. Nature 413, 247–248 (2001).