Mars: Dead or Alive?

Recent missions to Mars have focused on the search for water, past or present, as a surrogate for life itself. But now a British-led team is working to renew the search for life directly, fueled by doubts about the equipment that prompted NASA to declare Mars a dead world some 26 years ago.

If all goes according to plan, a Soyuz-Fregat booster rocket will lift off from Baikonur cosmodrome next month carrying an extremely compact and sophisticated life detection probe that might finally settle one of the most intriguing questions in science: did Mars once harbor microbial life—and is it still there?

The probe is hitching a ride on the European Space Agency's (ESA's) Mars Express orbiter as part of the agency's first home- grown mission to the Red Planet. Named Beagle 2, in honor of the HMS Beagle in which Charles Darwin made the historic voyage of discovery that led him to the theory of evolution, it was designed by scientists from Britain's University of Leicester and Open University in collaboration with Martin-Baker Aircraft and Matra Marconi Space Systems. Once the orbiter reaches Mars, Beagle 2 will be sent down to dig around on the planet's surface.

But even after it has dropped off its passenger, the Mars Express orbiter will not be idle. It will use a sounding radar called Marsis to search below the surface for water. It will have an ultraviolet and infrared spectrometer called Spicam to study the atmosphere over the course of a Martian year. And it will relay data transmitted from the lander back to Earth.

Did Viking get it wrong?

The first spacecraft with dedicated equipment to look for life on Mars were NASA's twin Viking landers, which touched down on the surface in 1976. Why send another now?

On board both Viking landers were miniature life detection laboratories, and some of the data they returned could indeed be interpreted as evidence for life on Mars. Yet the majority of the project's scientists became convinced that inorganic oxidants in the soil were responsible for the ambiguous data. The next year, NASA publicly announced its conclusion: that Viking had found no life.

Was the U.S. agency jumping to conclusions? In recent years, questions have been raised about the effectiveness of a key instrument—a combined gas chromatograph and mass spectrometer (GCMS)—that swayed most of the Viking scientists into the no-life camp. The GCMS failed to detect any organic molecules on the Martian surface at all, which posed something of a puzzle, as even the barren surface of the moon is host to some organic molecules. To explain the anomaly, scientists postulated a harsh chemical environment that supposedly made the planet self-sterilizing by actively destroying organic matter [see "Why NASA Said No to Life on Mars"].

To find out if this picture is correct, Beagle 2 is designed to search for organic material below, as well as on, the surface of Mars. In addition, it will study the inorganic chemistry and mineralogy of the landing site, says Mark Sims, the Beagle 2 mission manager who is based at Leicester University.

Without question, the Beagle 2 lander manifests an enormous leap of scientific engineering. It costs only US $40 million versus Viking's $1 billion, and weighs in at a mere 60 kg at launch, as opposed to 661 kg for each fully fueled Viking lander. In its set of scientific instruments are the first ever optical microscope to fly to Mars, as well as a gas analysis package (GAP) that will directly challenge or confirm the results of Viking's gas chromatograph-mass spectrometer (GCMS).

Beagle 2's destination on Mars is a region known as Isidis Planitia [see map]. This relatively flat basin may have been formed by sedimentary deposits and was chosen not just for the chances of finding life there but with a view to the safety of the lander as well. A rocket engine will not be used for a soft landing. Instead, like NASA's Mars Pathfinder mission in 1997, it will use parachutes, along with a system of airbags to absorb the shock of landing.

There are differences between the Beagle 2 and Mars Pathfinder approaches, however. "Mars Pathfinder had a series of 24 interconnecting airbag spheres, whereas the Beagle 2 will only have three," says Colin Pillinger, the lander's project scientist at the Open University, headquartered in Milton Keynes, England.

On-site workbench

The Beagle 2's primary science mission will last 180 Martian days, or sols (a sol is about 37 minutes longer than an Earth day). The probe will stay put wherever it lands, but will be able to obtain and analyze samples of rock and soil from within a 75-cm radius, thanks to a 2.5-kg robotic arm known as the position-adjustable workbench (PAW). Although some of the Beagle 2's instruments are housed in the lander's body, such as the GAP, several are located on the adjustable workbench itself [see "A Miniature Marvel"].

The PAW is the brainchild of a multidisciplinary science team from the Space Research Center of the University of Leicester led by Mark Sims. The original idea was for the robotic arm to pick up and use different instruments in turn, but "now it carries an entire science instrument module, with six instruments as well as a brush, scoop, and wide-angle mirror," says Derek Pullan, the lander's instrument manager. Discrete electronic interfaces between each instrument and the lander would have been complex to build and heavy as well. So the PAW uses a single interface with a field-programmable gate array that can reconfigure itself to match each instrument's needs.

Stereo cameras built into the PAW will help researchers identify suitable soil and rock samples. They will obtain a number of overlapping stereo images of the landing site, from which a computer back on Earth will construct a three-dimensional representation of the site, called a digital elevation model. Pullan explains that "although the [model] is useful for creating 3-D images of the landing site, it becomes particularly important when...we wish to maneuver the PAW arm. The 3-D model... is used to plan our close-up experiments on rocks and soils."

At 0.75 milliradian per pixel, the stereo cameras have 1.3 times the resolution of those on board Mars Pathfinder, according to Andrew Griffiths, the stereo camera project manager. Normally they focus from 1.2 meters to infinity, but one camera is also equipped with a close-up lens for inspecting objects only 10 cm distant.

Close-up viewing

The optical microscope aboard Beagle 2 allows even closer visual inspections. Without supplementary lighting, though, samples placed in front of it will be less than adequately lit because Mars receives only 43 percent as much sunlight as Earth. Consequently, the instrument is equipped with 12 light-emitting diodes (LEDs)—three each of red, green, and blue, plus three that emit ultraviolet light. UV might cause any organisms present to fluoresce, but in any case UV fluorescence is crucial in the identification of some minerals.

Focusing the microscope is done by moving the whole thing back and forth. Its 4-8-µm resolution is too coarse to distinguish individual bacteria but could detect biofilms, the layers of slime produced by many bacterial colonies, if they were present.

Beagle 2's microscope is largely based on an optical instrument that was designed for the canceled NASA 2001 Mars Lander, explains Peter Smith, a co-investigator on the experiment, who comes from the Lunar and Planetary Laboratory of the University of Arizona (Tucson) and is the only U.S. scientist working on the mission. Smith was the lead scientist on the Mars Pathfinder lander camera in 1997. He is supplying the lens and LEDs for Beagle 2's microscope. Surprisingly, his work is not being funded by NASA or the European Space Agency, but by a grant from the University of Arizona Foundation.

Once the microscope is positioned by the PAW in front of a soil or rock sample, a monochrome charge-coupled device camera will digitally image the sample through the microscope lens. To create color images, it will use the red, green, and blue LEDs in sequence to illuminate samples with each color in turn. True color images can later be constructed on Earth by superimposing the separate images.

To obtain that soil or rock sample, the PAW is also equipped with a set of digging, drilling, and grinding tools. A subsurface soil sampler called Pluto (PLanetary Underground TOol) will dig to a depth of about 2 meters, whereas the Viking lander robotic arm could scoop no deeper than 20 cm. The heart of the Pluto device is a soil penetrator and sampler called the Mole [see illustration, next page]. Mounted in a metal tube and powered through a tether attached to Pluto, the Mole can burrow into Martian soil or rock and capture a 0.24-cm³ sample. After crawling out at most 3 meters in any direction, the Mole can be reeled back into the PAW and positioned over a funnel to release its samples for analysis in the body of the Beagle 2.

The decision to include a grinder on the Beagle 2 mission followed on the failure of the Mars Pathfinder Rover's X-ray spectrometer to positively identify rock samples through their coating of Martian dust. The PAW's rock corer-grinder and the Mole's sampling tip were designed by a Hong Kong dentist and precision toolmaker named TC Ng [see "A Dentist's Drill on Mars?"]. Any dust adhering to rock surfaces is removed by the corer-grinder, whose rotating chipping bit produces a flat surface 30 mm in diameter. The X-ray and Mossbauer spectrometers built into the PAW can then be positioned to analyze the pristine rock surface.

Derek Pullan explains that following Mars Pathfinder's difficulties in analyzing samples,"we knew we had to clean rock surfaces if we wanted to obtain good data about rock composition." Otherwise, dust coatings can obscure the chemical and mineralogical signature of the underlying rock.

The corer-grinder can also sample the interior of rocks, using a hammering and rotating action to obtain cores about half a centimeter long. No previous mission to the planet has obtained a sample from within a Martian rock. Although NASA is sending similar drilling equipment along with its 2003 Mars Rover mission [see "Drilling Holes in Mars," Spectrum, April 2002, p. 22], it won't land on Mars until January 2004, whereas the Beagle 2 will arrive around Christmas this year, giving the British probe another first.

Life's lighter signature

Apart from the possibility that the microscope will provide direct visual evidence of biofilms or fossils on Mars, the most promising instrument for confirming life's isotopic signature is the gas analysis package. This instrument is three times as sensitive as the Viking's GCMS. If it detects any organic molecules in the soil or atmosphere, whether biological in origin or not, it will compel a significant reworking of our understanding of the Red Planet's chemistry.

Among the gases the package can detect are hydrogen, nitrogen, oxygen, and carbon dioxide. It uses a method known as stepped combustion, in which gas is incrementally extracted from samples over a series of temperature intervals and passed into a mass spectrometer.

The underlying principle is that a living creature's metabolism leaves isotopic signatures in four kinds of materials: organic carbon, carbonates, sulfates, and sulfides. Metabolic reactions that process these substances utilize lighter isotopes than do nonliving processes. The relative amounts of isotopes measured in a sample can then be compared with those in surrounding inorganic rocks found by the Beagle 2 spectrometers—an increase in lighter isotopes in the sample relative to the inorganic rocks would offer evidence of life on Mars.

The gas analyzer will operate in one of two ways: either by direct atmospheric sampling, or by using samples gathered by the subsurface soil sampler, Pluto, and fed into one of 12 sample ovens mounted on a rotating carousel. Whatever form the carbon takes, the package will be able to measure it in nanogram quantities.

Beagle 2 will also carry environmental sensors to measure UV radiation, temperature, atmospheric pressure, wind speed and direction, and the momentum and amount of atmospheric dust.

Vying for the prize

If Beagle 2 does find evidence for either extant or extinct life on Mars, will that mean the British can claim they found it first? NASA has insisted for nearly 26 years that the Viking landers found no life on Mars, but within the last two years has been reevaluating both the GCMS findings and the Viking biology experiment known as labeled release.

Labeled release used a radioactive isotope, carbon-14, to test for metabolic activity of micro-organisms in the soil. Nine soil samples were moistened with a nutrient labeled with the isotope. Each sample was then incubated for up to 10 days, time enough for any micro-organisms present to consume the nutrient and produce gases detectable by the rise in radioactivity level.

"The results from our experiment on Mars looked very much like results we obtained in our Earth tests with California Death Valley soils" and their cargo of bacteria, says Gilbert Levin, principal investigator for the Viking labeled release experiment. "Had the [GCMS] detected organic molecules, NASA might have announced the discovery of life on Mars 25 years ago."

Is NASA publicly re-evaluating its Viking data in preparation for what the Beagle 2 might find in 2004? Clearly, if the British lander does find life on Mars, a scientific symposium will have to be convened to sort out who may have discovered it first: NASA or ESA.