The search for evidence of past or present life

The more we find out about life on Earth, the more likely it seems that life could exist elsewhere. In recent years, microorganisms have been found in the most inhospitable places on Earth, where it previously had been thought nothing could survive. Life has been found thriving close to hydrothermal vents on the sea floor, where temperatures and pressures are high, in saline ponds and in permafrost. Water and energy seem to be the only prerequisites.

There is evidence that water once flowed freely on Mars. Like Earth, Mars has the Sun as an energy source and also had, early in its history at least, its own internally generated heat as well. The odds are therefore favourable that primitive life existed some time during the planet's history. The Mars Express lander, Beagle 2, was designed to hunt for evidence of this life.

One way that Beagle 2 was intended to search for evidence of long-dead or still-living life on Mars was by measuring the abundance ratio between two different types of carbon in rocks. On Earth, biological processes favour the lighter isotope of carbon, carbon-12, over the heavier, carbon-13. A high carbon-12 to carbon-13 ratio has been found in rocks up to four billion years old and is taken as evidence that there was life on Earth when they formed. Finding a high carbon-12 to carbon-13 ratio on Mars would suggest the presence of biological processes there as well.

On Earth, some biological activity produces another characteristic signature - methane. The simplest biological sources, such as those associated with peat bogs, rice fields and ruminant animals, continuously supply fresh gas to replace that destroyed by oxidation. Methane also has a very short lifetime on Mars because of the oxidising nature of the atmosphere, so its presence would indicate the presence of a replenishing source, which might be life. Such a source could be detectable even if it was far from the detector or buried beneath the surface.

The only previous landers to look for evidence of life on Mars were NASA's Viking spacecraft in 1976. They failed to find any life indicators in the soil samples they took from the surface, but it is now thought that the oxidising atmosphere would have destroyed any such signs long ago. Beagle 2 hoped to overcome this problem by taking samples from beneath the surface using a 'mole' to retrieve samples of soil from more than one metre below ground level. It would also have used a corer and grinder to study the interior of rocks.

Many of Beagle 2's experiments were situated in its 'PAW' (position adjustable workbench) at the end of a robotic arm. The PAW experiments included a pair of stereoscopic cameras, two types of spectrometer (Mössbauer and X-ray), a microscope, the corer/grinder and the mole.

Beagle 2 is equipped with a suite of instruments designed to look for evidence of life on Mars.

Shortly after Beagle 2 had landed and set itself up, the arm was intended to deploy and the two stereoscopic cameras were to take panoramic shots of the landing site, followed by close-up images of near-by soil and rocks as potential candidates for further analysis.

When a suitable rock had been chosen, the PAW would have been rotated until the grinder was in position to grind away the weathered surface. The PAW could then be repositioned for the microscope and/or the spectrometers to analyse the freshly exposed material. If a rock looked particularly interesting, a sample could be drilled out with the corer and taken to the gas analysis package (GAP) inside the shell of the lander by means of the robotic arm. The mole was designed to collect soil samples and return them to the GAP in a similar way.

Beagle 2 Lander configuration on the surface of Mars with the PAW instruments.

The gas analysis package

This is where experiments most relevant to detecting past or present life were to be conducted. The instrument was equipped with twelve ovens in which rock or soil samples could be heated gradually in the presence of oxygen. The carbon dioxide generated at each temperature would have been delivered to a mass spectrometer to measure its abundance and determine the ratio of carbon-12 to carbon-13. The mass spectrometer also had the capability of studying other elements and looking for methane in samples of atmosphere. "Different carbon bearing materials combust at different temperatures. At 300-400º C, organic material burns, at 600-700º C, carbonate rocks break down and at higher temperatures, gas trapped in rocks diffuses out. So the temperature at which the carbon is generated also reveals something about its origins," said Ian Wright, who led the GAP team at the Open University, Milton Keynes, United Kingdom.

The environmental sensors

Several tiny sensors were intended to measure different aspects of the Martian environment and so help to determine whether life could have, or could still, exist there. The meteorological sensor package was designed to measure atmospheric pressure, air temperature and wind speed and direction. Other sensors would have helped determine how hospitable the Martian environment is to life by measuring ultra-violet (UV) radiation and oxidising gases such as ozone and hydrogen peroxide in the atmosphere. Dust fall-out rates were also to have been measured, as was the density and pressure of the upper atmosphere during Beagle 2's descent. "It is a tiny package, but a powerful one," said John Zarneki, also from the Open University, United Kingdom. "The UV flux has never before been measured directly on the surface of Mars - but it's very important for life."

The PAW instruments

The PAW instruments.

"The design of the PAW has been a challenge in miniaturization and mass optimisation. It weighs only 2.5 kg, yet will play a crucial role in imaging objects of interest close up, conducting measurements of rocks and soils and supplying the Gas Analysis Package with samples," said Derek Pullan, scientific payload manager.

Two stereoscopic cameras

The cameras' main task was to construct a 3D model of the area within reach of the robotic arm, allowing the other instruments on the PAW to be moved accurately into position alongside target rocks and soil. "As we don't have the telemetry to send images back to Earth in real time (and it would take too long even if we did), we have to operate the PAW using a 3D model of the local area. We can generate the model shortly after Beagle 2 lands, from a few stereo images taken from different angles. Provided the features in the landscape don't move around, it will be valid for the whole mission!" said Andrew Coates, who worked on the camera at the Mullard Space Science Laboratories, University College, London, United Kingdom.

Microscope

The microscope had the ability to pick out features just four thousandths of a millimetre across in rock surfaces exposed by the grinder. This is small enough to pick out features that could be bacteria. A set of light emitting diodes (LEDs) would have illuminated the sample in red, blue, green and UV light. "We'll turn the visible light LEDs on one-by-one to see what the rock looks like in different colours and then combine them to see it in white light," said Nick Thomas, the microscope's principal investigator from the Max Planck Institut für Aeronomie, Lindau, Germany. The microscope would have been able to reveal the shape and size of dust particles, the roughness and texture of rock surfaces and the microscopic structure of rocks. The UV LED was intended to test whether rocks fluoresce. Some inorganic rocks fluoresce naturally. However, another common fluorescent material on Earth is chlorophyll and hence, fluorescence could be an indicator of life. "If a rock's fluorescing, we'll want to get a chunk of it for gas analysis," said Thomas.

Mössbauer spectrometer

The Mössbauer spectrometer was designed to investigate the mineral composition of rocks and soil by irradiating exposed rock and soil surfaces with gamma rays emitted by a radioactive source (cobalt-57), and then measuring the spectrum of the gamma rays reflected back. As the way in which gamma rays are reflected depends on the electronic environment of atoms, this technique can reveal much about how atoms are bound chemically, and hence about the mineral composition of the rocks and soil.

"We will be looking for iron atoms and determining the environment in which they're bound. We'll be able to get information on the different kinds of iron oxide," said Göstar Klingelhöfer, who developed the Mössbauer spectrometer at the Johannes Gutenberg-University, Mainz, Germany. The type of iron oxide depends on the conditions under which it forms, so this information would reveal much about Mars's history.

The Mössbauer spectrometer could also examine the weathered surface of rocks and the oxidation state of the soil to help determine the oxidising nature of the present atmosphere. "If we find exotic iron oxidation states, for example iron with six excess positive charges, we'll have a good idea of what the oxidising species are," said Klingelhöfer.

X-ray spectrometer

The X-ray spectrometer was intended to measure the element composition of rocks by bombarding exposed surfaces with X-rays from four radioactive sources (two iron-55 and two cadmium-109). Under such a bombardment rocks fluoresce and emit lower energy X-rays characteristic of the elements present. "We will measure the percentages by weight of three types of constituent: first, the bulk constituents such as silicon and iron; second the trace elements, such as strontium, which tell us about the rocks' origins and history; and third, we'll measure potassium which will give us the first radioisotope date for Martian rock taken from the surface," said George Fraser, who built the X-ray spectrometer at Leicester University, United Kingdom.

Measurements of potassium were to have been combined with measurements of argon performed by the GAP to date rocks by determining the abundance ratio between the radioactive isotope potassium-40 and its decay product, argon-40.

Mole

The mole (or PLUTO for PLanetary Underground TOol) was designed to collect soil samples for return to the GAP. Using a compressed spring mechanism to propel a drive mass, it would have crawled horizontally across the surface or penetrated vertically below the surface to a maximum distance of 1.5 m. Samples were to be collected in a cavity in the tip and retrieved by reeling in the tether using a winch.

"We will start operations by deploying the mole straight down beneath the surface. The first soil sample will be taken 10-20 cm below the surface and then delivered to the gas analyser. The next sample will be taken 1 m and the third 1.5 m down. Then, depending on what the terrain looks like, we'll do lateral deployment," said Lutz Richter from the Deutsches Zentrum für Luft- und Raumfahrt (DLR - the German centre for aviation and space flight), Cologne, who was responsible for development of the mole.

The mole was intended to travel along the surface until it came to a rock, which it would then proceed to burrow beneath. "We'll be sure that the soil under the rock was sitting there for a billion years or so because a large rock (0.5 m or more in diameter) won't have been moved by any geological processes for at least that length of time," said Richter. Soil without such protection, however, could have been moved around the planet by dust storms much more recently.

Corer/grinder

The corer/grinder was designed to grind the weathered rind off the surface of rocks and then drill down 4 mm to acquire a sample of rock powder for analysis in the GAP. When removing the rind, the corer/grinder would have chiselled across the rock face to remove a 3 mm layer over a small circular patch. When the lateral movement was completed, the corer/grinder would have drilled into the rock.

"The drill head is a clever design, consisting of two parts," said Richter. "When drilling, it generates powder. Once you've reached the drilling depth you can close the drill head to collect the sample." The corer/grinder was intended to collect three or four samples in this way for analysis in the GAP.

Consortium Leader: Prof. Colin Pillinger, Open University, Milton Keynes, UK.

For further information see related links.

SPICAM: UV and IR Atmospheric Spectrometer