Mars rocks

With a terrestrial symmetry, Christmas and New Year should be a busy period on Mars, too. After six years without a single visitor, the red planet is due to receive three robotic spacecraft from Earth within a month of one another. Beagle 2, a lander from the European Space Agency (ESA), is scheduled to arrive first, on 26 December, followed by two rovers from Nasa, Spirit, on 4 January, and Opportunity, three weeks later. 

The three robotic explorers, which took off six months ago, are equipped with optical and chemical sensors to probe the Martian environment for signs of life as we know it. They will search for evidence that water once flowed on Mars and therefore that life existed, or even continues to exist, somewhere on this seemingly inhospitable planet. 

Space spectrometers

European teams in Germany and the UK developed the two types of spectrometer, Mössbauer and X-ray, that each craft will use to analyse the composition of rocks and soil. On board Beagle 2 there is also a ’Tardis-like’ gas processor that reflects the lander’s central mission to detect organic material; the main task of Spirit and Opportunity is to assess whether conditions on Mars ever favoured life. 

The catalyst for this flurry of missions to Mars is that over the summer Mars and Earth came within 35m miles of each other, the closest the two planets have been for around 60,000 years. Taking advantage of the relatively short distance, ESA launched its Mars Express mission on 2 June 2003. After releasing Beagle 2, the Mars Express spacecraft will orbit Mars and provide data on the planet’s atmosphere, structure and geology (see Box Analytical Instruments below). Nasa’s Mars Exploration mission also took advantage of the red planet’s proximity: Spirit was launched on 10 June 2003 and Opportunity followed on 7 July 2003. Nasa already has two spacecraft orbiting Mars: Global Surveyor (since September 1997) and Odyssey (since October 2001), which have provided a great of deal of information about the planet’s geology and composition. 

Indeed, says Nasa, these two spacecrafts’ observations have provided much of the evidence that water may once have flowed over the surface of Mars. 

Global Surveyor has identified many geographical features that could be ancient river beds or lakes, while Odyssey has uncovered evidence that large quantities of frozen water still exist near Mars’ two poles. 

Information provided by the orbiters was also used by Nasa scientists to identify prime landing sites for Spirit and Opportunity. These sites are 6000 miles apart, on opposite sides of Mars, but both contain evidence suggesting that liquid water may once have been present. 

Landing sites

Spirit will land in the 150km diameter Gusev Crater, which is at the end of a 900km meandering valley, known as Ma’adim Vallis. This valley possesses many of the visual hallmarks of erosion by running water and may once have been host to a river, meaning that the Gusev Crater may once have been a giant lake. 

Opportunity will land in the Meridiani Planum, which is one of the smoothest and flattest places on Mars. Its appeal as a landing site comes from the detection by Global Surveyor of high levels of gray haematite, an iron oxide mineral, in the soil. On Earth, gray haematite is often formed as a precipitate of iron-rich water. 

Beagle 2 will land at Isidis Planitia, a flat sedimentary basin near the Martian equator. ESA scientists chose this site primarily because it looks to be a fairly safe place to land - not too rocky and at a low enough elevation that Beagle 2’s parachutes will have time to slow its descent. Also, its surface seems to be fairly young, meaning that it could potentially have retained any traces of life. 

Although the ESA and Nasa missions overlap somewhat in terms of timing and scientific objectives, their precise natures are very different. The Nasa rovers will be much more mobile than the Beagle 2 lander. Each rover is about the size of a sit-down lawnmower and is expected to travel up to 40m every Martian day (which is around 40 minutes longer than an Earth day). Based on images taken by each rover’s panoramic camera and thermal emission spectrometer, which will be able to detect carbonates, silicates and organic molecules, Nasa scientists will identify interesting-looking rocks for the rovers to approach and study. Both rovers will operate for around three months. 

Detection sites

The Beagle 2 lander is essentially static and restricted to studying the area where it lands. However, it does have an instrument called a mole, which will be able to crawl horizontally for up to 1.5m to collect samples of rocks and soil. The mole will also be able to drill below the surface of Mars and take samples from soil that may have remained undisturbed for hundreds of millions of years. Beagle 2 will operate for around six months. 

Despite the difference in mobility, Beagle 2 and the two rovers possess many of the same instruments, including optical cameras, microscopes and abrasion tools to expose fresh rock. They also possess two different kinds of spectrometer, Mössbauer and X-ray, to analyse in detail the composition of Martian rocks and soil. On Beagle 2, the spectrometers are two of the instruments that make up the Paw (Position Adjustable Workbench, although it also looks like a paw), a 2.5kg extendable device that also contains two stereo cameras, a microscope, the mole and a corer and grinder. On Spirit and Opportunity, the two spectrometers are on an extendible arm, so that they can be placed close to the rock that is being studied. 

The Mössbauer spectrometers on the rovers and Beagle 2 were developed by researchers from the Johannes Gutenberg University in Mainz, Germany. They work by irradiating rock and soil with γ-rays emitted by a radioactive cobalt source. By measuring the spectra of the γ-rays reflected by a rock, specifically the Doppler shift in the velocity of the rays, the spectrometers can identify the different kinds of iron oxides that are present. The exact combination of iron oxides found in the rock will depend on the environmental conditions to which it has been exposed. 

The X-ray spectrometer on Beagle 2 has been developed by researchers at the University of Leicester. It will bombard exposed rock with X-rays from four radioactive sources (two iron and two cadmium). The energy levels of the X-rays re-emitted by the rock will provide information on the abundance of both the major and minor rock-forming elements, such as magnesium, silicon, sulphur and strontium. These data will help to identify the rocks and provide information on their origins, history and age. 

The rovers’ X-ray spectrometers, which were developed by researchers at the Max Planck Institute for Chemistry in Mainz, Germany, will perform the same function. However, they will bombard exposed rock with both X-rays and α-particles emitted by a radioactive source of curium. In alpha mode, the spectrometer will be able to detect lighter elements, such as carbon and oxygen. 

X-ray results

The results from the X-ray spectrometers will also help to interpret and constrain the findings from the Mössbauer spectrometers (and the thermal emission spectrometer on the rovers). Using these spectrometers, ESA and Nasa scientists are hoping to detect minerals that contain water or that may have been deposited by precipitation, evaporation or hydrothermal activity, such as phyllosilicates, iron oxyhydroxides, zeolites, halides, and hydrated sulfates and carbonates. The presence of such minerals would be good evidence that water once flowed over that area of Mars. 

The fact that Beagle 2 and the two rovers contain many of the same sensors and instruments is not surprising; they are operating in the same environment and are trying to answer many of the same questions. However, the detection of past or present life on Mars is much more central to the Beagle 2 mission than to the rovers’, whose main task is to detect whether water once flowed on Mars and from this to determine whether conditions may once have been favourable for life. Beagle 2, unlike the rovers, therefore contains an instrument whose prime purpose is to detect evidence of past or present life, making it the first Mars mission with this objective since the Nasa Viking landers in 1976 (see Box right). 

Gas analysis

The instrument charged with detecting ’life’ on Beagle 2 is the Gas Analysis Package (Gap), which is housed in the centre of Beagle 2’s main body. The Gap mainly consists of 12 sample ovens and a mass spectrometer, but also incorporates two ion pumps, two chemical pumps, a gas processing manifold, several solid-state chemical reactors, a few tanks of reference gases and variable-volume bellows. All of these instruments are contained within a wedge-shaped space 25cm long and 15cm deep. ’This space is like the inside of Dr Who’s Tardis’, jokes Ian Wright of the Open University’s Planetary and Space Sciences Research Institute, who leads the Gap development team. 

The Gap will gradually heat samples of Martian soil and rock collected by the mole and use its mass spectrometer to identify and analyse any gases that are emitted, particularly carbon dioxide. ’We will be on the lookout for the presence of non-equilibrium gases (such as methane) and also differences in isotope ratios (fractionations) that may imply the action of processes that could have involved Martian biota’, explains Wright. 

On Earth, biological processes preferentially favour the use of 12C, the lighter of the two isotopes of carbon, rather than 13C. If the Gap detects an excess of 12C in carbon dioxide released from the stepped combustion of Martian soil samples, then this would be good evidence for the presence of biological material in the soil. In addition, the ratio of the two isotopes could give an indication of the specific biological process, such as photosynthesis or methanogenesis (production of methane by biological processes), that caused the fractionation. 

Martian atmosphere 
Methane should be quickly broken down by the aggressively oxidising nature of the Mars atmosphere (the red surface of Mars is caused by the extensive oxidation of iron in the soil, making the whole planet rusty). The Gap spectrometer will also be used to analyse the Martian atmosphere; if it detects significant amounts of methane then something, possibly microbes, must continuously be releasing the gas into the atmosphere. 

Whatever Beagle 2, Spirit and Opportunity detect it won’t be conclusive proof for the presence or absence of Martians (unless the rovers run one over) or whether water once flowed over the surface. Any data will probably be open to a number of interpretations and may even be contradictory. Nevertheless, the missions should greatly enhance our understanding of Mars and its history. 

Source: Chemistry in Britain

Jon Evans