Recent years have seen a spectacular renaissance in the search for habitable spaces and traces of life in the universe. This revived interest in astrobiology has been inspired both by the discovery of hundreds of extrasolar planets (C&I, 2010, 6, 22) and new insights into the extreme habitats here on Earth, both suggesting that there may be many more habitable places in the universe than science considered possible until very recently.
In our closer neighbourhood, Venus, with clouds of sulfuric acid and surface temperatures that can melt lead, can safely be ruled out of the competition, but Mars remains, despite numerous studies suggesting it is too hostile an environment to support life. This is in part due to the findings of the exploration rover Opportunity, currently in its ninth year of activity on the Red Planet, which has supplied planetologists with unprecedented details, including clear evidence of past liquid water.
Now a new rover mission is about to land on Mars, with a much larger scientific payload and more ambitious aims. For the first time since 1977, when the twin Viking landers obtained ambiguous results from their biology experiments, which were eventually interpreted as negative, a probe will specifically look for chemical traces of life on Mars. The Mars Science Laboratory (MSL) rover, renamed Curiosity, carries the most sophisticated suite of analytical instruments sent to space so far. So what can we expect to get back, once Curiosity has safely landed?
Views of Mars
First, we will see pictures of those faraway landscapes that none of us is likely to visit. Curiosity carries three imaging systems. One, the Mars Descent Imager (MARDI), will film the terrain of the landing site from above to help the team back at mission control both in making sure the landing goes according to plan and in their subsequent decision-making when they send the rover roaming around.
The second is the mast camera, Mastcam, which, as the name suggests, is on the lookout from an elevated position on top of the rover, taking still images and videos of the rover’s immediate surroundings. The Mastcam equipment actually includes two cameras, a moderate resolution and a high resolution one, along with various filters to produce images at specific wavelengths. It also comes with its own memory to store images until they can be beamed back to Earth. Expect spectacular videos, as James Cameron, director of Titanic and the third person to reach the bottom of the Mariana Trench, is officially associated with this imaging system as a mission scientist.
The third and final imaging system is a close-up camera designed to do the work that human geologists would normally do with a hand lens, for example, inspecting minerals in detail to assess their textures and structures, and identify their composition. This instrument, the Mars Hand Lens Imager (MAHLI), can resolve details down to 12.5μm. It carries its own light sources, enabling it to shine white or ultraviolet light at its targets. Thus it can work during the Martian night, and it can identify minerals based on fluorescence produced by UV irradiation. The main goal of the MAHLI team is to get deeper insights into Martian geological history from close-up analysis of rock samples.
In addition to the four cameras in the three imaging systems, there is a long lens camera, the ChemCam. Designed to analyse surfaces that the rover can’t reach from a distance of up to 7m, this newly-developed instrument can fire a laser beam at such surfaces and vaporise the exposed material to analyse its composition spectroscopically. The laser and camera telescope are installed on the mast of the rover, but light from the camera is transferred to the processing unit in the body of the probe via fibre optics, for spectral analysis. The ChemCam can rapidly identify the nature of rocks and analyse their elemental composition, detect water both as ice and within the crystal structure of minerals, assess the degree and depth of weathering on exposed rock surfaces, and provide help in selecting samples for other analytical tools.
Analytical prowess on Mars
Along with the imaging and remote analysis work, Curiosity will also do some traditional ‘dirty work’ to analyse the soil it finds in more detail. Like any terrestrial geologist on a field trip, it carries a drill to access deeper layers of rocks and a scoop to collect samples of dust and loose soil. It can sieve the collected materials to obtain finely powdered samples for analysis. The Chemistry and Mineralogy instrument (CheMin) will use X-ray diffraction and fluorescence to analyse the crystal structures and elemental composition found in such powdered samples.
The largest part of the scientific payload is the Sample Analysis at Mars (SAM) suite of instruments, which includes a mass spectrometer, gas chromatograph, and tunable laser spectrometer. Mainly, the SAM suite will be used to look for carbon compounds, including methane, which has already been detected remotely by the European Space Agency’s Mars Express Orbiter. NASA researchers hope to work out where that methane comes from – with current knowledge they cannot exclude that it might be of biological origin.
‘The prospect of methane in Mars’ atmosphere is a very exciting one,’ says Lewis Dartnell, an astrobiologist at University College London. ‘Much of the methane in Earth’s air is produced biologically. Also, since methane is scrubbed out of the atmosphere pretty quickly by the action of sunlight, if there is methane present on Mars, something must have released it recently, if not still producing it.’
However, Dartnell isn’t quite convinced yet that the results from the orbiter are 100% certain. ‘The problem is that the measurements so far have been right on the limit of detection, and so we’re not sure if there is actually any Martian methane at all,’ he cautions. ‘This is hopefully what Curiosity will be able to resolve, and more importantly, may be able to find evidence that the methane was generated by microbial life.’
The SAM suite will also look for other light elements associated with life, including hydrogen, oxygen, and nitrogen, and analyse their abundance and isotope ratios.
The Russian Federal Space Agency has contributed an instrument to the mission, namely the Dynamic Albedo of Neutrons (DAN) tool. It is based on the observation that neutrons colliding with light nuclei – which, given the paucity of helium, lithium and beryllium in the planetary crusts, essentially means hydrogen – pass on much of their kinetic energy to the atom and are slowed down. By contrast, on collision with heavier nuclei they bounce elastically, thus retaining their kinetic energy.
The DAN instrument will generate neutron beams and direct them at the ground. Neutrons will penetrate 1–2m deep and scatter as they collide with atoms. Measuring the kinetic energy of neutrons reflected back at the probe, the DAN instrument will be able to detect hydrogen atoms – almost certainly in the form of water molecules – even if the water only occurs at an abundance of 0.1 %. Water ice is plentiful at the Martian poles, but its distribution at lower latitudes is yet unclear. Even liquid water may still exist underground, thanks to geological heat and pressure. Such locations, if found, might be habitats where microbial life could have survived on the Red Planet.
Probing the environment
Finally, the remaining scientific instruments in Curiosity’s arsenal serve to analyse the environmental conditions at its landing site. The Radiation Assessment Detector (RAD) is unique in that its analyses are specifically designed to prepare for future visits by human explorers.
RAD will measure the radiation that astronauts will be exposed to when they walk around on Mars. As the Martian atmosphere is much thinner than ours and the planet lacks both a magnetic shield – like Earth’s Van Allen belt – and an ozone layer, the surface is exposed to hard UV light, γ-rays and high energy particles, including protons, neutrons, and ions. Measuring the radiation will enable scientists to get a better idea of how humans would have to protect themselves in future explorations of Mars. It will also help astrobiologists to understand how and where microbial life on Mars may have survived the grilling from space.
‘The cosmic radiation bombarding down onto the unprotected Martian surface is one of the main hazards to dormant Martian microbes near the surface, or the preservation of signs that it was ever there, and is unlike any natural environment on Earth,’ says Dartnell, who investigates the effects of such radiation in his own research at UCL. ‘Curiosity will be able to measure this radiation flux on the surface for the first time – we’ve measured it from Martian orbit in the past, and ironically this detector instrument was wiped out by a pulse of radiation from the sun – and this is enormously important for working out how best to shield our astronauts when we send them to explore the dusty plains, or for the prospects of native Martian life.’
Another instrument that gathers information for future missions is the MSL Entry Descent and Landing Instrument (MEDLI), which measures technical data during the approach and landing with a view to making future missions safer and more efficient.
Finally, Spain’s Centro de Astrobiología (CAB) in Madrid, a joint research centre of the Consejo Superior de Investigaciones Cientifícas (CSIC) and the Instituto Nacional de Técnica Aeroespacial (INTA), has contributed a weather station for the mission, the Rover Environmental Monitoring Station (REMS). This station will provide daily and seasonal reports on atmospheric pressure, humidity, ultraviolet radiation at the Martian surface, wind speed and direction, air temperature, and ground temperature in Curiosity’s surroundings.
The search for life on Mars has only just begun.
Mars on Earth
Spanish researchers from the Centro de Astrobiología in Madrid have tested a life-detecting technology designed for Mars at one of the most Mars-like locations available – Chile’s Atacama desert – and successfully discovered life on Earth.
Astrobiologist Victor Parro, together with Chilean colleagues, carried out a ‘Mars-like’ drilling project in the desert and analysed several samples from depths of up to 5m with a Life Detector Chip (LDChip) carrying 300 different antibodies against various kinds of biomolecules (Astrobiology, 2011, 11, 969).
They found a hypersaline habitat 2m below ground, populated by bacteria and archaea. Hygroscopic salts, including halite and traces of perchlorate, attract moisture even in the extremely dry environment of the Chilean desert and provide thin water layers on which the microbes can thrive. The researchers also found nutrients, including organic acids, sulfate, and nitrates. Parro and colleagues believe that the findings, which they later confirmed with detailed laboratory studies, validate their instrument, known as SOLID, as a tool for future life-searching missions to Mars. ‘If there are similar microbes on Mars or remains in similar conditions to the ones we have found in Atacama, we could detect them with instruments like SOLID,’ Parro said.
Michael Gross is a science writer based at Oxford, UK