Phoenix Special > The Role of MECA, TEGA and the Robotic Arm

We are now less than a month away from the arrival of Phoenix at the Martian arctic plains. Excitement surrounding this unique mission has been mounting steadily since its launch last August, when I last wrote on Phoenix, and at long last the wait is almost over. You can already read on spaceEurope the thoughts of many of the key players intimately involved in the design of the probe and its operation on the frosty martian surface over the coming months, and what I’d like to give here is an insight into what astrobiologists like myself are hoping for.

Although Phoenix is not designed to detect signs of life itself, it will carry several instruments to Mars for the first time and provide crucial insight into whether life as we know it could ever survive in this environment – whether there is a potential subsurface habitable zone. Three major factors on the potential habitability of a site are the availability of liquid water, the presence of carbon and organic molecules, and access to energy sources. Phoenix is also the first landing probe to be sent far from the martian equator, and so will be exploring a truly unique environment: the martian arctic plains.

Robotic probes are our emissaries, sent forth bristling with instruments to explore strange lands – a remote extension of our own arm-reach and senses. For Phoenix, the science payload makes up about 8% of the total mass of the lander at launch. Which instruments aboard Phoenix promise to provide some truly exciting results on the habitability of the red planet? Here is the spaceEurope guide to the eyes and ears (and microscopes, electrochemistry analyzers and mass spectrometers!) of Phoenix.

Robotic Arm
Phoenix, like its machine cousins the Mars Exploration Rovers Spirit and Opportunity and Beagle 2, comes equipped with a robotic arm able to reach out and position certain tools. These include a powered rasp for breaking up the frozen soil and a close-up camera. Most importantly, however, Phoenix will be the first probe since the Viking landers in the 1970s to be able to dig beneath the martian surface. Phoenix’s robotic arm is 2.35 m long, complete with an elbow joint in the middle, and will be able to scrape a trench up to half a meter down into the frozen soil.

It is expected that in the arctic plains where Phoenix is landing there is permafrost water very close to the surface, and the great hope is that the probe will be able to dig deep enough to find this ice layer. Once the arm reaches this ice, it will use a powered rasp to extract samples and then reach back to deliver them to the analysis instruments on top of Phoenix. This will constitute the first direct detection of water on the planet Mars – previous missions have only observed water remotely from high above in orbit, or inferred its prior presence from the geochemistry and morphology of ancient minerals.

The robotic arm will therefore be penetrating beneath the UV-sterilised surface of Mars and potentially coming into direct contact with underground water ice and the subsurface habitable zone. For this reason, it is especially important that the robotic arm is as clean and sterile of earthly microbial life or even organic molecules as is reasonably possible. To achieve this, Phoenix’s robotic arm was sealed before launch in an impenetrable bag – a biobarrier – then thoroughly sterilized through heat treatment. This biobarrier will not be opened again until after Phoenix has landed on Mars.

This effort is part of the precautions of Planetary Protection – that is, trying to prevent the forward contamination of extraterrestrial locations we visit with microbes from Earth. Most parts of the Phoenix mission, such as the bulk of the lander, the parachute, the entry shell, and so on, have been sterilized to a level of no more than 300 bacterial spores per square meter. The robotic arm has been cleansed to much stricter standards, allowing no more than a single spore per square meter of surface area. This is because the arm is the only part of the mission expected to penetrate down to the potentially-habitable ice layer in the martian ground. There remains the risk, however, of some form of failure during the descent sequence and Phoenix dumping itself at high speed into the martian subsurface, allowing even the less-thoroughly sterilized components into contact with the habitable zone.

TEGA – Thermal and Evolved Gas Analyzer
TEGA will study substances within soil samples collected by the scooping arm that are vaporised when heated in one of eight tiny ovens. These whiffs of gas from the martian soil will be passed over a mass spectrometer able to determine the presence of different molecules, like a chemical nose. TEGA will be used to provide information on the presence of water and carbon dioxide ices in the martian arctic soil. The most exciting application of TEGA, however, at least for astrobiologists, will be in the first detection of organic molecules on Mars. The mass spectrometer will be able to identify, and also quantify, any organics vaporized out of the soil samples. The presence of organics in the martian subsurface is a crucial factor in its potentially habitability for extraterrestrial life.

TEGA was designed and built by research teams at the University of Arizona and University of Texas, with William Boynton as the Principle Investigator. When I caught up with Prof. Boynton last month he told me that he “hopes TEGA will find both the ice beneath the surface and that it contains significant amounts of organic compounds, the sorts of molecules required by life”. As has already been mentioned, Mars is bathed in lethal levels of ultraviolet radiation from the Sun and this is though to have destroyed any organic molecules lying too close to the surface. “We are hopeful that the ice layer may protect the organic compounds from the oxidizing layer hypothesized to be on the martian surface based on the Viking lander results,” he said. Prof. Boynton is also PI on the Gamma Ray Spectrometer (GRS) instrument that is still flying aboard the Mars Odyssey orbiter. It was this GRS, along with a related instrument, which detected the first evidence of water ice lying very close to the surface near the martian north and south poles. As Boynton explains, “we found great amounts of water ice in the ground where Phoenix will be landing, and hopefully close enough to the surface that the probe will be able to scoop it up. But even more importantly for astrobiology, this ice may experience periods of thawing.”

One necessary design constraint with the TEGA is that it is strictly limitted in the number of different soil samples it can analyse. The instrument has only eight single-shot ovens – eight chances at detecting something ground-breaking within the soil samples. The mission controllers are going to need to be very careful in using these shots as wisely as possible as the robotic arm gouges deeper and deeper into the subsurface.

MECA – Microscopy, Electrochemistry and Conductivity Analyzer
MECA is another instrument that Phoenix will use to examine the samples of soil scooped up by the robotic arm. MECA is made-up of several related pieces of equipment, but for astrobiology the most exciting is the wet chemistry laboratory. This experiment will stir warm water into the handfuls of martian grit and analyse the kinds and concentrations of the chemicals that dissolve out of this muddy sample. Like TEGA, though, MECA is severely limited in the number of times it can be used – there is space for just four once-only teacup-sized beakers. This means that Phoenix will only be able to use MECA on a single sample from the martian surface, and then from three further depths as the robotic arm digs underground.

Around the inner surface of these analysis cups are various sensors for different chemical species. Some of these will test the pH of the soil – the concentration of hydrogen ions, and therefore how acidic or alkaline the arctic environment is. The pH is a very important determinant of how well life can survive in different potential niches.
Other sensors will measure the concentration of chloride, bromide, magnesium, calcium and potassium ions, and thus how salty the water is. On Earth, salinity is again a major factor for which forms of life can survive. Much life on Earth survives on chemical energy, running redox reactions to power themselves. MECA’s wet chemistry lab will also be able to identify potential energy sources in the martian arctic, such as soluble sulphate minerals.

Assessing the acidity and salinity of the martian ground is crucial in determining its ability to host microbial life. Hardy lifeforms are known on Earth that can survive extremes of these physical parameters, but combinations can be especially challenging. Andrew Knoll is the Fisher Professor of Natural History at Harvard University and works with the Mars rovers Spirit and Opportunity. Prof. Knoll recently discussed with me the latest discoveries these explorers have made about the likely ancient martian environment and how it affects the chances of life. He explained that while the water at Meridiani plains was very acidic, there are terrestrial microorganisms that can tolerate comparable pH levels. “But more recently we have been able to work out the saltiness of the water, and it seems that as the waters percolated through Meridiani sediments they became steadily more saline until most, if not all, terrestrial organisms would be challenged to survive,” he says.

Phoenix will be able to tell us much more about the environmental chemistry and potential habitability of a very different region in the martian arctic. Watch this space as the results come in!

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