Chemistry on Mars: using innovative analytical techniques, Canadian scientists are gaining insight into the chemical makeup of a world beyond our own.
Laboratories in Space
In 1976, NASA's Viking landers carried out the most famous--or infamous -chemistry experiments done on Mars. They scooped up a sample of soil then added a solution of amino acids and other nutrient molecules that were radiolabelled with [sup.14]C. After a few hours, the sample gave off C[O.sub.2] that was also radiolabelled with [sup.14]C, an indication that the molecules from Earth were being broken down by something in the soil, possibly something alive.
But the excitement wasn't to last. The addition of more organic molecules a week later failed to elicit another spike in C[O.sub.2], something that would have been expected if the purported life forms had multiplied. Today most scientists believe that inorganic salts in the Martian soil--possibly superoxides that contain [O.sub.2]- ions--were responsible for the observed oxidation, not biological action.
Still, missions like Viking did provide indisputable evidence that Mars once had flowing liquid water, both from satellite images of dried-out riverbeds and the detection of minerals like silicates, which can only form by crystallizing out of a liquid medium. Later missions like the Mars Exploration Rovers (MER)--known to the public as Spirit and Opportunity--were built around this finding. "The theme for MER was 'follow the water,' " says Ralf Gellert, an associate professor of physics at the University of Guelph who still works on the MER missions.
Gellert's job was to build an instrument that could probe the elemental composition of Martian rocks to see whether they were formed in or altered by water. One way to do this is with laser spectrometers, which fire a short pulse at a spot of less than a millimetre diameter, up to seven metres away. This excites the least-bound electrons; when those electrons return to their ground state, they give off observable light at specific frequencies. But there are other, complimentary methods. "Ultraviolet or visible light is quite weak, so it only kicks out the outer, least-bound electrons," Gellert says. "What we do is strip the inner electrons out of the atom, by getting close to the sample and hitting it with either an alpha particle or an X-ray. Compared to laser spectroscopy, we measure a penny-sized spot--more representative of the whole rock--with higher accuracy and very high sensitivity for many important geological elements like sulphur, chlorine, nickel and zinc."
On Earth, creating high-energy alpha particles or X-rays can take room-sized pieces of equipment. Gellert's alpha-particle X-ray spectrometer (APXS) does the same thing, but it's only the size of a pop can. The secret is curium-244 (244Cu), which naturally gives off all the X-rays and alpha particles you need. True, it takes a little longer than in the lab, but time is not a problem when you're working on another planet.
Spirit and Opportunity landed on Mars in 2004; the latter is still sending back data from the Meridiani Planum, a vast plain near the Martian equator. The results indicate past water, but not the kind you'd like to swim in. "The sulphate component [in the bedrock] is something like 30 weight per cent and very homogeneous," says Gellert. Those sulphates likely precipitated out when a body of very acidic water dried up. Although there are terrestrial organisms called extremophiles that can survive under those kinds of conditions, they would be toxic to most life.
The latest version of the APXS--the one now being wielded by Curiosity --is smaller, faster and more sophisticated and was also designed by Gellert. This robot headed to a very different region, the Gale crater, which straddles the border between Mars' mountainous south and the relatively flat and low-lying north. Satellite images and infrared spectra suggested this site would be interesting, but what Curiosity has found so far has surpassed expectations. "Our first rock was a complete surprise: a high-potassium, high-feldspar rock that no rover has yet seen anywhere," Gellert says. "The diversity of what we've seen so far is quite high."
This diversity makes it hard to summarize the current results from Curiosity, but there are a few general patterns. "The bedrock on Meridiani was chock full of sulphur; the bedrock we've seen so far at Gale is very low in sulphur," says Gellert. Yet it still contains minerals like clays, which indicate a liquid origin. Currently, the thinking is that this part of Gale crater was once filled with water, which laid down the sediments that Curiosity is looking at. However, this water was much more neutral than what once covered the Meridiani plains. "Very likely, this was earlier in Martian history, about 3.7 billion years ago," says Gellert. "Everything that's so acidic likely happened later on." If that's true, it means that Mars wasn't always as harsh and inhospitable as it is now, or seems to have been in its recent past. "When you have near-neutral water, it's a different kind of habitability, and that's what Curiosity found evidence for," says Gellert.
Messengers from Mars
Space laboratories like MSL provide incredible insight, but they don't come cheap: the total budget of the mission is estimated at $2.5 billion. A more cost-effective method might be to wait for Mars to come to us, which is not as impossible as it sounds. Shergottites are a type of meteorite named after Shergotty, India, where the first example was recovered in 1865. These space rocks contain glassy inclusions which themselves hold small pockets of gas. By carefully extracting those pockets and running them through gas chromatography columns, scientists have established that they all contain the same ratio of gaseous chemical species: about 95 per cent C[O.sub.2] with a little bit of nitrogen, argon and other trace gases, including about 210 parts per million of water vapour. The composition is an exact match to that of the Martian atmosphere, which proves that shergottites are pieces of Mars knocked off by cosmic impacts and later captured by Earth's gravity. More than 60 confirmed Martian meteorites (mostly shergottites) have been found on Earth.
Earlier this year, a shergottite called NWA 5298 sat under a high-precision field emission scanning electron microscope (FE-SEM) in Desmond Moser's lab at Western University. "The beam tip is one nanometre," says Moser, an earth scientist specializing in geochronology. "It can go up to half a million times magnification." Moser and his team need this kind of precision to hunt their elusive quarry: tiny crystals of mineral called baddeleyite (Zr[O.sub.2]), a rare zirconium oxide that provides clues about Mars' past.
Most rocks are dated using uranium isotopes that decay into lead over millions of years at a known rate. Measuring the ratio of uranium to lead levels can provide information about when the rock formed, but this relies on assumptions about how much uranium and lead was in the rock to begin with. Baddeleyites are supremely useful in that they exclude lead from their crystal structure, but they can include uranium. This means if a baddeleyite crystal contains any lead, it can only have come from the decay of uranium, thus the mineral provides a more accurate clock.
In a recently published paper in Nature, Moser's team showed that the baddeleyite crystals in NWA 5298 must have crystallized from molten rock only 200 million years ago, a fairly recent date compared to other estimates of up to four billion years for Martian shergottites. "This means the giant volcanoes that we see on the surface today were active at that time," says Moser. Such findings indicate that Mars was recently--and indeed may still be--more geologically dynamic than it seems.
Meteoric messengers also provide tantalizing clues to the possibility of past, present or future life forms. In 1996, a meteorite known as Allan Hills 84001 made headlines when it was suggested that tiny bumps on carbonate globules observed by electron microscopy were fossilized microbes, although most scientists now believe these to be abiotic--physical rather than biological--in origin. On the other hand, simple carbon-based molecules similar to those created by terrestrial life have been found inside many Martian meteorites, and were the subject of a paper published in Science last year.
The investigators used a technique called Raman spectroscopy, which uses laser light to excite molecules either individually or in small groups, causing vibrations. This vibrational energy is then released as light at other frequencies. Detecting this light allows investigators to distinguish between chemical species, including organic molecules. Confocal Raman spectroscopy allows researchers to determine the 3D location of those species with great accuracy. "That's important because it allowed us to see that these carbonaceous materials were actually inside the rock, and not something that was stuck on the surface," says Chris Herd, a professor of Earth and Atmospheric Sciences at the University of Alberta and co-author of the study.
In the meteorites, Herd and his colleagues detected organic ring-bearing structures like polycyclic aromatic hydrocarbons (PAHs). On Earth, these are usually associated with oil and bitumen, which have a biological origin. However, PAHs are also known to form abiotically as solar radiation interacts with space dust, which means they could have been present in the rocks that initially came together to form Mars. Eons later, such molecules could have been expelled onto the surface in the form of lava. "When a magma crystallizes, the very first crystals that form will trap little bits of the magma in them; these are called magmatic inclusions," says Herd. In the paper, the researchers show that all the organic carbon found in the Martian meteorites were within these inclusions. This means they had to be present in the fundamental building blocks of Mars, not deposited later by biological processes.
The holy grail of Mars exploration would be a sample-return mission, what Gellert calls "a rover with a rocket in its backpack." Such a mission could return a specific, well-documented, in-context sample--unlike the random samples returned by meteorites. But there's a catch. "You would need to carry so much fuel with you that you wouldn't be able to take off," says Janusz Kozinski, a materials scientist and founding dean of the Lassonde School of Engineering at York University.
A few years ago, Kozinski worked on a project with the European Space Agency. The goal was to see if any of the elements naturally present on Mars could be used as fuel for a possible return trip. "There is perhaps 10 times more aluminum and magnesium on Mars than there is on Earth," says Kozinski. "We proposed a set of experiments looking into the possibility of mixing aluminum and magnesium from the Martian crust with CO2 from the Martian atmosphere, in order to initiate ignition and then sustain it," Kozinski says. The results, published in Proceedings of the Combustion Institute, were intriguing but not earth shattering. Aluminum did indeed oxidize in C[O.sub.2], especially when the particle size was small, but not at rates that would sustain an Earth-bound rocket.
Still, experiments like Kozinksi's represent the kind of creative thinking that could bring us to new heights in Mars exploration. Planning for future missions to Mars is already underway, including NASA's InSight project, set for 2016, which will drill deep into the Martian crust. On this mission, as with previous work, chemical techniques will play a vital role in pushing back the frontiers of knowledge.
Caption: NASA's Mars Science Laboratory rover, also known as Curiosity, is unearthing evidence that the Red Planet could have supported microbial life.
Caption: Euriosity's two-metre-long robotic arm contains a variety of tools including cameras, a drill, a soil scoop and an alpha particle X-ray spectrophotometer (APXS), visible here as a can-shaped device located at the 10 o'clock position. Designed and built by Canadian scientist Rail Gellert, the APXS uses radioactive curium-244 to produce both alpha particles and X-rays that probe the elemental composition of Martian rocks.
Caption: Northwest Africa (NWA) 5298 is a Martian shergottite meteorite discovered in Morocco and acquired by the Royal Ontario Museum in 2008. Inside, tiny crystals of the mineral baddeleyite act like a geological clock. By examining the ratio of uranium to lead within these crystals, scientists have shown that the rock crystallized from a lava flow on the surface of Mars about 200 million years ago, much more recently than many researchers thought. This important new finding tells us that Mars has been volcanically active for most of its existence.
Please note: Illustration(s) are not available due to copyright restrictions.
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|Publication:||Canadian Chemical News|
|Date:||Sep 1, 2013|
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