24 CAnAdiAn ChemiCAl news www.cheminst.ca/magazine

Ch
24 CAnadian Chemical News www.cheminst.ca/magazine
chemistry | mars exploration
hemistry
on
Mars
Using innovative analytical techniques, Canadian
scientists are gaining insight into the chemical
makeup of a world beyond our own.
By Tyler Irving
NASA/JPL-Caltech
NASA’s Mars Science Laboratory rover, also known as Curiosity, is unearthing
evidence that the Red Planet could have supported microbial life.
September | october 2013 CAnadian Chemical News 25
chemistry | mars exploration
J
ust over a year ago, the biggest
Mars space exploration vehicle,
or rover, yet built was gently
lowered from a rocket-powered
‘sky crane’ onto the surface of the
red planet. Known to the public as
Curiosity, the rover’s proper name
is Mars Science Laboratory (MSL).
Built by the National Aeronautics and
Space Administration (NASA), the
rover fairly bristles with equipment
for doing analytical chemistry: a gas
chromatograph, a mass spectrometer, a
laser-induced breakdown spectrometer
and much more. MSL is truly impressive, but the use of analytical chemistry
techniques to learn more about the
geological history — and potential
habitability — of Mars has actually been
going on for decades. And whether it’s
sending a lab like Curiosity into space
or examining Martian samples here on
earth, Canadian scientists have played
an important role.
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 14 C. After a few
hours, the sample gave off CO2 that was
also radiolabelled with 14C, 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 CO2, something that would have been
26 CAnadian Chemical News www.cheminst.ca/magazine
expected if the purported life forms had multiplied. Today most
scientists believe that inorganic salts in the Martian soil —
possibly superoxides that contain O2¯ 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 alphaparticle 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
chemistry | mars exploration
NASA/JPL-Caltech
Curiosity's two-metre-long robotic­arm
contains a variety of tools including­
cameras, a drill, a soil scoop and an
alpha­particle X-ray spectrophotometer­
(APXS), visible here as a can-shaped
device located at the 10 o’clock position. Designed and built by Canadian
scientist Ralf Gellert, the APXS uses
radioactive curium-244 to produce
both alpha particles and X-rays that
probe the elemental­composition of
Martian rocks.
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 lowlying 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
September | october 2013 CAnadian Chemical News 27
chemistry | mars exploration
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 CO2
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
28 CAnadian Chemical News www.cheminst.ca/magazine
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 (ZrO2), 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 carbonbased 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
chemistry | mars exploration
NWA 5298 © Royal Ontario Museum
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.
Coming home
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
Northwest Africa (NWA) 5298 is a Martian shergottite
meteorite­discovered in Morocco and acquired by the Royal
Ontario Museum­in 2008. Inside, tiny crystals of the mineral
baddeleyite­act 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
researchers­thought. This important new finding tells us that
Mars has been volcanically active for most of its existence.
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 CO2, 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.
September | october 2013 CAnadian Chemical News 29