Last Wednesday, the European Space Agency (ESA) released the results
of measurements conducted by the Rosetta space probe, currently in
orbit of a 2.5-mile wide comet designated as 67P/Churyumov-Gerasimenko.
The measurements are the newest development in the quest to determine
the origins of Earth's water - specifically, whether it was delivered by impacts from comets or asteroids.
The findings revealed that the chemical composition of the water on the
comet 67P was significantly different than that found in Earth's
oceans.
"Wait, what? Isn't water H2O, no matter where you find it?"
If you find yourself wondering how comet water could be different from
Earth water, or if you're interested in how chemistry can be applied in space, read on.
Isotopes: Not All Atoms Are Created Equal
Atoms are composed of three basic subatomic particles: positively-charged protons, negatively-charged electrons, and neutrons, which have no charge.
Atoms of different elements always have different numbers of protons,
and atoms of the same element always have the same number of protons.
Electrons are much messier, but they don't play much of a role in this
story, so we'll leave them for another day.
 |
| Hydrogen (H) and deuterium (D) both form water, but they are made of different stuff on the subatomic level. |
Neutrons add to the mass of an atom without changing its chemical
identity. Atoms of the same element can have different numbers of
neutrons. The simplest (and most relevant) example is the difference
between hydrogen (H) and deuterium (D). The average hydrogen atom
contains only a proton and an electron. Deuterium, on the other hand,
has a proton, an electron, and a neutron, so it is heavier than a
regular hydrogen atom, but because it still has only one proton, it
still acts like hydrogen. Hydrogen and deuterium are called isotopes of one another - same element, different mass.
You may have heard the word "isotope" connected with radioactivity, such as the famous plutonium-239 used in nuclear bombs, or carbon-14 used for radiocarbon dating.
Radioactive isotopes are unstable and decompose at predictable rates.
However, there are plenty of stable, non-radioactive isotopes that do
not decompose on their own. Hydrogen and deuterium are two examples. A
very important consequence of their stability: once a sample containing
hydrogen and deuterium is isolated from its environment, the ratio of
hydrogen to deuterium will never change.
Stable isotopes have (almost) the same chemistry as one another. Hydrogen can react with oxygen to form water, and so can deuterium. H2O, D2O, and HDO all exist, and all are colorless, odorless, tasteless liquids at room temperature. On Earth, about 16 of every 1,000 hydrogen atoms
are actually deuterium. So, if these three kinds of water are so
similar to each other, how do scientists figure out the difference?
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| A mass spectrometer at the University of Michigan Department of Chemistry. This instrument also uses a gas chromatograph (GC) in conjunction with the MS to identify complex mixtures of compounds. |
One very common method for determining the amount of an isotope present
in a sample is to measure the mass of a molecule of that sample using a
technique called mass spectrometry, often abbreviated MS. In
mass spectrometry, molecules are hit by a high-energy spark of
electricity to charge them up. Then, the molecular ions (charged
molecules) are flown through a tube in the presence of an electric or
magnetic field. The field is adjusted to allow only ions of a certain
mass-to-charge ratio to pass through to the detector, which measures how
many of the ions got through. Different mass-to-charge ratios are
permitted through the tube one-at-a-time as the instrument continually
adjusts the field strength. In this way, an analyst can measure the
relative amounts of chemicals of different masses within the original
sample. Mass spectrometry can be coupled with other separation or
chemical analysis techniques to give even more information about the
sample.
The ESA's Rosetta probe contains two very sophisticated mass spectrometers.
Since there is no easy way to bring the water on the comet to Earth for
analysis, the ESA brought the mass spectrometer to the comet. The
readings that were sent back to Earth showed that the water on comet 67P
contains three times more deuterium than the average value in Earth's
oceans. Since deuterium is stable, and since the comet is an isolated
system that does not exchange water with other bodies in the solar
system, the findings suggest that water from comet 67P and related
comets was not the primary source for Earth's water.
Problem Solved! ...Right?
Of course not! Science is never over!
The Rosetta measurement is just one piece of an extremely large and
complicated puzzle. It is strong, but not conclusive, evidence that
Earth's oceans could not have come from cometary collisions. For
starters, Rosetta has only measured the composition of the comet's outer
layer, so the deeper water could potentially be different. Secondly,
67P is only one comet among many. According to Dr. Nicholas Dauphas at
the University of Chicago, quoted by Dan Vergano at National Geographic, this "is a very exciting study that raises more questions than it answers."
One intriguing facet of the Rosetta measurement, though, is its
simplicity. Even though the ESA spent millions on launching the probe to
comet 67P in an unprecedented feat of aeronautics,
the chemistry that the probe examined in these measurements is fairly
basic. The next time you're sitting in General Chemistry picturing a
chemist toiling behind a benchtop somewhere in a sterile lab, maybe try
picturing that scene in space.
Everything is cooler in space.
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