How can we tell organic molecules on meteorites came from outer space?

Last week I attended one of Café Scientifique’s series of public forums at the Royal Society in London, in which Dr Zita Martins, a University Research Fellow at Imperial College, London, gave a short talk entitled “What’s left to explore in the solar system?” Her talk and extended Q & A session actually focused on her analysis of meteorites–rocks of non-terrestrial origin that reach the surface of the Earth–and covered a wide-range of fascinating subjects.

Meteorites, as opposed to meteors that disintegrate in the atmosphere, give us a unique glimpse into the material universe away from our planet, and have important things to tell us about asteroids, solar system formation, and possibly even the origin of life. During the Q & A session I asked Dr Martins how we could be sure that the organic material found within a special class of metorites called carbonaceous chondrites were of extraterrestrial origin.

Before I relay what she told me, I’m going to answer another question first: how can we tell if a rock is a meteorite which has undergone a spectacular multi-million mile journey through the depths of space, surviving the fireball entry through Earth’s scorching atmosphere, or is, well, a darn boring rock?

Well, first off, it’s not if a burning fireball ploughs through your roof and sets alight your home. That probably just means it’s November Fifth. Check your calendar. Unlike the Hollywood depiction of meteorites being red-hot lances of fire, meteorites are actually fairly cold when they reach the ground. The high-temperatures that burn up the smaller pieces occur at altitudes over seven miles, and by the time the meteorite hits the ground its travelling at terminal velocity–much like an ordinary stone thrown up into the air.

However, the friction and superheating of the compressed air in front of the meteorite, will lead to a effect called fusion crust over the rock’s surface. A thin black rind, approx. the thickness of apple skin, sometimes shiny, sometimes matte black, is good evidence of this phenomena.

Second, most meteorites are attracted to magnets due to their high iron content–which also leads to their third distinctive property–they are noticeably heavier than most terrestrial rocks. Fourth, they never have vesicles–small holes throughout the structure–which are common in volcanic rocks due to escaping gas when lava cools. Five, six, and seven are the “thumbprints” caused by the surface melting while travelling through the atmosphere, the metallic flakes and grain-like chrondules of the interior, and acquistion of a yellow/ochre patina caused by oxidation on Earth’s surface.

As you can see, identifying a meteorite is a complex task, and if a strange rock turns up in your yard chances are it’s been kicked up by a passing truck’s drive belt rather than hurled from the asteroid belt.

So, now we’ve confirmed we’ve got a genuine rock from the big dark deep on our hands, how can we tell that any organic matter it carries actually came with it?

Dr Martins informed me there were three major ways scientists could tell if the carbon compounds, including amino acids, seemingly present in the meteorite samples came from outer space. First, the organics in meteorites exhibit racemic chirality. They do what? Chirality is a property of objects that lack an internal plane of symmetry–like hands. Your left hand and your right hand are mirror images of one another, and even though they can perform the same functions, they are not identical. Many carbon molecules exhibit a similar quality, and can be divided into left-hand and right-hand versions. For reasons that are still not fully understood, it turns out Earth-based organics like amino-acids and proteins predominately show one or other of the two forms, whereas extraterrestrial samples show equal mixtures of both forms, racemic in other words.

The second method involves determining the isotopic ratios of the carbon present in the organics. Many elements come in naturally occuring “flavours” or isotopes where the nucleus contains equal numbers of protons (and therefore electrons making them chemically near-identical) but differs in the number of neutrons present. Carbon-12 and carbon-14 are examples. Terrestrial and extraterrestrial samples will exhibit differing ratios of these isotopes and can therefore be used to determine whether an organic compound is truly of off-world origin.

Lastly–phew!–a comparison of the soil organics where the meteorite fell and the organic matter purported to be in the meteorite should show detectable differences, furthering the evidence the meteorite wasn’t contaminated on Earth.

A question to finish with: where do you think the best place to hunt for meteorites is?

(Clue: you’ll want to take your thermals . . .)


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