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Tagish Lake Meteorite Delivers Different Composition

This is one of the Tagish Lake meteorite fragments. Credit: Michael Holly, Creative Services, University of Alberta

We’re all familiar with the hypothesis of panspermia – that life can be “seeded” from the contents of asteroids, comets and planetoids vis-a-vis meteorite impacts – but so far no direct evidence has been found. So why should we even consider meteorites to be potential parents? The truth is out there – they contain the essentials – right down to amino acids. Up until now, what we’ve recovered has been considered structured. Then along came Tagish Lake…

In January, 2000, a large meteoroid exploded in Earth’s atmosphere over northern British Columbia, Canada, resulting in a debris fall over frozen Tagish Lake. It was a rare observed fall, and the meteorites were meticulously gathered, documented and preserved in their frozen state. The reason was twofold: to preserve the integrity of the space stones and to ensure no contamination could occur either to Earth or to the specimens.

“The Tagish Lake meteorite fell on a frozen lake in the middle of winter and was collected in a way to make it the best preserved meteorite in the world,” said Dr. Christopher Herd of the University of Alberta, Edmonton, Canada, lead author of a paper about the analysis of the meteorite fragments published June 10 in the journal Science.

For meteorite collectors, we’re well aware of the value of an observed fall and equally aware of the documentation needed to make a meteorite valuable both to market and scientific study. It’s more than just writing down the date and time of the observation and where the fragments were collected. To be done properly, the field needs to be measured. Each fragment needs to be photographed in the position in which it was found. The depth measured and more. Nothing is left to speculation.

“The first Tagish Lake samples – the ones we used in our study that were collected within days of the fall – are the closest we have to an asteroid sample return mission in terms of cleanliness,” adds Dr. Michael Callahan of NASA’s Goddard Space Flight Center in Greenbelt, Md., a co-author on the paper.

What the scientists found was the Tagish Lake meteorites are rich in carbon – and contain an assortment of organic matter including amino acids. While these “building blocks of life” aren’t new to meteoritic structure, what was out of the ordinary was different pieces had greatly differing amounts of amino acids. This varies way off the beaten path.

“We see that some pieces have 10 to 100 times the amount of specific amino acids than other pieces,” said Dr. Daniel Glavin of NASA Goddard, also a co-author on the Science paper. “We’ve never seen this kind of variability from a single parent asteroid before. Only one other meteorite fall, called Almahata Sitta, matches Tagish Lake in terms of diversity, but it came from an asteroid that appears to be a mash-up of many different asteroids.”

The team set to work on the recovered fragments – identifying different minerals present in each meteorite. What they were looking for was to see how much each had been changed by the presence of water. What they found was the different fragments each had a different water signature not accounted for from their landing on Earth. Some had more interaction and others less. This alteration may explain the diversity in amino acid production.

“Our research provides new insights into the role that water plays in the modification of pre-biotic molecules on asteroids,” said Herd. “Our results provide perhaps the first clear evidence that water percolating through the asteroid parent body caused some molecules to be formed and others destroyed. The Tagish Lake meteorite provides a unique window into what was happening to organic molecules on asteroids four-and-a-half billion years ago, and the pre-biotic chemistry involved.”

How does this change the way we look at the panspermia theory? If future falls continue to show this widespread variability, scientists are going to have to be a bit more reserved in their judgements about whether or not meteorites could deliver enough bio-molecules to make the hypothesis viable.

“Biochemical reactions are concentration dependent,” says Callahan. “If you’re below the limit, you’re toast, but if you’re above it, you’re OK. One meteorite might have levels below the limit, but the diversity in Tagish Lake shows that collecting just one fragment might not be enough to get the whole story.”

While the Tagish Lake samples are undoubtedly some of the most carefully preserved specimens collected so far, there is still a possibility of contamination from both Earth atmosphere and their lake landing. But don’t simply write off these new findings just yet. In one fragment, the amino acid abundances were high enough to show they were made in space by analyzing their isotopes. These versions of elements with different masses can tell us a lot more about the story. For example, the carbon 13 found in the Tagish Lake samples is a much heavier, and less common, variety of carbon. Because amino acids prefer lighter forms of carbon, the enriched and heavier carbon 13 deposits were most likely created in space.

“We found that the amino acids in a fragment of Tagish Lake were enriched in carbon 13, indicating they were probably created by non-biological processes in the parent asteroid,” said Dr. Jamie Elsila of NASA Goddard, a co-author on the paper who performed the isotopic analysis.

The team compared their results with researchers at the Goddard Astrobiology Analytical Lab for their expertise with the difficult analysis. “We specialize in extraterrestrial amino acid and organic matter analysis,” said Dr. Jason Dworkin, a co-author on the paper who leads the Goddard laboratory. “We have top-flight, extremely sensitive equipment and the meticulous techniques necessary to make such precise measurements. We plan to refine our techniques with additional challenging assignments so we can apply them to the OSIRIS-REx asteroid sample return mission.”

We look forward to their findings!

Original Story Source: NASA / Goddard Spaceflight News.

About 

Tammy is a professional astronomy author, President Emeritus of Warren Rupp Observatory and retired Astronomical League Executive Secretary. She’s received a vast number of astronomy achievement and observing awards, including the Great Lakes Astronomy Achievement Award, RG Wright Service Award and the first woman astronomer to achieve Comet Hunter's Gold Status.

Comments on this entry are closed.

  • Anonymous June 12, 2011, 10:05 PM

    I wonder, did they find a preference for levo aminoacids like in many other meteors? I’m still puzzled by the chirality of life.

    • Sam Regenbogen June 13, 2011, 7:48 AM

      That’s a really good question. I need to check out the paper to see if they address it. The chirality of life makes sense – nucleic acids and most proteins depend on specific arrangements of hydrogen bonds, van der Waals interactions, etc., to create the specificity and reproducibility that allows any strand of DNA to be exactly duplicated, or for any given sequence of amino acids to fold the exact same way. If both stereoisomers of any amino acid could be utilized, either the cell would need some way to distinguish between 40 possibilities (instead of 20), or it would have to deal with the 50% chance that every residue or nucleotide added to a polymer would alter or eliminate its intended function.

      Now, the question of why levo instead of dextro is a different story… As far as I know, there’s no *inherent* reason for a bias, so I’d imagine AAs formed through non-biological processes in space would be pretty evenly split. If not, the implications would be pretty darn interesting…

      • Anonymous June 13, 2011, 10:58 AM

        The only other reason to have dextrochirality comes with the utilization of carbohydrates i.e. glucose. Organisms to live so they require a substrate.
        In this case, nature seem to say, at least in our version of the universe,
        that the building blocks or protein substrate be the levo variety and the metabolic fuel be dextro in character. This would make it easier to survive having two strategies to recognize the different molecules instead of having to determine if two competing levo or dextro structures were both metabolic and anabolic. Maybe, in another universe, the organisms would depend on dextrochiraltiy for their amino acid meal and levochiraltiy for their metabolic needs.

        Henry

        • Anonymous June 13, 2011, 1:09 PM

          Yes but the question is, is it homochirality a necessity for life or a consequence of it? The former could imply some kind of parity violation or some local (like in this solar system) condition. Some meteorites do have some enantiomeric excess (p.e. http://bit.ly/krsrHZ ) which could suggest a prebiotic origin. Some researchers are working on the effects of circularly polarized light on organic molecules and show that some enantiomers react preferentially with it (p.e. http://bit.ly/kBYz6j).
          But I also found a paper which proposes a biological process of enantiomeric cross-inhibition in which a polymer chain of aminoacids stops growing if a monomer of the wrong handedness attach to it (I think this is the paper I read: http://bit.ly/lveFE2).

          So yes, it is pretty darn interesting :)

          “Maybe, in another universe, the organisms would depend on dextrochiraltiy for their amino acid meal and levochiraltiy for their metabolic needs.”
          Why, of course. Turians and Quarians use dexo-amino acids.

          • Torbjörn Larsson June 13, 2011, 9:33 PM

            As for chirality, on Earth L forms comes mainly in amino acids, and D forms in carbohydrates and IIRC as well as in heteropolymers: amino acids (many protein folds), carbohydrates (glycolipids, cellulose) and nucleotides (DNA spirals).

            But there are several different forms of chirality, chemical stereo centers (chirality set by definition), L/D forms (chirality set by mirror forms), optical (chirality set by ability to rotate light). Molecules can go in and out of chirality by chemical reactions (say, an L and D form combine to a non-chiral molecule).

            So at various times chirality plays different roles and have different selective pressures in extant forms. Mostly today these pressures would be to conform to metabolism AFAIK, where enzymes may operate on limited sets of molecules and among them chiral forms. (Which is why for example thalidomide was so poisonous for fetuses.)

            So there are selective pressures such as enzyme function in proteins active sites that form and tip chiral balances. But there are also many other more or less chemo-physical systems that with various degrees of initial imbalances reinforce (or at least maintain) them. Too many to put in a comment or link to – a good way to get an informative tour would be to look into an astrobiology text book.

          • Torbjörn Larsson June 13, 2011, 9:33 PM

            As for chirality, on Earth L forms comes mainly in amino acids, and D forms in carbohydrates and IIRC as well as in heteropolymers: amino acids (many protein folds), carbohydrates (glycolipids, cellulose) and nucleotides (DNA spirals).

            But there are several different forms of chirality, chemical stereo centers (chirality set by definition), L/D forms (chirality set by mirror forms), optical (chirality set by ability to rotate light). Molecules can go in and out of chirality by chemical reactions (say, an L and D form combine to a non-chiral molecule).

            So at various times chirality plays different roles and have different selective pressures in extant forms. Mostly today these pressures would be to conform to metabolism AFAIK, where enzymes may operate on limited sets of molecules and among them chiral forms. (Which is why for example thalidomide was so poisonous for fetuses.)

            So there are selective pressures such as enzyme function in proteins active sites that form and tip chiral balances. But there are also many other more or less chemo-physical systems that with various degrees of initial imbalances reinforce (or at least maintain) them. Too many to put in a comment or link to – a good way to get an informative tour would be to look into an astrobiology text book.

    • Anonymous June 13, 2011, 1:40 PM

      I tend to find this mysterious as well. Generally most molecular biologists consider this to be a matter of randomness. This appears to be the obvious hypothesis, but it appears troublesome with the conjecture that the origin of life involved a large number of random molecular “experiments,” which may have merged in some instances.

      The L-D chiral breaking, or homochirality, for amino acids and sacharrides is thought by some to be an aspect of polarized radiation incident on molecules in space. The conjecture there is that radiation has some selected polarization state in the presence of comet dust or by some other mechanism. Consequently there is a demolition of the D-chiral forms of amino acids. However, this does shove the problem of the chiral symmetry breaking from Earth to “out there.” Of course there may be some advantage to this, for it might permit the problem to be examined in certain ways.

      I am not sure if there are birefringent properties to dust on comets. If this does exist, and a search this morning has not revealed any research on this, then there might be some mechanism for entanglement in singlet and triplet phases. If there is then some breaking of quantum degeneracy, there could be a mechanism for this chiral breaking. The triplet entanglement has a set of states which can be split, whereas the singlet state does not.

      LC

  • Torbjörn Larsson June 13, 2011, 9:12 PM

    Well, now I know what to do next time I see a meteoroid debris fall – phone the Dust Busters. =D

    That said, it was a while since I saw the panspermia “writ large” hypothesis. Nowadays it seems people mainly classify it along the transpermia (deep frozen cells or possibly RNA) – late veneer delivery (of volatiles) axis instead.

    Nitpick:

    “Because amino acids prefer lighter forms of carbon, the enriched and heavier carbon 13 deposits were most likely created in space.

    “We found that the amino acids in a fragment of Tagish Lake were enriched in carbon 13, indicating they were probably created by non-biological processes in the parent asteroid,””

    I believe the 2nd sentence contradicts and explains the 1st. It isn’t so much that amino acids or other chemical compounds prefer lighter isotopes, but isotopes become fractionated during mainly non-biological thermo-chemical and biological metabolic processes. Especially the most modern and efficient photosynthesis of land plants is good at this, which shows through the food chain.

    Only one other meteorite fall, called Almahata Sitta, matches Tagish Lake in terms of diversity, but it came from an asteroid that appears to be a mash-up of many different asteroids.

    That was the 2nd time I read that and didn’t understand what it meant. Turns out Almahata Sitta is an interesting case.

    From Planetary Society [“Asteroid Tracked in Space, Its Remains Recovered on Earth”, Amir Alexander] I get a geological description:

    “Almahata Sitta, it turned out, is an extremely rare type of space rock, known as an F-class “ureilite.” A close analysis showed that it is extremely porous and fragile, which is why it had exploded at such a high altitude. All ureilites are thought to have originated from a single primordial source, and the structure of Almahata Sitta indicates that it was a volcanically active body in which gas bubbles were trapped inside the porous rock.” [My bold.]

    From Planetary Science Research Discoveries I get the following illuminative description under a nifty stick figure Almahata Sitta life graphic:

    “Researchers propose that the story begins with a carbonaceous-chondrite-like parent body that was heated and partially melted. This hot body was smashed to smithereens by a major impact and the wreckage cooled and was mixed with debris from other asteroidal collisions. The re-accreted debris assembled into a rubble pile of all sorts of materials, with a regolith accumulated on its surface. Further impacts ejected debris off this rubble pile, sending our asteroid 2008 TC3 careening through space and eventually through Earth’s atmosphere where it broke apart and scattered itself in the Nubian Desert.”

    So, it was a large enough parental body (> 200 km diameter) that it was a) volcanic b) gravitationally mixed and reassembled as breccia after being smashed up, before c) getting hit again into ejecting Almahata Sitta.

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