Geoscientist Online

Geoscientist 17.5 April 2007

On 1 December 2006, The Guardian ran the headline "Older than the sun, the meteorite scientists call 'the real time machine'". Joe McCall recounts what is known about a unique meteorite.

On 18 January 2000 residents in Western Canada saw a fireball as bright as the Sun streak across the morning sky, exploding with an estimated yield of 5-10 thousand tonnes of TNT (or one fifth of the Hiroshima nuclear explosion), such that defence satellites and seismic monitoring stations were alerted2. The brilliant meteor was seen by anyone outdoors within about 800km of its path. This was a very long-lasting trail and estimates of the amount of meteoritic material involved have ranged from 56 through 150 to 200-250 metric tonnes. It is probable that the lower figure is nearer the mark. Dust clouds from terminal fragmentation were widely observed. The explosions on atmospheric entry were recorded infrasonically in Manitoba (Figure 1) and seismically at Haines Junction and Whitehorse (Figure 2)³. There were several fragmentation explosions. .

That same month a local resident, Jim Brook, was driving across the frozen Tagish Lake when he saw some black rock upon the snow-covered ice. Luckily he knew how to collect and placed 12 samples in clean bags and kept them frozen - untouched by human hand. He eventually recovered several dozen meteorites from the Taku Arm of Tagish Lake. This find was better in one respect than the numerous Antarctic meteorite finds (McCall et al. 2006) which have been sitting on or in ice for a long time (even as much as 10,000 years), for this was a meteorite fall recovered on (and in some cases encased in) ice. To cap it all, the object proved to be a unique stony meteorite, a very primitive carbonaceous chondrite. Such primitive meteorites are very fragile, disintegrating rapidly on exposure to air (Figure 3).

Many more samples were later recovered by scientists, the masses littering the ice along a narrow ellipse trending northwest. Dr Peter Brown of the University of Western Ontario went out on the lake on 20 April and encountered a hole in the snow with dark material in the bottom like wolf droppings: but it was, in fact, another meteorite fragment. In the next few days his party harvested more than 400 fragments from the Taku Arm and a nearby small unnamed lake, the largest piece weighing 200-300g, the total mass 5-10kg. All were frozen in the ice. Exposure to water would have turned this type of carbonaceous chondrite to a black organic sludge.

The fragments were probably cold when they hit the ground – though the outer layers were heated by friction in their approach, this type of meteorite does not conduct heat well and the interior beneath the thin fusion crust was probably still ice cold on arrival, just as in space. No other meteorite fall has provided such unthawed specimens. The last collection was on 8 May. Some 200 fragments were selected, priority being given to those fragments of most mass and least disaggregation. 

Statistics of Tagish Lake Meteorite

• Latitude and longitude of first recovery: 59o 42’ 15.7”N: 134o 12’ 4.9” W
• Strewn field: at least 16 x 3 km, oriented –S30E
• Fell: 2000 January 18 at 08.43.42 PST (16 43 42 UT)
• Type: Carbonaceous chondrite (C2 ungrouped, probably CI 2)
• Shock stage: S1
  
  

Petrography

Chondrites⁸ are undifferentiated meteorites characterised by chondrules, millimetre- sized spherical rocky beads. There are several schools of thought about their origin but the most likely origin is partial to complete melting of clumps of silicate mineral dust in the solar nebula. Chondrules are never found in terrestrial rocks, nor in the SNC achondrite meteorites attributed to a source on Mars; but very rare, round bodies in lunar surface breccias have been supposed to be chondrules. Many chondrites display secondary metamorphism, dry heating from the original partly glassy object (Type 3) and the progressive metamorphic stages are classified from Types 4-7, the chondrules being entirely lost by recrystallisation in the latter. In contrast some of the carbonaceous chondrites are of Type 3 or even higher, but the really primitive ones display increasing low temperature alteration in the presence of water, to form Ca-Mg-Fe carbonate minerals and sulphates such as epsomite, pseudomorphing the high temperature minerals olivine and pyroxenes. In the CM chondrites (named after Murchison fall, Australia) there are chondrules, but in the most primitive type of all, the CI chondrites (named after Ivuna fall, Tanzania, though the first and most famous is the Orgeueil meteorite fall, France7) there are no chondrules. Tagish Lake^3,4,5,12 has very few chondrules. It is matrix-dominated, this matrix being mainly of phyllosilicates, with also Fe-Ni sulphides and refractory Calcium-Aluminium Inclusions (CAIs). The olivine is highly magnesian (peak Fa1), as is the pyroxene (peak Fs2).

This meteorite is composed of two different rock types, one poor in carbonate minerals, the other rich in them12. Both are largely composed of low temperature minerals pseudomorphing original high temperature phases, though some of the latter are preserved residually. Though the presence of chondrules distinguishes this meteorite from the most primitive CI class such as Orgueil, the much fewer chondrules than Murchison (CM2) suggests that this meteorite is unique and should be allotted a new class CI2. Tagish Lake also has a lower density than any other chondrite meteorite known.

The chemical composition is also used to classify meteorites. On one parameter, the oxygen isotopic composition, Tagish Lake is very close to the most primitive CI1 chondrites, but different^4,9. The bulk composition reflects the degree of fractionation from the original solar nebula composition of the cloud of dust and gas at the start of the solar system10. The CI1 chondrites such as Orgueil are nearly unfractionated. Tagish Lake is closest to the CI1 and CM2 carbonaceous chondrites, and rather closer to the CM2 class.

Carbon studies

Much of the carbon content of carbonaceous chondrites is contained in carbonate minerals, but ~ 44% of the carbon in Tagish Lake is contained in organic molecules, either formed in the interstellar medium or remnants of such compounds modified during alteration on asteroidal parent bodies10. Either way, a portion of the material in Tagish Lake was never strongly heated in the nascent solar nebula.

Minute 'nanodiamonds', a few micrometres in size, have been recognised in this material4, believed to be dust from other stars that travelled through the interstellar medium and ended up in the dust that formed our solar system - specifically in this case an asteroidal parent body. Opinions differ about the ultimate origin of these diamonds, but one theory is that they formed in the expanding shell of a Type II supernova. Nanodiamonds occur in all types of chondrites, but are most abundant in Types CI and CM carbonaceous chondrites.

All the petrographic and chemical characteristics of Tagish lake suggest formation in the outer reaches of the asteroid belt, and there is other evidence to support this as we will see below.

Amino acids & aromatic hydrocarbons

The Murchison CM meteorite contained some 74 amino acids in its insoluble residue, whereas the Orgueil and Ivuna CI meteorites contained a very simple mixture, mainly glycine and beta-alanine. Tagish Lake contains no amino acids, but does contain aromatic hydrocarbons⁶. The amino acids in the CM and CI meteorites had the extraordinary characteristic that the optical isomerism of the organic compounds was equally right- and left-handed, whereas terrestrial biogenic organic compounds are all left handed. Kminek et al.⁶ conclude that the absence means that Tagish Lake originated in a different type of parent body to the CI and CM carbonaceous chondrites. Some 44 alkyl dicarboxylic acids were recognised in Tagish Lake¹¹, closely matching the CM carbonaceous chondrites. The data collected restricts the possible source regions of this meteorite to either interstellar or very cold nebular regions.

A further publication by Binet et al.¹ reports that, like Orgueil and Murchison, the aromatic hydrocarbons in the Tagish Lake insoluble residues are anomalous in the distribution of free organic radicals compared with terrestrial coals and other polyaromatic macromolecules - and also in the high concentration of ‘diradicaloids’. These anomalies may indeed be additional markers of the synthetic process which occurred in the interstellar medium.

Orbital and spectral characteristics

The orbital characteristics of Tagish Lake were determined from observation of the fireball trace (only the sixth fall to be so determined) and its perihelion was in the outer part of the main asteroid belt between Mars and Jupiter (Figure 4)⁹, where C, P and D type asteroids predominate. It is by no means certain that such an orbit actually fixes the orbital position of the parent body, because such Earth-passing orbits are secondary due to violent change from the original planet-like orbit within the asteroid belt, by resonance interactions with Jupiter. However Takahiro Hiroi and colleagues⁵ measured the spectrum of light reflected by samples of Tagish Lake and compared it with spectra of various asteroids, finding a good match with the D- type asteroids of the outer region of the asteroid belt.

Asteroid 368 Haidea is a possible match: asteroids 336 and 773 are also possible spectral analogues, but are less likely. Such spectral matches, though increasingly being developed to eliminate such problems as space weathering due to solar wind, are by no means certain indicators of precise asteroidal source. The parent bodies of many meteorites may no longer exist having been completely fragmented. This evidence does, however, tend to confirm that Tagish Lake was originally formed in the outer regions of the solar nebula, where the sun’s heat was diminished. It may be a sample of D-type asteroid. If so it is the first meteorite to be linked to this low-albedo group.

Organic globules

The organic carbon content of Tagish Lake is 2.6 wt %. The excitement in the press in December 2006 was caused by the publication by Nakamura and others in Science¹⁰ . Whereas there have been numerous claims to find organic inclusions, (and even possible microscopic life forms) in primitive carbonaceous chondrites such as Orgueil⁷, and although extrasolar system inclusions such as nanodiamonds and SiC are also well documented, the refinement of coordinated transmission electron microscopy has allowed study of the very small (submicrometre) hollow organic globules in Tagish Lake to a degree never before possible. What this has shown is that these inclusions have astonishing nitrogen-15 to 14 and deuterium to hydrogen isotope ratios - anomalies that can only have been produced by mass fractionation at extremely low temperatures (10-20 Kelvin)¹⁰.

Similar globules have been described from other carbonaceous chondrites, but their origin remained uncertain. The values are variable, but, where globules are in contact with one another, the values are identical, suggesting aggregation before incorporation in the meteorite body. These globules are now believed to have originated as organic ice coatings on pre-existing grains. They resemble cometary materials and give some idea of what the material that began the solar system's formation was like. Very likely they originated far out in the protosolar disk, in the region of the Kuiper Belt. Such globules may have been the common form of prebiotic organic material delivered to Earth by comets and meteorites. Further studies may show whether these objects constituted important building blocks in the origin of life.

Life, the universe and Tagish

The Tagish Lake meteorite is unique in many respects and is immensely important. The classifiers in their wisdom⁴ have allocated it to CI2, but detailed findings surely suggest that it is neither CI nor CM, and it is my feeling is that Tagish should perhaps have a category ("CT2") all to itself. It is important to realise that the microscopic globules have not been identified as life forms: There is a school of thought, panspermia - promoted by the late Fred Hoyle among others - that envisages life being universally seeded throughout the cosmos and transported to Earth from space.

To my mind this concept only shifts the conundrum of the origin of life to an earlier date, and farther away – a mirror image of the suggestion that humankind's ultimate future involves the colonisation of the galaxy. The treatment of Tagish Lake in the media recently was fair, though as ever a little overdramatic for specialist taste! Tagish is unique in many respects, but it doesn’t solve the mystery of the origin of life, and nor is the presence of organic carbon globules unique. Improved instrumental techniques have merely allowed us the opportunity to find out more about them.

References 

1. Binet, L, Gourier, D, Derenne, S, Pizzarello, S and Becker, L 2004 Diradicaloids in the mobile organic matter from the Tagish lake Meteorite: comparison with the Orgueil and Murchison meteorites Meteoritics and Planetary Science 39(10); 1649-1654
2. Brown,P G and others 2000 The fall, recovery and composition of the Tagish Lake meteorite: a new type of carbonaceous chondrite Science 290; 320-325 
3.  Brown, P G, ReVelle, D O, Tagliaferri, E and Hildebrand, A R 2002 An entry model for the Tagish Lake fireball using seismic, satellite and infrasound records Meteoritics and Planetary Science 37 (5); 661-675
4. Grady, M M, Verchovsky, A B, Franchi, I A, Wright, I B and Pillinger, C T 2002 Light element geochemistry of the Tagish lake CI2 chondrite Comparison with CI1 and CM2 meteorites Meteoritics and Planetary Science 37 (5); 713-735
5. Hiroi, T Zolensky, M E & Pieters, C M 2001 The Tagish Lake Meteorite: a possible sample from a D-type asteroid Science 293; 2234-2236
6. Kminek, G, Botta, O, Glavin, D P and Bada, J L 2002 Amino acids in the Tagish Lake meteorite Meteoritics and Planetary Science 37 (5); 697-701
7. McCall, G J H 2006 Pride and Prejudice: the Orgueil meteorite fraud comes full circle Geoscientist 16/1; 6-11
8. McCall, G J H 2006 Chondrules and calcium-aluminium-rich inclusions (CAIs) In McCall, G J H, Bowden, A J & Howarth, R J The History of Meteorites and Key Meteorite Collections: Fireballs, falls and finds Geological Society of London, Special Publication Np 256; 345-361
9. Mittlefehldt, D W 2002 Geochemistry of the ungrouped carbonaceous chondrite Tagish Lake, the anomalous CM chondrite Bells, and comparison with CI and CM chondrites Meteoritics and Planetary Science 37 (5); 703-712 (see also www psrd Hawaii edu/Dec02/TagishLake html
10. Nakamura, K, Messenger, S, Keiler, S P, Clemett, S J and Zolensky, M E 2006 Organic globules in the Tagish Lake meteorite: remnants of the protosolar disk Science 314; 1439-1442
11. Pizzarello, S and Huang, Y 2002 Molecular and isotopic analysis of Tagish Lake alkyl dicarboxylic acids Meteoritics and Planetary Science 37 (5); 687-696
12. Zolensky, M E, Nakamura, K, Gounelle, Mikouchi, T, Kasama, T, Tachikawa, O and Tonui, E 2002 Mineralogy of Tagish Lake: an ungrouped type 2 carbonaceous chondrite Meteoritics and Planetary Science 37 (5); 737-761