The fall of the Murchison meteorite on September 28, 1969 in Victoria, Australia is perhaps one of the most serendipitous events in the history of science. (See photo 1) This single meteorite has revolutionized our understanding from the formation of the solar system to the origins of life on Earth. This information is contained in a single element namely carbon!
Carbon in the Murchison CM2 carbonaceous chondrite occurs in several forms from elemental to organic. Organic carbon refers to carbon chemically bonded to hydrogen, nitrogen and oxygen.
Organic molecules such as amino acids, carboxylic acids, fatty acids, nucleotides and sugars have been identified in this magnificent meteorite. See Figure 1. Living terrestrial organisms utilize amino acids to form the building block of proteins, fatty acids in forming cell membranes and nucleotides to build the genetic code. Sugars are utilized as a source of energy and in forming the backbone of RNA & DNA. Chemically sugars are a group of compounds with several hydroxyl (-OH) groups attached to a carbon skeleton. See Figure 2 The most abundant three carbon sugars identified in the Murchison meteorite are dihydroxyacetone, glycerol and ethylene glycol.
A recent paper from Yoshihiro Furukawa et al entitled: Extraterrestrial ribose and other sugars in primitive meteorites published in PNAS | December 3, 2019 | vol. 116 | no. 49 | 24445 is available online at https://www.pnas.org/content/pnas/116/49/24440.full.pdf. The authors confirm the sugars D-ribose along with D-arabinose, D-xylose, D-lyxose and dihydroxyacetone were produced extraterrestrially in the Murchison CM2 carbonaceous chondrite. See Figure 2. Sugars such as ribose are essential as a building block for RNA. In addition, two terrestrialized meteoritic finds NWA 861 and NWA 7020 CR2 were also analyzed. (See Photo 2 &3). NWA 861 was found in 2001 and NWA 7020 was found in 2011. Scientists are now able to determine extraterrestrially synthesized molecules in meteoritic finds using the δ13C technique. δ13C is a measure of the ratio of stable isotopes 13C: 12C, reported in parts per thousand (per mil, ‰). Organic compounds formed in the cold environments of space are typically 13C rich with δ 13C values greater than zero, while terrestrial organics derived from biology are much lighter in their carbon isotopic composition with δ 13C values less than zero. See Figure 3 & Table 1.
The authors analyzed the Murchison CM2 and NWA 861 & NWA 7020 CR2 carbonaceous chondrites via gas chromatography/mass spectroscopy. This instrument separates individual organic compounds from a complex mixture and then determines their mass and chemical structure. Four 5-carbon sugars (pentoses) were detected namely ribose, arabinose, xylose and lyxose in concentrations of 2.3 ppb to 11 ppb (parts per billion) from NWA 801 and 6.7 ppb to 180 ppb from the Murchison meteorite. Unfortunately, no sugars were detected in NWA 7020 since they were below the detection limit (<0.5ppb) of the instrumentation.
The δ 13C values of ribose and xylose in NWA 801 and ribose and arabinose in Murchison exhibited significant 13C enrichment ranging from +8‰ to +43‰. However, xylose in Murchison was lower at -1‰. The δ 13C positive values of the extraterrestrially produced sugars are clearly distinct from biologically synthesized sugars from algae, higher plants and secondary producers which have negative values ranging between -32‰ to -1‰. See Table 1.
The Insoluble Organic Matter (IOM) was also analyzed in all three meteorites. The IOM is an indicator of aqueous alteration in the parent body. The Murchison meteorite is phyllosilicate-rich and thus experienced significant aqueous alteration. NWA 801 was determined to have experienced limited low temperature aqueous processing in the parent body. In contrast, the matrix of NWA 7020 is dominated by phyllosilicates, serpentine and saponite suggesting pervasive aqueous alteration.
Carbonaceous type asteroids such as Ryugu and Bennu can potentially serve as the parent body for such meteorites. NASA’s OSIRIS-Rex mission https://www.nasa.gov/osiris-rex is currently orbiting and mapping asteroid Bennu with a sample return mission in mind. Sometime during the summer of 2020, the OSIRIS-Rex spacecraft will touch Bennu’s surface to collect a sample the surface regolith for return to Earth. The sample of Bennu’s surface is scheduled to arrive back to Earth in 2023. JAXA’s Hayabusa 2 spacecraft http://www.hayabusa2.jaxa.jp/en/ has already collected samples of asteroid Ryugu and is currently headed back to earth and scheduled to arrive in December 2020. Future analyses of pristine samples returned from these two asteroids will provide the first opportunity to measure the extent of extraterrestrially synthesized organic molecules on a parent body.
Figure 1: Summary of Organic and Inorganic compounds detected in the Murchison Meteorite. https://www.geocaching.com/geocache/GC6EK8H_murchison-meteorite-the-building-blocks-of-life?guid=994f64f6-12c5-4449-a1b8-4cc2653c3559
Figure 2: Fisher projection structures of sugars detected by Furukawa Y et al. All sugars are shown in their D- conformational isomer. Note that living organisms utilize only D- isomer sugars and L-isomer amino acids.
Figure 3: δ13C (pronounced “delta carbon thirteen”) is an isotopic signature, a measure of the ratio of stable isotopes 13C: 12C, reported in parts per thousand (per mil, ‰). An isotope of an element has the same number of protons but a different number of neutrons within the nucleus of the atom. For further information see https://en.wikipedia.org/wiki/Δ13C . Organic compounds formed in the cold environments of space are typically 13C rich with δ 13C values > 0, while terrestrial organics derived from biology are much lighter in their carbon isotopic composition (δ 13C < 0)
Table 1: Stable carbon isotopic values of sugars detected by Furukawa Y et al. Note that terrestrial organic molecules have δ13C with negative values ranging from – 1, ‰ to -58 ‰ while extraterrestrially produced organic molecules have positive δ13C values ranging from +5 ‰ to 60 ‰.
References:
1) Cooper G. et. al. Carbonaceous meteorites as a source of sugar-related organic compounds for the early Earth. Nature December 2001 vol. 414 pp 879-882.
2) Shanos, G. Sweet & Sour Meteorites Meteorite! August 2002 vol.8 no. 3 May 2002 pp 18-19.
3) Yoshihiro Furukawa et al entitled: Extraterrestrial ribose and other sugars in primitive meteorites published in PNAS | December 3, 2019 | vol. 116 | no. 49 | 24445 is available online at https://www.pnas.org/content/pnas/116/49/24440.full.pdf.
4) δ13C https://en.wikipedia.org/wiki/Δ13C
