Extraterrestrial ribose and other sugars in primitive meteorites

Sugars are indispensable molecules for life, working along essential metabolic pathways and as constituents of sugar phosphate backbone of genetic molecules (DNA and RNA). Ribose is particularly essential as a building block of RNA, which could have both stored information and catalyzed reactions in primitive life on Earth (1, 2). Thus, the availability of abiotic sugars and their precursors on the prebiotic Earth has been investigated for many years (3–5).

Numerous studies have been conducted on meteoritic organic compounds, particularly amino acids and nucleobases (6–10). However, unlike these organic compounds, an extraterrestrial origin for biological sugars in meteorites has not yet been proven. In the 1960s, researchers reported the detection of common biological sugars, including arabinose 2, xylose 4, glucose, and mannose (Fig. 1), in both carbonaceous and ordinary chondrites in comparable concentrations (11, 12). As noted by the authors and the authors of a subsequent review, a major problem was the possibility of terrestrial contamination of the detected aldoses (13). Since the 2000s, Cooper and coworkers carefully analyzed carbonaceous meteorites by gas chromatography/mass spectrometry (GC/MS) and detected the simplest ketose, dihydroxyacetone 1, in the Murchison and Murray meteorites with several sugar alcohols and sugar acids; however, dihydroxyacetone has a lesser role in biology, the phosphate is used in central metabolism, but it has no known structural or informational roles (14). Cooper and Rios (15) verified that several sugar acids and sugar alcohols are extraterrestrial in origin, having stable carbon isotope ratios that differ substantially from terrestrial sugar alcohols and sugar acids. Therefore, to date, no previous research has verified the presence of extraterrestrial aldoses or other sugar compounds related to biological macromolecules in any astronomical sample (16).

We detected all 4 types of aldopentoses (i.e., ribose 3, arabinose, xylose, and lyxose 5) in concentrations of 2.3 to 11 ppb from NWA 801 and 6.7 to 180 ppb from the Murchison meteorite (Fig. 2; Table 1; and SI Appendix, Figs. S2 and S3). Several hexoses are also found in the NWA 801 and the Murchison meteorites. Conversely, in NWA 7020, these sugars were below our detection limit for the amount of sample analyzed (<0.5 ppb). Tetroses, sugar acids and sugar alcohols containing 5 carbon atoms (e.g., ribitol and ribonic acid), as well as 2-deoxyribose and branched isomers of aldopentoses, were below our detection limit in both meteorites (<0.5 ppb). We also analyzed serpentine which had been baked in air at 500 °C overnight as an analytical blank, and no pentoses were detected.

The δ¹³C values of ribose and xylose in NWA 801 and ribose and arabinose in Murchison showed significant ¹³C enrichment (+8 to +43‰), whereas xylose in Murchison was lower (−1‰) (Figs. 3 and 4 and see also SI Appendix, SI Text). The δ¹³C values of other sugars were not calculated because of low intensity or poor peak separation. The positive δ¹³C values are clearly distinct from biologically synthesized sugars (17, 18). For example, pentoses and hexoses in algae, higher plants, and secondary producers all have negative δ¹³C values ranging between −32 and −1‰ (Fig. 4). We also analyzed the carbon isotope composition of pentoses extracted from a soil sample that was collected in 1999 from the original Murchison meteorite’s 1969 fall site in Australia (9) to indicate the δ¹³C values of likely contaminants. The δ¹³C values of ribose (−46‰), arabinose (−52‰), and xylose (−44‰) are even lower and all well outside the range of sugars we detected in meteorites. The δ¹³C values of ribose and xylose in NWA 801 and ribose and arabinose in Murchison are comparable to those of sugar acids and sugar alcohols detected previously (i.e., +5 to +82‰ in GRA 95229 and Murchison meteorites) (15) and those of α-amino acids in many carbonaceous chondrites, which typically range from +3 to +44‰ (10). This is very strong evidence that NWA 801 and Murchison contain extraterrestrial pentoses. While enriched ¹³C (δ¹³C > 0‰) is usually an indicator of extraterrestrial origin, slightly depleted (δ¹³C ≤ 0) does not always indicate terrestrial contamination. For example, many extraterrestrial carboxylic acids have negative δ¹³C values (19). Xylose in the Murchison meteorite has a distinct δ¹³C value from xylose in Murchison soil, although that is insufficiently distinct from biological sugars reported previously (17). Although no δ¹³C value could be determined for lyxose, this sugar may also be extraterrestrial due to its rarity in the biosphere. Furthermore, the composition of sugars in the NWA 801 and Murchison meteorites are distinct from the compositions of sugars in extracellular carbohydrate polymers of desert soil algae, showing the presence of ribose and lyxose in the meteorites and absence of these sugars in desert soil algae also support an extraterrestrial origin for these sugars (20–22). The enantiomeric ratios of chiral molecules are sometimes used to evaluate the extent of biological contamination in abiotic synthesis products. However, this may not be useful for the evaluation of biological sugar contamination in meteorites, since chiral sugar-related compounds in Murchison and other meteorites have been observed to have large d-enantiomeric excesses (15).

The concentrations of detected extraterrestrial sugars are approximately 3 orders of magnitude lower than those of amino acids (8) and comparable to those of purines found in CR2 meteorites (9) and those of 5-carbon sugar acids and sugar alcohols in the Murchison meteorite (15).

The molecular structures of insoluble organic matter (IOM) were analyzed with solid-state ¹³C-NMR (SS-NMR). A chemical shift attributed to aliphatic carbon is clearly more abundant than that from aromatic carbon in NWA 801, whereas the aliphatic carbon signal was comparable to the aromatic carbon signal in NWA 7020 (Fig. 5D). The ratio of aliphatic/aromatic chemical shifts is 1 indicator of the extent of low-temperature chemical oxidation of meteorite organics associated with water (23). The carbon and nitrogen isotope ratios of IOM were analyzed with a sensitivity-modified elemental analyzer/isotope ratio mass spectrometer (EA/IRMS) (24). The δ¹³C values of IOM in these 3 meteorites are comparable to those of typical carbonaceous meteorites (25) (Table 1). The nitrogen isotope composition (δ¹⁵N_{vs. Air}) of the IOM in the 2 NWA meteorites showed significantly positive values (i.e., δ¹⁵N = +66.0 and +93.4‰ for NWA 801 and NWA 7020, respectively), which is characteristic of CR chondrites (Table 1).

We also explored the minerals in both meteorites using synchrotron X-ray diffraction (S-XRD) and field-emission scanning electron microscopy (FE-SEM) to assess the environments these meteorites experienced in their parent body asteroids. FE-SEM observation of the NWA 801 fragment showed that it contains large chondrules and metal grains with a small amount of fine-grained matrix. Chondrules and mineral fragments are not aqueously altered and are predominantly composed of olivine and pyroxene. Grain boundaries of olivine in type 1 porphyritic olivine chondrules are filled with unaltered iron-free mesostasis glass (Fig. 5C). X-ray diffraction analysis of the matrix indicates that it is composed mainly of anhydrous silicates and kamacite and does not contain phyllosilicates (Fig. 5B and SI Appendix, Fig. S4). A small reflection at around 7.2 Å is not serpentine but probably akaganeite (iron hydroxide made by terrestrial weathering of kamacite); if it was serpentine, then prism reflections should have been detected, as was observed in the NWA 7020 matrix (Fig. 5A). Metals are partially altered to iron hydroxide by terrestrial weathering, but magnetite is absent. Based on the reported classification of CR chondrites, the subtypes of NWA 801 would be 3.0 to 2.8, indicating that this meteorite experienced very limited and low-temperature aqueous processing in its parent body (26). NWA 7020 also contains large chondrules. Grain boundaries of olivine in type 1 porphyritic olivine chondrules are filled with unaltered iron-free mesostasis glass. On the other hand, fine-grained matrix materials are dominated by phyllosilicates, serpentine, and saponite, suggesting that aqueous alteration was pervasive in the matrix of NWA 7020, in contrast to NWA 801. Metals are partially altered to iron hydroxide by terrestrial weathering. The presence of magnetite is not clear. Based on the reported classification of CR chondrites, the subtypes of NWA 7020 would be 2.8 to 2.5 (26). The Murchison meteorite is also phyllosilicate-rich and thus has experienced significant aqueous alteration. The difference in the aqueous alteration history recorded in minerals is consistent with the recorded history of molecular structure of IOM.

The formose reaction is a thermally driven aqueous process producing a number of sugars, including ribose, from aldehydes with alkaline catalysts (3, 5). Formose-like reactions have been hypothesized to be related to IOM formation (27, 28). Formose-like reactions are also capable of forming sugars in natural environments (5). The fluid in the parent bodies of carbonaceous chondrites is thought to be alkaline and contains many cations, including Mg²⁺ (29). Thus, a formose-like reaction would have been possible during aqueous processing in many of the parent asteroids of carbonaceous meteorites. We also conducted a laboratory experiment of the formose-like reaction to compare the product composition with the composition of sugars detected in this study (Fig. 6). The relative composition of aldopentoses in the product of the formose-like reaction is mostly consistent with the relative content of sugars detected in the NWA 801 and Murchison meteorites. This indicates that the sugars in the meteorites detected in this study could be the products of formose-like reactions.

The mineralogy and IOM structure of NWA 801 indicate that aqueous alteration of this meteorite was very limited. Thus, sugars in NWA 801 could have been formed before accretion of the CR2 parent body or during an early parent body aqueous alteration stage. The formation of sugars before the accretion of meteorite parent bodies has been suggested in previous studies (30, 31). The IOM structure and mineral compositions of NWA 7020 show a higher extent of aqueous alteration in its parent body. Thus, sugars in the NWA 7020 meteorite might have degraded in the parent body during aqueous alteration. The CM2 Murchison meteorite experienced a much higher degree of aqueous alteration compared to the CR2 NWA 7020. However, compositional differences in the mineralogy and in the fluid between these parent bodies may have resulted in different formation and preservation conditions for sugars. For example, many CR2 meteorites have a high NH₃ content relative to other carbonaceous chondrites (32), and NH₃ can react with sugars and consume them.

The detection of ribose and other bioessential sugars in NWA 801 and Murchison that are isotopically distinct from terrestrial sugars provides clear evidence of an extraterrestrial origin for these sugars in primitive meteorites. Further, this work provides evidence that prebiotic sugars could have been delivered to ancient environments on the Earth and possibly on Mars. The detection of extraterrestrial sugars in meteorites establishes the existence of natural geological routes, outside of the laboratory, to make and preserve them. The absence of deoxyribose as well as the presence of ribose in the natural geological routes further implies much more availability of ribose than deoxyribose on the prebiotic Earth. This would be geological support of the RNA world hypothesis.