We have detected a concentration of boron in martian clay far in excess of that in any previously reported extra-terrestrial object. This enrichment indicates that the chemistry necessary for the formation of ribose, a key component of RNA, could have existed on Mars since the formation of early clay deposits, contemporary to the emergence of life on Earth. Given the greater similarity of Earth and Mars early in their geological history, and the extensive disruption of Earth's earliest mineralogy by plate tectonics, we suggest that the conditions for prebiotic ribose synthesis may be better understood by further Mars exploration.

Funding: This material is based upon work supported by the National Aeronautics and Space Administration through the NASA Astrobiology Institute under Cooperative Agreement No. NNA09DA77A issued through the Office of Space Science. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Copyright: © 2013 Stephenson et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Comparison data was from chondrite meteorite phases, martian meteorite primary and alteration phases (including clays), terrestrial clays and coals. Measured data from the Sutter’s Mill chondrite and from martian meteorite MIL 090030 show similar or reduced boron concentrations to their previously measured equivalents. The degree of boron enrichment in MIL 090030 clay data is most closely comparable to terrestrial clays, it is highly enriched compared to other phases in the same meteorite and to previously measured martian clays. Error bars are 2σ.

Clays have long been proposed as excellent locations for prebiotic catalysis [7] , [8] , polymerization [9] , [10] , and compartmentalization [11] because of their ability to absorb and protect necessary reactants [12] . These properties are evident in the popularity of clays as catalysts within industry [13] . Boron is commonly concentrated as borate (BO 3 3− or BO 4 3− ) in terrestrial clays and organic-rich sediments (∼80–800 ppm), but it has never been found at concentrations above 20ppm in any extraterrestrial source ( Figure 1 , Table S1 ). Here we use secondary ion mass spectrometry to show that Earth-like boron concentrations exist in martian clay.

The sugar ribose is central to metabolism, most notably as the derivatized sugar component of RNA. Any theory of life’s origins focused on RNA must therefore include a plausible prebiotic ribose production pathway [1] , [2] . Borate minerals have been shown to stabilize ribose [3] , [4] synthesized via the formose reaction [5] , making boron a potentially important chemical element connecting geoscience to organic chemistry. One of the main objections to this mode of ribose accumulation on the early Earth is that evaporitic borate deposits (e.g. colemanite, ulexite and kernite) may not have been present on the early Earth (>3.5 Ga) [6] . Our research suggests boron-enriched clay as an alternative site for ribose production.

Results and Discussion

Martian meteorite MIL 090030 was collected in the Miller Range region of Antarctica during the 2009/10 ANSMET field season. It belongs to the nakhlite group of martian meteorites - basaltic lavas that crystallized ∼1.3 billion years ago [14], [15]. Subsequent to crystallization these meteorites were aqueously altered on Mars, which produced narrow (generally <100 µm wide) alteration veins containing evaporite salts, amorphous silicate, Fe-oxides and smectite clays [16]–[19]. We determined the abundance of boron in the alteration veins of MIL 090030, via in-situ Cameca ims 1280 ion-microprobe analyses at the University of Hawaii (Materials and Methods). For comparison we also analyzed the primary igneous minerals of MIL 090030, as well as various primary and aqueous alteration phases in Sutters Mill – a carbonaceous chondrite meteorite observed to fall in 2012 [20].

The abundances of boron measured in Sutter’s Mill olivine and matrix material (Table 1) are comparable to data previously reported for boron in the carbonaceous chondrites (0.004–0.7 ppm) [21]. Furthermore, boron abundances in the igneous primary minerals (olivine and pyroxene) of MIL 090030 are comparable to those of the carbonaceous chondrites (Table 1). In contrast, boron is concentrated in martian alteration veins (∼160 ppm), to levels rivaling those found in terrestrial clays and marine sediments (Figure 1). The level of boron concentration in the alteration veins of MIL 090030 cannot be explained by terrestrial contamination, as no atmospheric reservoir on Earth approaches the abundance of boron measured in these alteration veins (Figure 1). However, as smectite clays and amorphous materials can readily adsorb atmospheric and aqueous contaminants, both an exterior (MIL090030,25) and interior (MIL 090030,23) sample from the MIL 090030 meteorite stone were measured. We found no measureable difference between boron abundances in the alteration veins of the internal and external areas, implying terrestrial boron is negligible. In addition, heavy pre-sputtering prior to each ion-probe analysis (Figure S1 and Figure S2) removes any surface contamination which may result from sample preparation (Materials and Methods). Therefore, the boron concentration in MIL 090030 alteration veins must be the result of secondary alteration processes on Mars.

Boron is readily adsorbed onto clay surfaces [22], [23], and has a greater affinity for smectite and illite than for other clay species [24]–[26]. Given that MIL 090030 alteration veins contain smectite it follows that boron enrichments should be found in these veins. However, the degree of enrichment reported here is somewhat surprising - an order of magnitude higher than any other extraterrestrial phase investigated to date [20], [27], [28].

A straightforward geochemical interpretation of our results is that boron, a relatively volatile and soluble element, was first concentrated in the fluid dregs of lava (4–7 ppm boron has been detected in the late stage mesostasis of other nakhlites [28]) and then became further concentrated by any groundwater or hydrothermal fluids that came into contact with the rock. Due to the oxidizing conditions of martian clay the boron most likely exists as borate, possibly in isolation or else bound to cations such as calcium, magnesium or sodium.

Borates may be crucial to prebiotic chemistry due to their ability to stabilize ribose, a key component of RNA. Without them, ribose degrades, reducing it to only a small fraction of the formose reaction products [29]–[31]. With borate ribose can last months. This results in a relative enrichment of ribose and other aldopentoses [32] compared to other formose products [3], . Clays have long been implicated as ideal catalytic surfaces for prebiotic reactions [7], hence borate enriched martian clay may represent a potential site for ribose synthesis.

MIL090030 offers a glimpse into the chemistry of Mars at the time of its clay’s formation, but coupled with other data it also offers some insight into the potential of earlier clays. Remote sensing observations of the martian surface [33], [34] suggest an early (>3.7 Ga) wet environment, compared to the later, relatively dry conditions under which the MIL 090030 clays formed. During this earlier and wetter clay-forming period Fe-smectite clays similar to those found in MIL 090030 were dominant [34]. Hence, there is no reason to assume the earlier clays were comparatively boron depleted. Gale crater, which is currently being explored by the Curiosity rover, contains Fe-smectite units within its layered deposits [35], which may have formed in a lake environment on early Mars (3.6–3.8 Ga) [36].

This old (>3.7 Ga), relatively wet period in Mars history is contemporaneous with current estimates for the origin of life on Earth. Similar clay deposits to those produced in this wet martian environment appear to have formed on the early Earth as well. For example, intensively studied ancient (3.8 Ga) tourmaline grains from western Greenland appear to have formed from boron-rich clay precursors [37], [38]. Given the greater similarity of Earth and Mars early in their geological history, further exploration of Mars may prove invaluable in answering how ribsose may have accumulated on the Earth prebiotically.