Meteorites and the RNA world II: Synthesis of Nucleobases in Carbonaceous Planetesimals and the Role of Initial Volatile Content

Abstract

Prebiotic molecules, fundamental building blocks for the origin of life, have been found in carbonaceous chondrites. The exogenous delivery of these organic molecules onto the Hadean Earth could have sparked the polymerization of the first RNA molecules in Darwinian ponds during wet-dry cycles. Here, we investigate the formation of the RNA and DNA nucleobases adenine, uracil, cytosine, guanine, and thymine inside parent body planetesimals of carbonaceous chondrites. An up-to-date thermochemical equilibrium model coupled with a 1D thermodynamic planetesimal model is used to calculate the nucleobase concentrations. Different from the previous study (Pearce & Pudritz 2016), we lower the pristine initial abundances, as measured in comets, of the most volatile ices compared to the bulk water ice. This represents more accurately cosmochemical findings that carbonaceous chondrite parent bodies have formed inside the inner, ∼ 2-5 au, warm region of the solar system. Due to these improvements, our model was able to directly match the concentrations of adenine and guanine measured in carbonaceous chondrites. Our model did not reproduce per se the measurements of uracil and the absence of cytosine and thymine in these meteorites. Therefore, we provide a combined explanatory approach that could explain this deficiency. In conclusion, the synthesis of prebiotic organic matter in carbonaceous asteroids could be well explained by a combination of radiogenic heating, aqueous chemistry involving a few key processes at a specific range of radii inside planetesimals where water can exist in the liquid phase, and a reduced initial volatile content (H 2 , CO, HCN, CH 2 O) of the protoplanetary disk material.

Possible Ribose Synthesis in Carbonaceous Planetesimals

“…Figures 5 and 6 show the resulting ribose abundances for several parameters and are a representative selection among all the available thermal profiles. We also calculated the abundances for the other available planetesimal models with our chemical simulations, but the results share the same characteristics and trends as for the shown planetesimal model in Figures 5 and 6 (for the other available planetesimal models see also the previous study [31]). The resulting ribose abundances for different catalyst correction factors (see Table 2) are plotted as dashed lines with symbols (see legends).…”

“…In the framework of the previous studies of Pearce and Pudritz [ 40 , 41 ] and Paschek et al [ 31 ], an ID-number was assigned (maximum two digits) to each considered reaction pathway (there reaction pathways for the formation of nucleobases). We extended this numbering scheme to the formose reaction and assigned the no.…”

“…[85]. The temperature evolution for the other available model planetesimals was described in the previous study [31].…”

“…Our study sought to model the formose reaction pathway leading to the synthesis of ribose inside carbonaceous planetesimals. We used the same model as developed in our previous study by Paschek et al [ 31 ], where we studied the formation of nucleobases inside the parent bodies of carbonaceous chondrites. Our model comprised an up-to-date thermochemical equilibrium model coupled with a 1D thermodynamic planetesimal model to calculate the abundances of prebiotic molecules via abiotic synthesis pathways under realistic conditions inside parent body planetesimals.…”

“…First, we introduce our approach to model the formose reaction pathway, focusing on the synthesis of ribose. Further, we briefly recall our new concept to estimate the initial concentrations of the volatiles in carbonaceous planetesimals, which was explained in more detail in Paschek et al [ 31 ]. Next, in Section 2.2 , we outline our computational methods.…”