## 5 Conclusions  
The mechanochemical degradation of ribonucleosides, promoted by metals (Ni\({}^{2+}\), Al\({}^{3+}\), Fe\({}^{2+}\)/Fe\({}^{3+}\)), into the respective nucleobases was achieved. These results can clarify some aspects of the organic composition of extraterrestrial samples, since those inorganic species are main components of carbonaceous meteorites. It is commonly assumed that the organic content found in extraterrestrial environments was synthesized in the primordial solar system, and further modifications have been a marginal subject. However, several mechanochemical events could have led to several chemical transformations. Minerals and other inorganic species are used as fingerprints on formation processes of extraterrestrial bodies [57]; however, organic molecules are much more reactive and prone to ulterior transformations. Oxidized metal species (such as the ones used in this work) are expected in carbonaceous chondrite samples, due to the presence of water (i.e., by aqueous alteration). In addition to that, carbonate is widely available in extraterrestrial samples (e.g., as carbonate minerals), therefore being one of the major contributors to the alkaline media in parent-body aqueous alteration. It is important to note that this \"degradation model\" is oversimplified and that other aspects should be accounted for. Despite being more effective than thermal or radiative energy, mechanochemical energy (impact/shock events) is not strictly the main energy source. Mechanochemical, as well as shock/impact experiments, have accomplished the formation of molecules detected in extraterrestrial samples, such as cyclo(glycolglycine) \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{3}{*}{**Nucleobase**} & \multicolumn{6}{c}{**Concentration (ppb)**} \\ \cline{2-7}  & \multicolumn{6}{c}{**Murchison**} & \multirow{2}{*}{**Tangish Lake *} & \multirow{2}{*}{**Murray *} & \multirow{2}{*}{**Murchison Soil *} \\ \cline{2-7}  & \#1 ** & & & & & & \\ \hline Adenine & 15 & 1 & 7 & 5 & 6 & 3 & 40 \\ Guanine & 72 & 6 & 34 & 56 & 2 & 25 & 141 \\ Cytosine & 4 & 4 & 4 & n.d & 5 & 2 & 30 \\ Uracil & 15 & 1 & 7 & n.d & n.d & n.d & \textless{}138 \\ Thymine & 5 & 1 & 3 & n.d & 5 & n.d & \textless{}19 \\ Xanthine & 39 & 3 & 18 & 60 & 0.4 & 3 & 1 \\ Hypoxanthine & 24 & 1 & 11 & 26 & 1 & 1 & 2 \\ Purine & 0.4 & 0.02 & 0.2 & 3 & n.d & 0.2 & n.d \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Nucleobase concentration (part per billion, ppb) for Murchison, Tangish Lake, and Murray meteorites. Adapted from [PERSON] et al. [8]. (i.e., produced by the dimerization of glycine) [27; 54], and several amino acids [22; 54]: hence, extending these experiments could reveal new pathways for the interconversion of extraterrestrial organic matter. As stated before, the direct condensation of ribose with nucleobase (found on carbonaceous chondrites) has not yet been achieved [2]; hence, these results could contribute to explaining the gap in the pre-biological synthesis of RNA and the organic molecules found in extraterrestrial samples, suggesting that prebiological syntheses of ribonucleosides could have occurred by steps [3] and more complex prebiological molecules could be synthetized outside of Earth. Conceptualization, G.P.M. and J.A.L.d.S. ; X-Ray, V.A. and A.G.; Mechanochemistry, G.P.M., V.A. and A.P.C.R. ; HPLC analysis: G.P.M. and P.F.P. ; investigation: G.P.M., A.J.C. and C.G. ; writing--original draft preparation, G.P.M. and J.A.L.d.S. ; writing--review and editing, G.P.M., J.A.L.d.S., V.A., C.G., A.G., A.P.C.R., P.F.P. and A.J.C. All authors have read and agreed to the published version of the manuscript.