Enigmatic tubular features in impact glass: REPLY

Abstract

We thank McLoughlin and Grosch (2014) for their Comment, which provides us with the opportunity to expand upon issues surrounding the concept of biogenicity. In our manuscript, "Enigmatic tubular features in impact glass" (Sapers et al., 2014), we present morphological and geochemical evidence suggesting a biogenic origin for tubule features hosted in meteorite impact glass. McLoughlin and Grosch argue that a biogenic origin is unsupported by the data, and that astrobiological implications are unwarranted. In their Comment, they raise three main issues: (1) additional possible abiotic mechanisms; (2) inappropriate comparison between volcanic and impact glasses; and (3) tenuous, or incomplete evidence for endogenicity.McLoughlin and Grosch suggest the products of shock metamorphism could account for the tubules specifically citing ballen silica presented in Ferrière et al (2009). Ferrière et al. (2009 figure 2) present a shocked silica clast containing ballen α-cristobalite with (open) fractures/veinlets of, in part, altered coesite. These features differ from the Ries tubules in (1) appearance-ballen silica are not tubular but approximate spheroidal bodies that interpenetrate each other; and (2) composition-ballen textures occur exclusively in pure silica; the Ries tubular features occur in chemically complex glasses. McLoughlin and Grosch also suggest ambient inclusion trails as a possible abiotic mechanism. This hypothesis can be discounted due to the absence of the main diagnostic criteria (see McLoughlin et al., 2010): terminal inclusions, longitudinal striae, and consistent rhombohedral cross sections. Finally, McLoughlin and Grosch correctly point out that not all of the tubules depicted in our paper are clearly associated with fractures or vesicles. This is expected, as the process of petrographic sectioning commonly truncates the three-dimensional distribution of the tubules, an artifact that is also clearly depicted and addressed by Fisk and McLoughlin (2013, their figure 16a).Analog studies are fundamental to the study of astrobiology. The biogenicity criteria established by McLoughlin et al. (2007) necessitates proxy comparisons; i.e., to demonstrate homology to biological morphology and behavior. As such, the comparison between volcanic and impact glasses for the purpose of comparing alteration textures is valid. Not only are both amorphous substrates, they are also both produced by the quenching of a super-cooled lithic melt; it is only the process leading to the initial melting that differs. We are surprised by the suggestion that we have misapplied the recommendations from Fisk and McLoughlin (2013), as they explicitly state: "The atlas is intended as an illustrated guide for geologists, microbiologists, and astrobiologists studying glass alteration" (p. 317). Within this atlas, an example of alteration from a terrestrial rhyolitic tuff (Fisk et al., 1998) is discussed in the context of abundance and distribution of submarine volcanic glass alteration features. Considering the precedent in the lit...

Critically testing olivine‐hosted putative martian biosignatures in the Yamato 000593 meteorite—Geobiological implications

“…In this study, we argue that impact‐induced brecciation and fracturing in a martian impact crater is responsible for the microtextures found in olivines of Y000593, and this leads us to consider the prevalence of such processes in terrestrial impact sites. For example, in a previous discussion concerning enigmatic tubular microtextures found in impact glasses of the 14.6 Ma Reis impact structure, which were compared to terrestrial bioalteration textures of argued microbial origin by Sapers, Osinski, Banerjee, and Preston (), Sapers, Banerjee, Osinski, Preston, and Ferrière (), Sapers, Banerjee, and Osinski (), it was suggested that shock‐related processes needed to be more fully explored as an alternative origin for these microtextures and associated organics (McLoughlin & Grosch, ; Sapers, Banerjee, et al, ; Sapers, Osinski, et al, ). This current study reasserts that shock‐related processes can generate complex microalteration textures in impact rocks, albeit on Mars rather than Earth and that there are a range of abiotic processes for generating and redistributing organics within rocks of the impact site.…”