Samuel H Halim
We investigate the potential for terrestrial material (i.e., terrestrial meteorites) to be transferred to the Moon by a large impact on Earth and subsequently survive impact with the lunar surface, using computer modelling. Due to the near constant geological activity in the history of the Earth, organic and biological markers (biomarkers) are effectively non-existent in the geological record >3.8 billion years ago. Comparatively, the Moon has experienced limited alteration due to processes such as hydrological activity or plate tectonics. Therefore, the Moon could act as a “safe-haven” of sorts and might preserve a record of early Earth which the Earth itself no longer provides. Three-dimensional impact simulations show that a typical basin-forming impact on Earth can eject a small but significant mass of solid fragments at speeds sufficient to transfer from Earth to the Moon. We then use two-dimensional modelling to simulate terrestrial material impacting the lunar surface, tracking both pressure and temperature within the meteorite. Assuming that they survive launch from Earth, we show that some biomarker molecules within terrestrial meteorites are likely to survive impact with the Moon, especially at the lower end of the range of typical impact velocities for terrestrial meteorites (2.5 km/s). The survival of larger biomarkers (e.g., microfossils) is also assessed, and we find limited, but significant, survival for low impact velocity and high target porosity scenarios. Biomarkers within terrestrial meteorites that experience long durations of elevated temperatures will experience greater proportions of mass loss (thermal degradation). Shortly after impact, thermal degradation of biomarkers depends heavily upon where the meteorite lands, whether it is buried or remains on the surface, and the related cooling timescales.
Slide 2: Modelling methods. Pressure and temperature regimes within projectiles were compared to known thermal degradation parameters for some example biomarkers (arginine, valine, glutamine, tryptophan, and lignin), using a modified version of the Arrhenius equation. From this, we estimated a percentage of the original biomarker mass that survives after impact. Pressures and temperatures for a selection of microfossils which have survived in metamorphosed rocks were also used for comparison, including lycophyte megaspores (results displayed in the next slide, Figs. 2 and 3). We know that increasing pressure and temperature in the projectile will lead to less favourable conditions for biomarker survival. Therefore, we can make some broad conclusions from the suite of simulations produced. Biomarker survival potential decreases with increasing projectile porosity, increasing projectile velocity, and decreasing target porosity. We also see that sandstone projectiles experience slightly higher pressures and temperatures vs. limestone.
Slide 3: Results – biomarker survival. Fig. 2 displays post-shock temperature maps (left) and survival of selected biomarkers (right) in the worst- and best-case impact scenarios. Worst-case = 40% porous projectile impacting a solid target at 5 km/s and best-case = Solid projectile impacting a 70% porous target at 2.5 km/s. The fraction of surviving biomarker material in these scenarios is estimated by extrapolating cooling over 100 seconds for a 1 cm diameter fragment whilst radiatively cooling into space. The projectile plots show that in the worst-case scenario, the majority of the projectile reaches temperatures of 2000 K, where lignin is the only biomarker to survive after 100 s post-impact (7% of initial mass). In the best-case scenario, the majority of the projectile is heated to 600 K, with all of the selected biomarkers surviving in some proportion (lignin ≈ 70%, tryptophan ≈ 40%, arginine ≈ 32%, glutamine ≈ 10%, valine ≈ 4%). Fig. 3 displays survival maps for lycophyte megaspores in solid projectiles impacting increasingly porous targets (a = 10%, b = 20%, c = 30% d = 40%) at 2.5 km/s. Lycophytes survive in increasing volumes of projectile material with increasing target porosity, always concentrated at the back of the projectile.
Slide 4: Conclusions: Temperatures are higher than expected in all simulations, with lowest post-shock temperatures recorded being ~600 K. However, significant proportions of some biomarkers are still shown to survive post-impact, especially at lower impact velocities. Lignin and tryptophan survive well in a range of impact scenarios. Lycophyte megaspores survive in part of the impactor during only the most-favourable impact conditions. Long-term biomarker survival is highly dependent on the resulting location and size of ejected projectile fragments.
For further explanation of the modelling process and an expanded set of results, please see our recently published paper at https://doi.org/10.1016/j.icarus.2020.114026.