Revisiting the cosmic-ray induced Venusian radiation dose in the context of habitability (1911.12788v1)

Abstract: The Atmospheric Radiation Interaction Simulator (AtRIS) was used to model the altitude-dependent Venusian absorbed dose and the Venusian dose equivalent. For the first time, we modeled the dose rates for different shape-, size-, and composition-mimicking detectors (phantoms): a CO$_2$-based phantom, a water-based microbial cell, and a phantom mimicking human tissue. Based on a new model approach, we give a reliable estimate of the altitude-dependent Venusian radiation dose in water-based microorganisms here for the first time. These microorganisms are representative of known terrestrial life. We also present a detailed analysis of the influence of the strongest ground-level enhancements measured at the Earth's surface, and of the impact of two historic extreme solar events on the Venusian radiation dose. Our study shows that because a phantom based on Venusian air was used, and because furthermore, the quality factors of different radiation types were not taken into account, previous model efforts have underestimated the radiation hazard for any putative Venusian cloud-based life by up to a factor of five. However, because we furthermore show that even the strongest events would not have had a hazardous effect on putative microorganisms within the potentially habitable zone (51 km - 62 km), these differences may play only a minor role.

• The paper employs 3D AtRIS simulations to model altitude-dependent radiation doses, revealing that GCR protons and alpha particles contribute approximately 75% and 20% respectively.
• The paper finds that traditional CO2-based phantoms can underestimate radiation exposure by up to 60% for water-based life in Venus’ habitable zone.
• The paper assesses solar energetic particle events and concludes that, despite temporary dose spikes, such events are unlikely to pose lethal risks to radiotolerant organisms.

An Analysis of Cosmic-ray Induced Radiation Effects on Venusian Habitability

This paper presents a thorough investigation into the cosmic-ray (CR) induced atmospheric ionization and radiation dose experienced in the Venusian atmosphere and its implications for habitability. Employing the Atmospheric Radiation Interaction Simulator (AtRIS), the authors have modeled altitude-dependent absorbed doses, focusing on different phantoms representative of terrestrial life. The paper's relevance is underscored by its evaluation of the impacts of both galactic cosmic rays (GCRs) and solar cosmic-ray events on the habitable zone of Venus.

The research builds upon the well-established mechanism of cosmic-ray interaction with planetary atmospheres that results in atmospheric ionization and the potential formation of secondary particle showers. In this context, previous models assessing these effects on Venusian biosignature have predominantly considered CO$_2$-based phantoms, omitting precise evaluations of potential biological radiation hazards to water-based organisms. This paper addresses this gap by modeling three distinct phantoms: a CO$_2$-based phantom, a water-based microbial cell, and a phantom mimicking human tissue.

Significant findings include the critical role of GCR protons and alpha particles in the Venusian radiation environment within the potential habitable zone (51 km - 62 km altitude). Specifically, the paper reports that these particles contribute approximately 75% and 20% to the absorbed radiation dose, respectively. In contrast, heavier GCR nuclei (Z>2) contribute less than 10% and can be essentially disregarded in this context. This highlights the importance of focusing on GCRs' lighter constituents when assessing radiation risks to life.

The modeled results suggest that the previous estimations, which were based on Venusian-air phantoms, potentially underestimate the absorbed doses received by water-based life by up to 60%. The AtRIS models indicate a significantly higher radiation hazard within the cloud deck, particularly for water-reliant biota, suggesting these past models omitted critical variables that might influence the presence and persistence of life.

Furthermore, this research evaluates the episodic influence of solar energetic particle (SEP) events, most notably ground-level enhancements (GLEs), on the Venusian dose rates. Through simulations of historic GLEs and extremal solar phenomena such as the Carrington event and the AD775 event, the paper argues that, while these could temporarily augment radiation to levels two orders of magnitude higher than the GCR baseline, they are unlikely to have posed significant lethal radiation threats to potential life forms, given their occurrence rate and the resilience of radiotolerant organisms.

The paper's use of detailed 3D simulations marks an advancement in understanding planetary radiation environments, moving beyond traditional scalar approximations. By articulating discrepancies in existing methodologies, this research represents a critical appraisal that will guide future explorations into not only Venusian microbiology but also astrobiology on other CR-exposed planetary bodies.

Finally, the implications of this research extend to recognizing the effect of potent stellar flares on exoplanetary atmospheres, as inferred from observational parallels in stellar astronomy. Future steps may involve richer simulation frameworks incorporating temporal variations in atmospheric conditions and particle flux to model more accurately the episodic exposure and adaptive resilience of life in extraplanetary environments.

This work thus provides a rigorous scientific basis for reassessing the radiation conditions on Venus, essential for astrobiological studies aimed at understanding life's potential beyond Earth.