The Case (or Not) for Life in the Venusian Clouds.

Dirk Schulze-Makuch

doi:10.3390/life11030255

Abstract

The possible detection of the biomarker of phosphine as reported by Greaves et al. in the Venusian atmosphere stirred much excitement in the astrobiology community. While many in the community are adamant that the environmental conditions in the Venusian atmosphere are too extreme for life to exist, others point to the claimed detection of a convincing biomarker, the conjecture that early Venus was doubtlessly habitable, and any Venusian life might have adapted by natural selection to the harsh conditions in the Venusian clouds after the surface became uninhabitable. Here, I first briefly characterize the environmental conditions in the lower Venusian atmosphere and outline what challenges a biosphere would face to thrive there, and how some of these obstacles for life could possibly have been overcome. Then, I discuss the significance of the possible detection of phosphine and what it means (and does not mean) and provide an assessment on whether life may exist in the temperate cloud layer of the Venusian atmosphere or not.

Highlights

Ever since Morowitz and Sagan [1] suggested that life may exist in the lower Venusian atmosphere, many authors looked into that possibility [2,3,4,5,6,7,8,9,10,11] and made incremental progress understanding the environmental conditions in the Venusian atmosphere, and the possibility of it being inhabited by microbial life
The constraints on the required reproduction rate to maintain a constant microbial population in the Venusian clouds would be less limited in the Seager et al model [11], but it would require that the proposed microorganisms can form spores to counter the extreme environmental conditions when sinking into the lower haze layer of the Venusian atmosphere
If these observations are due to abiotic chemistry, how could PH3 be continuously produced in the oxidizing atmosphere of Venus? If they are due to biology, this is: how could life permanently cope with the difficulties of being airborne, and with hyperacidity, extreme lack of water, and a possible lack of critical nutrients? These challenges to life are very high, and Cockell et al [64] argued that there is no good reason to entertain the biological hypotheses

Summary

Introduction

Ever since Morowitz and Sagan [1] suggested that life may exist in the lower Venusian atmosphere, many authors looked into that possibility [2,3,4,5,6,7,8,9,10,11] and made incremental progress understanding the environmental conditions in the Venusian atmosphere, and the possibility of it being inhabited by microbial life. An earlier field study seemed to indicate that bacteria can grow and reproduce in cloud droplets [15] and earlier laboratory studies indicated that limited cell divisions of the facultative anaerobic bacterium Serratia marcescens occurred in an airborne state [16,17] It would not be surprising if Earth’s atmosphere is not utilized as a permanent habitat, because the environmental conditions on Earth’s surface are well-suited for life and any microorganisms in the atmosphere would be deposited back on the surface by precipitation, usually within a few days [18]. A first stock of where we are standing and what should be the steps for Venusian research, about 7 months after the initial announcement

The Environmental Conditions in the Venusian Clouds

Proposed Adaptations of Microbial Life to the Venusian Cloud Environment

The Claimed Detection of Phosphine

Discussion and Next