We revisit the hypothesis that there is life in the venusian clouds to propose a life cycle that resolves the conundrum of how life can persist aloft for hundreds of millions to billions of years. Most discussions of an aerial biosphere in the venusian atmosphere temperate layers never address whether the life—small microbial-type particles—is free floating or confined to the liquid environment inside cloud droplets. We argue that life must reside inside liquid droplets such that it will be protected from a fatal net loss of liquid to the atmosphere, an unavoidable problem for any free-floating microbial life forms. However, the droplet habitat poses a lifetime limitation: Droplets inexorably grow (over a few months) to large enough sizes that are forced by gravity to settle downward to hotter, uninhabitable layers of the venusian atmosphere. (Droplet fragmentation—which would reduce particle size—does not occur in venusian atmosphere conditions.) We propose for the first time that the only way life can survive indefinitely is with a life cycle that involves microbial life drying out as liquid droplets evaporate during settling, with the small desiccated “spores” halting at, and partially populating, the venusian atmosphere stagnant lower haze layer (33–48 km altitude). We, thus, call the venusian lower haze layer a “depot” for desiccated microbial life. The spores eventually return to the cloud layer by upward diffusion caused by mixing induced by gravity waves, act as cloud condensation nuclei, and rehydrate for a continued life cycle. We also review the challenges for life in the extremely harsh conditions of the venusian atmosphere, refuting the notion that the “habitable” cloud layer has an analogy in any terrestrial environment.

中文翻译：

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金星低层大气雾霾作为干燥微生物生命的基地：金星空中生物圈持久性的拟议生命周期

我们重新审视金星云中存在生命的假设，以提出一个生命周期，以解决生命如何在高空持续数亿到数十亿年的难题。大多数关于金星大气温带大气层中的空中生物圈的讨论从未涉及生命——微生物类的小颗粒——是自由漂浮还是局限于云滴内的液体环境。我们认为，生命必须存在于液滴中，这样才能避免液体向大气中的致命净损失，这对于任何自由漂浮的微生物生命形式来说都是不可避免的问题。然而，水滴栖息地构成了生命周期的限制：水滴不可避免地（在几个月内）生长到足够大的尺寸，在重力的作用下，它们会向下沉降到更热、无法居住的金星大气层。（在金星大气条件下不会发生液滴破碎——这会减小颗粒尺寸——不会发生。）我们第一次提出，生命可以无限期生存的唯一方式是生命周期涉及微生物生命在沉降过程中随着液滴蒸发而变干，随着干燥的小“孢子”在金星大气停滞的低雾层（33-48公里高度）处停留并部分填充。因此，我们将金星下雾层称为干燥微生物生命的“仓库”。孢子最终通过重力波引起的混合引起的向上扩散返回云层，充当云凝结核，并重新水化以继续生命周期。我们还回顾了金星大气极端恶劣条件下的生命挑战，