Could Life Exist in Water Droplet Worlds in Venus’ Atmosphere?

It’s a measure of human ingenuity and curiosity that scientists debate the possibility of life on Venus. They established long ago that Venus’ surface is absolutely hostile to life. But didn’t scientists find a biomarker in the planet’s clouds? Could life exist there, never touching the planet’s sweltering surface?

It seems to depend on who you ask.

We’ll start with phosphine.

Phosphine is a biomarker, and in 2020, researchers reported the detection of phosphine in Venus’ atmosphere. There should be no phosphine because phosphorous should be oxidized in the planet’s atmosphere. According to the paper, no abiotic source could explain the quantity found, about 20 ppb.

Subsequently, the detection was challenged. When others tried to find it, they couldn’t. Also, the original paper’s authors informed everyone of an error in their data processing that could’ve affected the conclusions. Those authors examined the issue again and mostly stood by their original detection.

At this point, the phosphine issue seems unsettled. But if it is present in Venus’ atmosphere and is biological in nature, where could it be coming from? Venus’s surface is out of the question.

That leaves Venus’ cloud-filled atmosphere as the only abode of life. While the idea might seem ridiculous at first glance, researchers have dug into the idea and generated some interesting results.

In a new paper, researchers examine the idea of microscopic life that lives and reproduces in water droplets in Venus’s clouds. The title is “Necessary Conditions for Earthly Life Floating in the Venusian Atmosphere.” The lead author is Jennifer Abreu from the Department of Physics and Astronomy, Lehman College, City University of New York. The paper is currently in pre-print.

“It has long been known that the surface of Venus is too harsh an environment for life,” the authors write. “Contrariwise, it has long been speculated that the clouds of Venus offer a favourable habitat for life but regulated to be domiciled at an essentially fixed altitude.” So, if life existed in the clouds, it wouldn’t be spread throughout. Only certain altitudes appear to have what’s needed for life to survive.

The type of life the authors envision aligns with other thinking about Venusian atmospheric life. “The archetype living thing <being> the spherical hydrogen gasbag isopycnic organism,” they state. (Isopycnic means constant density; the other terms are self-explanatory.)

Here’s how the authors think it could work.

Venus is shrouded in clouds so thick we can only see the surface with radar. The clouds reach all the way around the globe. The cloud base is about 47 km (29 miles) from the surface, where the temperature is about 100 C (212 F.) At equatorial and mid-latitudes, they extend up to a 74 km (46 miles) altitude, and at the poles, they extend up to about 65 km (40 miles.)

The clouds can be subdivided into three layers based on the size of aerosol particles: the upper layer from
56.5 to 70 km altitude, the middle layer from 50.5 to 56.5 km, and the lower layer from 47.5 to 50.5 km. The smallest droplets can float in all three layers. But the largest droplets, which the authors call type 3 droplets with a radius of 4 µm, are only present in the middle and lower layers.

“It has long been suspected that the cloud decks of Venus offer an aqueous habitat where microorganisms can grow and flourish,” the authors write. Everything life needs is there: “Carbon dioxide, sulfuric acid compounds, and ultraviolet (UV) light could give microbes food and energy.”

Because of temperature, life in Venus’ clouds would be restricted to a specific altitude range. At 50 km, the temperature is between 60 and 90 degrees Celsius (140 and 194 degrees Fahrenheit). The pressure at that altitude is about 1 Earth atmosphere.

There’s a precedent for life existing in the clouds. It happens here on Earth, where scientists have observed bacteria, pollen, and even algae at altitudes as high as 15 km (9.3 miles.) There’s even evidence of bacteria growing in droplets in a super-cooled cloud high in the Alps. The understanding is that these organisms were carried aloft by wind, evaporation, eruptions, or even meteor impacts. But there’s an important difference between Earth’s and Venus’ clouds.

Earth’s clouds are transient. They form and dissolve constantly. But Venus’ clouds are long-lasting. They’re a stable environment compared to Earth’s clouds. In Earth’s clouds, aerosol particles are sustained for only a few days, while in Venus’ clouds, the particles can be sustained for much longer periods of time.

Add it all up, and you get stable cloud environments where aerosol particles can sustain themselves in an environment where energy and nutrients are available. The researchers say that though eventually aerosol particles and the life within them will fall to the surface, they have time to reproduce before that happens.

The idea of a microbial life cycle in Venusian clouds was developed by other researchers in their 2021 paper “The Venusian Lower Atmosphere Haze as a Depot for Desiccated Microbial Life: A Proposed Life Cycle for Persistence of the Venusian Aerial Biosphere.”

There are five steps in Venus’s proposed cloud lifecycle:

1. Dormant desiccated spores (black blobs) partially populate the lower haze layer of the atmosphere.
2. Updrafts transport them up to the habitable layer. The spores could travel up to the clouds via gravity waves.
3. Shortly after reaching the (middle and lower cloud) habitable layer, the spores act as cloud condensation nuclei, and more and more water gathers into a single droplet. Once the spores are surrounded by liquid with the necessary chemicals, they germinate and become metabolically active.
4. Metabolically active microbes (dashed blobs) grow and divide within liquid droplets (shown as solid circles in the figure). The liquid droplets continue to grow by coagulation.
5. Eventually, the droplets are large enough to settle out of the atmosphere gravitationally; higher temperatures and droplet evaporation trigger cell division and sporulation. The spores are smaller than the microbes and resist further downward sedimentation. They remain suspended in the lower haze layer (a depot of hibernating microbial life) to restart the cycle.

In this new work, the researchers focus on time.

“One of the key assumptions of the aerial life cycle put forward in Seager et al. 2021 is the timescale on which droplets would persist in the habitable layer to empower replication,” the authors write. “It is this that we now turn to study.”

The authors used E. Coli generation times under optimal conditions in their work. In aerobic and nutrient-rich conditions, E. Coli can reproduce in 20 minutes. So, the E. Coli population will double three times in one hour. Bacteria must reproduce faster than they fall to the surface to sustain itself. They need to form a colony.

The researchers calculated that to sustain itself, the time it takes for bacteria to fall from the habitable part of the atmosphere to the inhabitable has to be longer than half an Earth day. As droplet size increases, the droplets would begin to sink. “As the droplet size approaches 100 µm, the droplets would start sinking to the lower haze layers,” they explain. However, their detailed calculations show that reproduction outpaces the fallout rate.

According to the team’s work, a population of bacteria could sustain itself in Venus’ clouds.

There are, obviously, still some questions. How certain are we that nutrients are available? Is there enough energy? Are there updrafts that can loft spores into the right layer of the atmosphere?

But the real big question is how was this all set in motion?

“An optimist might even imagine that the microbial life actually arose in a good-natured surface habitat, perhaps in a primitive ocean, before the planet suffered a runaway greenhouse, and the microbes lofted into the clouds,” the authors write. If that’s the case, this unique situation arose billions of years ago. Is there any other possibility? Could life have originated in the clouds?

Much scientific investigation into Venus, phosphine, clouds, and life relies on scant evidence. Few are willing to go out on a limb and proclaim that Venus can and does support life. We need more evidence.

For that, we have to wait for missions like the Venus Life Finder Mission. It’s a private mission being developed by Rocket Lab and a team from MIT. Who knows what VLF and other missions like DAVINCI and VERITAS will find? Stronger evidence of phosphine? Better data on Venus’ atmospheric layers and the conditions in them?

Life itself?