The search for life on Venus
Is there life on Venus? For more than a century, scientists have pondered this question. Now, there is renewed interest in Venus as a place that could support it.
“We are trying to make the case for exploring Venus and to inspire and inform future missions to collect in situ data with satellites,” says Sanjay Limaye, University of Wisconsin–Madison Space Science and Engineering Center scientist and co-author of a recent collection of papers on Venus that he hopes will do just that.
Limaye is an expert on Venus. With more than 45 years of research behind him, he continues to investigate the planet’s dense atmosphere. “As one of our closest neighbors it is easier to reach Venus compared to other bodies like Titan, Enceladus or even Mars,” he says.
Published in 2021 in the journal Astrobiology, the series of articles targets Venus as a potential home for microbial life like bacteria and other organisms. More than 50 scientists convened in Moscow, Russia for the 2019 Venera-D Venus Cloud Habitability workshop to examine existing research on Venus in order to understand its potential to support life, both today and billions of years ago.
Formed around the same time as Earth, about 4.5 billion years ago, Venus orbits between Mercury and Earth. The planet rotates very slowly on its axis, taking 243 Earth days to complete a single rotation. Its massive atmosphere of mostly carbon dioxide makes Venus a planet of extremes with scorching temperatures, intense winds and volcanic activity.
Despite the features that make the planet’s surface inhospitable, its thick global cloud cover may present gentler conditions for some microbial life forms due to the availability of sunlight, nutrients and some water. All of which can create narrow, but habitable zones like those theorized to exist high in the atmosphere.
The Astrobiology series investigates these aspects and presents reasons to explore whether life may be present, perhaps hidden throughout the planet’s complex clouds. It explores Venus’ history as a potential life bearing planet, its habitable zones in the upper atmosphere and potential signatures of phototrophic activity – organisms that use light as an energy source.
Early formation and water
Venus was not always the pressure cooker it is today. Like Earth, its environment included volcanic activity and liquid water oceans. For nearly 3 billion years these thermal vents and oceans could have harbored microorganisms. But that oasis was lost.
“When life began on Earth the conditions on Venus were likely similar,” says Limaye. “Some modelling suggests water could have existed for 1 billion to 3 billion years. Mars on the other hand, appears to have had liquid water on the surface for a lot less time. The question of whether Mars was cold and dry or warm and wet is still being debated.”
In these early days, the Sun was dimmer but more intense, radiating the planets with high energy particles and ultraviolet rays. Around 3.5 billion years after its formation, Venus began to heat up and over the course of millions of years, its oceans evaporated. The runaway greenhouse with its atmosphere of carbon dioxide and water vapor trapped more of the Sun’s energy, increasing the surface pressure. This process resulted in the dense and hot atmosphere of today, with surface pressures more than 90 times that of Earth and surface temperatures exceeding 486 degrees Celsius (906 F), hot enough to melt lead.
While these conditions are inhospitable to most life on Earth, small and hardy microorganisms known as extremophiles have extraordinary abilities to survive in the worst imaginable places. It is possible these ancient organisms adapted to the changing conditions on Venus and eventually found a new home within its thick atmosphere.
Scientists are skeptical that fossil remains exist on the Venusian surface due to continuous and destructive volcanic activity. However, satellites orbiting the planet offer deep glimpses into its atmosphere and have presented clues of potential life drifting throughout narrow pockets high above.
Since the 1960s, dozens of space missions have examined Venus from all angles. The Pioneer Venus Orbiter and Multiprobe missions in 1978 gave scientists an in-depth look at the top of the atmosphere as well as through its many layers down to the surface. In 1981 the Venera 13 and 14 missions successfully sent back several color panoramas of the surface, information about its clouds, soil and the first ever recording of winds on the planet’s surface. A few years later, VeGa 1 and VeGa 2 balloons and landers recorded more information about the atmosphere, clouds, winds at 54 kilometers and surface composition at the landing sites.
Using data known as spectra – or measurements of how much light is either reflected or absorbed by a planet like Venus – researchers are able to identify chemical signatures such as carbon dioxide, sulfur dioxide and even trace amounts of water vapor. However, some spectral data show areas of strong absorption, creating inexplicable dark patches prominent in the planetary images. With some water, energy and nutrients like carbon, hydrogen, nitrogen, phosphorous and sulfur available in the cloud layer, Limaye and colleagues wondered whether microorganisms could be the source of that solar energy absorption, much like the algal patches in Earth’s oceans or lakes.
“Venus has a potential to harbor conditions for iron- and sulfur-centered metabolism,” says Limaye. “Together, our lines of reasoning suggest that particles in Venus’ lower clouds contain sufficient mass balance to support microorganisms, water, and solutes, and potentially sufficient biomass to be detected by optical methods.”
As the planet’s surface became more inhospitable, microorganisms may have migrated to calmer and less extreme conditions in Venus’ atmosphere. This could explain why microorganisms may be drifting aloft in the Venusian sky. At a range of 48 to 70 kilometers (30 to 43 miles) above its surface, atmospheric pressures are similar to Earth’s and its thick clouds reduce some of the Sun’s damaging ultraviolet radiation, creating milder conditions. Extremophiles on Earth have been found to withstand, and even thrive, in similar conditions.
“It’s not impossible these microorganisms could have migrated to the clouds and sustained life,” says Limaye.
Their peculiar absorption data compel researchers to look more closely at what could produce these signatures that go beyond common atmospheric and chemical reactions. A paper co-authored by Limaye indicates that phototrophic activity is possible in the Venusian atmosphere using sunlight, and also on the night side using the radiation emitted by the lower parts of the atmosphere and surface. They found that the acidity of the cloud droplets can be lower than believed due to the presence of some salts in the droplets.
Harvesting light, harboring life
Planetary atmospheres serve many functions, including the filtration of dangerous radiation. On Earth, our atmosphere (and magnetic field) blocks much of the Sun’s harmful rays like X-rays and strong ultraviolet energy. Plant life on Earth thrives from weaker forms of ultraviolet light, providing fuel for photosynthesis. Limaye and colleagues believe Venus’ atmosphere is capable of producing similarly balanced zones within its atmosphere, where Earth-like levels of radiation are consistent and also protected from destructive X-rays.
“Solar irradiances calculated across Venus’ clouds support the potential for Earth-like phototrophy and that treatment of Venus’ aerosols containing neutralized sulfuric acid favor a habitable zone,” says Limaye. “Therefore, Venus’ light, with limited ultraviolet flux in the middle and lower clouds, is likely quite favorable for phototrophy.”
Given this broad range of suitable atmosphere, it is possible drifting microbes could be responsible for producing some of the strong UV absorbing gases observed by researchers.
“Throughout this series of papers, we focus on finding similar conditions on Earth where we know life thrives, but this isn’t the only approach,” says Limaye. “The search for life is not limited to microbes as we know them on Earth, but there are many possibilities of extremophiles existing and thriving in ways we haven’t imagined.”
Since the 1960s, nearly 50 missions have been launched to study our neighboring planet. Some of them successfully landed on the planet but operated for less than two hours because of equipment failures.
The next decade; however, will herald seven missions to explore Venus and uncover its atmospheric and surface mysteries. Rocket Labs has announced a small probe to look for phosphine to be launched in 2023. The India Space Research Organization Venus orbiter is expected to carry radars and is anticipated to arrive around 2026, followed by NASA’s VERITAS radar orbiter in 2027 and in 2028, NASA’s DAVINCI mission will be launched featuring a dual-purpose spacecraft to collect atmospheric measurements from orbit, in tandem with an atmospheric probe that will sift through the dense atmosphere, collecting data during its descent. The Venera-D mission by the Russian Roscosmos State Corporation for Space Activities, proposed for 2029, will deploy an orbiter and lander designed to withstand the harsh conditions on the planet’s surface and collect valuable atmospheric and ground measurements. It will also include a long-lived small lander to be contributed by NASA. The decade of exploration will end with the European Space Agency’s EnVIsion Radar orbiter, also with contributions from NASA.
Limaye is encouraged that these future missions will ultimately answer many of the observed mysteries on the hostile planet. Another workshop, the Venera-D: Venus Cloud Habitability System Workshop, is scheduled for late November 2021 and will bring together more than 200 scientists and students from 31 countries to continue the conversation about whether there is life on Venus.