February 23, 2021
Of all the planets in our solar system, ancient Venus’ climate is believed to have been most similar to Earth’s when early, microbial life originated. Scientists think that, like Earth, Venus once contained an ocean of water — a necessary ingredient for life as we know it — that may have persisted for some 2–3 billion years. The two planets’ climatic trajectories would then diverge: Earth would retain its watery ocean and become host to a variety of higher lifeforms, while Venus, closer to the Sun, would dry up, its surface too hot to support living organisms.
The planets’ early parallels raise the tantalizing question of whether life ever arose on Venus. Last year’s possible discovery in the Venusian atmosphere of the chemical phosphine — a potential marker for life — now ups the ante. If microbial life did arise, does it still exist, if not on the planet’s scorching surface then perhaps in its sulfuric acid clouds? A new study led by the Johns Hopkins Applied Physics Laboratory (APL) in Laurel, Maryland, provides a framework for estimating that chance.
The Venus Life Equation calculates the probability that life currently exists on the second planet from the Sun by focusing on the likelihood of each of three factors: origination, robustness and continuity. Origination represents the chance that life ever began and gained a foothold on the planet; robustness is the odds that the amount and variety of life that developed was great enough to withstand dramatic climatic events; and continuity is the likelihood that conditions supportive of life have persisted continuously from its origins to present day.
Each factor is given a value from zero to one, with zero meaning there’s no chance and one signifying a certainty. The three factors are multiplied to produce an overall zero to one estimated probability for life.
The study was published Jan. 28 in the journal Astrobiology.
The Venus Life Equation is a subset of the famous Drake Equation, developed by the American astronomer Frank Drake, that provides a framework for estimating the number of intelligent civilizations in our Milky Way galaxy. “I’ve gone deep down into the factor of the Drake Equation that estimates the number of life-supporting planets and zeroed in on the factors particular to Venus,” said APL planetary scientist and lead author of the study Noam Izenberg. “Based on the current state of the science, we argue that there is a nonzero chance that Venus harbors life. But this study is really a jumping-off point for recognizing all the gaps of knowledge that need to be filled to arrive at a more meaningful probability.”
For the origination factor, there are two possibilities for how life emerged on Venus: It either arose on its own from nonliving matter or was transported there from elsewhere. That Venus is thought to have once had a similar climate to Earth, replete with a watery ocean, opens up the possibility that life could have begun there independently. Yet the transport theory is also in play.
It is plausible that life from Earth or elsewhere could have traveled to Venus on meteorites produced by planetary impacts that were more common during the early eras of the solar system. According to the study, the likelihood values for both considerations are subject to change based on further investigation. For example, more research into both the chemistry and climate of early Venus and the survivability of life stowed on meteorites over hundreds or thousands of years of travel through space would improve the accuracy of those estimates.
If life did take hold on Venus, the second factor, robustness, assesses whether the quantity and variety of life was substantial enough to withstand dramatic shifts in climate. Earth is the measuring stick for robustness because not only is it the only planet known to contain living organisms, but it also contained enough sheer biomass and diversity of living things that survive today. Venus’ possible early environment comparable with Earth’s raises the notion that it could have also harbored a vast amount and diversity of microbial life in its oceans. Yet the actual size of the ancient Venusian ocean — key to understanding just how much life may have emerged — is not currently known with any degree of certainty. Similarly, much remains unknown about a number of chemical and geological processes that also figure into the life equation.
Uncertainty extends to whether there was and could still be life in Venus’s sulfuric acid clouds, as evidenced by last year’s detection of the chemical phosphine, which suggests the possible presence of living organisms. The paucity of research on the possibility of earthly organisms that spend their entire lifecycles in the atmosphere, specifically the sulfate layer of the stratosphere, Earth’s closest analog, makes this an area ripe for more investigation.
“We need more field investigations to understand Earth’s variety of life better, especially in the atmosphere and other extreme niches,” Izenberg said. “And we need to understand Venus better. What are the constituents in its cloud layers? Do they have the resources that are required to support life?”
The third and final factor in the Venus Life Equation captures the necessity of an unbroken continuity of habitat from the origination of life up to the present. It is the factor that poses the greatest uncertainty because so little is known still about the planet’s geologic past and its chemical makeup. One key question is whether there was any or enough overlap between the evaporation of the oceans and the formation of the current sulfuric cloud cover for life to have adapted from surviving in one ecosystem to another. The shorter the overlap, the less likely that it happened.
“We don’t know enough about Venus’s evolution as a planet,” Izenberg said. “But that’s something we can discover if we learn more about its history through direct observation.”
New missions to Venus would illuminate the planet’s history.
Surveying the surface geology would provide a better estimate of ocean formation and evaporation. Measuring isotopes of gases would also inform the overall history of water on the planet and go a long way toward better understanding whether there is adequate circulation of materials into the atmosphere that may be necessary for the survival of microbes. Or to aim for the bull’s-eye, verifying a biological source for phosphine would settle the question of continuity — and the overall question of whether life currently exists — in one fell swoop.
“As human beings, we are really interested in whether life exists elsewhere,” Izenberg said. “We now know of a once-watery world possibly containing a chemical signature for life in its atmosphere, and now we also have a framework for working out the odds. Hopefully, this points to more renewed interest in Venus followed by missions to put the equation to more meaningful use.”
Media contact: Jeremy Rehm, 240-592-3997, Jeremy.Rehm@jhuapl.edu
The Applied Physics Laboratory, a not-for-profit division of The Johns Hopkins University, meets critical national challenges through the innovative application of science and technology. For more information, visit www.jhuapl.edu.