The planet Venus is fascinating, terrifying, ironic, beautiful, deadly and — just possibly — alive with organisms.
In many ways, Venus shows us an Earth that could have been; the twin that grew up in a different environment. As one of the few other rocky worlds in our solar system with a thick atmosphere and active volcanoes, Venus can help scientists on Earth learn valuable lessons on global carbon balance, ozone depletion and acid rain. Its wrinkly highlands probably conceal chapters in the origin story of life in the cosmos.
It is faster and easier to get to Venus than Mars. Despite Venus' hellish surface conditions, it may be practical for humans to explore — and someday settle — the second planet from the sun. Yet no active spacecraft have been sent to its surface since the Soviet Union's Vega 2 mission in June 1985.
As long as there have been human imaginations, Venus has inspired them. A feminine presence in the mythology of ancient Europe and the Middle East, and a masculine spirit in Mesoamerica, the duality of the "Morning/Evening Star" has launched wonder and inquiry about the universe. Interviewing 10 planetary scientists for our documentary film "Venus: Death of a Planet" and its follow-on series "Exploring Venus," we became entranced with the planet's promise. For limited time, Space.com readers can view these films for free at MagellanTV.
Earth and Venus appear to have assembled themselves out of matching materials, very near to one another in the protoplanetary disc of our early solar system. They are nearly identical in mass, gravitation and size; Earth is just 396 miles (638 kilometers) larger in diameter. As of this writing, astronomers have cataloged and confirmed 4,201 exoplanets. But the alien world that most closely resembles Earth's chemical composition, mass, diameter and gravity is actually the planet that orbits nearest to Earth.
"Everything points to Venus and Earth having been twins," planetary scientist David Grinspoon says in our film. "There's a lot of circumstantial evidence that Venus had a more Earthlike environment when it was young. They may have both had warm oceans and all the other conditions necessary for an origin of life at the time when Earth, apparently, had an origin of life."
But not anymore: The Soviet Venera and Vega landers of the 1970s and 1980s recorded temperatures around 864 degrees Fahrenheit (462 degrees Celsius) and pressures equivalent to being submerged 3,051 feet (930 meters) in an ocean on Earth. Studying when and how Venus and Earth diverged so radically from their initial similarities could be essential for sensibly managing our planet in this Anthropocene age.
Venus has been called the poster child for the greenhouse effect. Measurements of the ghastly conditions at its surface have alerted — and will continue to inform — Earthly climate science. Spoiler Alert: Humans will not succeed in liberating enough carbon to bring about a Venus-like hell on Earth. But, as an outlier in planetary climate data sets, Venus data calibrate and refine models of global warming upon which critical geopolitical decisions will be made for decades to come.
Understanding the cause of — and cure for — the so-called "ozone hole" over Antarctica arose directly from research into chlorine reactions observed in Venus' atmosphere. The worst acid rain in the solar system falls through Venus' heavy carbon dioxide air, where sulfuric acid solution replaces water as the cycling fluid. In our short film "Venus: Warnings of a Doomed Planet," scientists detail the lessons learned from comparing these macroclimates.
"We need to understand the molecular fossils that are left in Venus' atmosphere: the noble gases of argon, neon, xenon, krypton," Lori Glaze, Director of NASA's Science Mission Directorate's Planetary Science Division, says in our film. "Those gases don't react with anything else."
So, they carry a record of how fast the early Venus atmosphere escaped to space. In particular, Venus' ratio of deuterium (an isotope of hydrogen that makes "heavy water") to hydrogen today is roughly a hundred times higher than Earth's, which scientists interpret as evidence that Venus had oceans of water in the ancient past.
What washes over Venus now? Frozen waves of lava. "Most of the surface of Venus is actually covered by volcanoes," Rosaly Lopes, a planetary volcanologist at NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California, says in the film. "We think that the volcanism on Venus happened fairly recently. There's like this real zoo of volcanoes."
Some, like shield volcanoes, resemble their counterparts on Earth. Others are like nothing else in the solar system: ones with flat tops that researchers call "pancake domes," some with both radial and concentric fractures resembling spiderwebs and so nicknamed "arachnoids," others called "ticks" with blobby bodies and spiky, leg-like slopes, and widespread tiny cinder cones, some no bigger than a house.
At the other end of the volcanic size range are huge caldera-like circles, with concentric cracks, called "coronae." Oddly, at the edges of the larger coronae, some sections of Venus' crust seem to be diving under others. But Venus doesn't have the kind of crustal plates geologists see on Earth. "Why do we see evidence of subduction, yet it hasn't developed into plate tectonics?" asks Suzanne Smrekar, a planetary geophysicist at JPL, in our film. "[Perhaps] the lithosphere is too hot, and that prevents the huge faults that define the edges of the plate from being maintained over a long term. Another idea is that we need surface water to help lubricate those faults and allow plates to slip past each other."
With no tectonic means to gradually vent its internal heat, Venus may undergo spasms of planet-wide eruptions. "The vast majority of Venus' surface is geologically very young," says Anthony Del Genio, Research Scientist at NASA's Goddard Institute for Space Studies in New York City. "Something happened, 500 million to a billion years ago, that created tremendous amounts of volcanism and essentially overwrote most of Venus' surface."
Left standing above the global lava fields are creased and furrowed highlands called "tesserae" that may hold confirmation of ancient oceans. "That's where we want to go," says planetary geoscientist Martha Gilmore. "These rocks are the only rocks from the first 80% of the history of Venus. Were there evidence of ancient environments that would support life? Whatever the science is to form life on Earth, everything that we know about chemistry, about physics, about geology, suggests that those conditions were there on Mars and Venus too."
Gathering evidence of past life on Venus will not be simple. Mission planners are contemplating an armada of orbiters, balloons, landers and rovers. Our short film "Venus: Doing Science in Hell" explains the engineering challenges and some promising solutions.
But the biggest challenge in finding evidence of past life on Venus may be to assemble a critical mass of public and governmental support for big Venus missions.
There's no evidence of premeditated sedition against Venus in the science or space agency communities. Yet Mars, despite its smaller mass, clearly exerts stronger gravitational influence on policy and program planning. Why?
More Mars data is available, drawing more graduate students. Thus, more careers are building around the Red Planet. It's harder to land on Mars but much easier to do surface science: Optics and electronics much prefer the cold, thin Martian atmosphere to the viscous blast furnace of Venus. If we "follow the water," we won't find any on the landscape of Venus.
We've seen a lot more of Mars' surface and it looks like Earth; so much so that lay observers easily forget its fundamental unfriendliness to biology. Visiting — and populating — the landscape of Mars is going to be a lot more challenging for humans than those inspirational illustrations from SpaceX, NASA and others make it look. Still, a certain Martian chauvinism persists.
Search for life
In the quest for other bioworlds, Venus offers more frequent optimum launch windows and shorter trip time than Mars; much shorter than to Jupiter's moon Europa or Saturn's moons Enceladus or Titan. But can anything live on Venus? Likely not on the superhot surface! But rise up 25 to 45 miles (45 to 70 km) into the Venus clouds and you'll notice the temperature and pressure drop to that of sea level on Earth. And there's an anomaly floating in these temperate altitudes.
For more than 100 years, astronomers have photographed dark patches appearing in the ultraviolet region of the spectrum. "These patches are because something is absorbing sunlight in the clouds of Venus,' says Sanjay Limaye, a senior scientist at the Space Science and Engineering Center of the University of Wisconsin-Madison.
"Some of the properties of terrestrial bacteria mimic the spectral absorption that we see on Venus. And we hypothesize that maybe there are some microorganisms, given the fact that Venus maybe had liquid water. It had all the conditions to evolve life," Limaye added. Researchers have found bacteria in the clouds of Earth, as well as every other Earthly environment ever checked.
There's more discourse on this question, from Limaye and other scientists, in our short film "Is There Life in the Clouds of Venus?" on MagellanTV.
The cloud-climate appears to shift on a scale of decades. The dark absorption patches ebb and flow. And this "living mist" concept is not new: In 1967, biologist Harold Morowitz and astronomer Carl Sagan speculated on the possibility of cloud-borne biota floating over Venus. If they are there, it should be a simple matter to find them. No landing is required, just a mission to the cool cloud tops.
Science on the wind
"Venus winds are extremely fast," says Limaye. "On Earth it'll take you a month to fly around the Earth, whereas, on Venus, the clouds go around every four to five days and even less time than that at a higher latitude." In 1985, the Soviet Vega 2 balloon probe rode these turbulent hurricane-force thermal currents to travel more than 6,900 miles (11,100 km) at an average speed of 150 mph (240 km/h). It found the clouds at 33 miles (53 km) altitude to be warm; about 100 degrees F (38 degrees C).
Imagine what might be discovered using materials and electronics developed over the past 35 years. Geoffrey A. Landis, engineer and scientist at NASA's Glenn Research Center in Cleveland, Ohio believes we can do better than just floating at the mercy of the winds. Picture a semi-buoyant electric airplane, dispatched into the cloud layer. Sunlight, bouncing around the clouds, scatters in every direction so the plane can employ solar panels on many surfaces. In addition to analyzing the atmosphere — perhaps searching for microbial life — this flying platform can serve as a relay station, in contact with landers or rovers on the surface and with orbiters overhead.
"Most of the processing power, most of the computers, most of the things that run the mission would be in an airplane, that's flying 50 kilometers above, or maybe in a satellite," says Landis in our film. "And it controls the probe on the surface, almost like you'd be controlling a radio-controlled car."
Northrop Grumman Corporation's Venus Atmospheric Maneuverable Platform(VAMP) proposes a dirigible flying wing, plying the atmosphere. NASA's High Altitude Venus Operational Concept (HAVOC) envisions the beginnings of crewed sorties to the temperate zones of Venus' atmosphere.
"You could float habitats in the atmosphere of Venus," says Landis, "And the habitats could be very large. They could be kilometers in scale. You wouldn't even need hydrogen or helium. Because the atmosphere of Venus is mostly carbon dioxide, oxygen and nitrogen — ordinary breathable air — would float. The air that's holding you up is also the air that you can breathe. The lifting gas is your environment."
"I love the idea of a human crewed mission to a cloud city on Venus," says Jonathan Sauder, senior mechatronics engineer at JPL. "You would just need to wear some type of suit that would provide you with oxygen to breathe as well as protection from the chemical air. But you wouldn't necessarily need a pressure suit."
Sauder, however, also sees the downside: "Humans tend to not like the idea of not being able to be on firm ground. And the idea that you have to stay floating above this furnace essentially, in some ways is a hard sell!" It also requires a very powerful rocket to launch back out of the atmosphere — and to pull away from Venus' Earth-level gravity — for your return to our planet.
As long as we're stretching our imaginations into advanced technologies, what would it take to break the planet's global greenhouse and let the heat out? On Earth, rain pulls carbon out of the sky and into the crust, then a few active volcanoes release CO2 back into the air; a (usually) healthy balance between what scientists call "sources and sinks."
"If you wanted to scrub the Venus atmosphere, you would need an ocean, and you would need weathering," says Gilmore. "Then the carbon dioxide in the atmosphere can link with calcium and form rock. That's what sequesters our CO2. Four and a half billion years ago, Earth's atmosphere was also CO2 rich; that is the original atmosphere of the terrestrial planets."
Over the border between currently practical engineering and science fiction lies the idea that we may, someday, be able to remodel the entire second planet to be more like the third. Terraforming — the transformation by technology of an object to have Earth-like climate characteristics — can be an entertaining mental exercise. But is it remotely realistic for Venus?
"Well, you know, our more immediate task is to avoid "Venera-forming" Earth right now, says Grinspoon. "However, the mental exercise of imagining how we would terraform another planet is, I think, very valuable for learning how we would manage ourselves on Earth better, because it forces us to ask: How would we interact constructively with the planet?"
For Venus, constructive interaction means collapsing the heat trap, perhaps by dusting the air and getting the CO2 to condense out onto the landscape. To bind it into rock, we would need water; lots and lots of water. "If this was your goal," says Grinspoon, "there are a lot of stray icy objects in the solar system; I would take some large number of them and crash them into Venus."
In 1991, British scientist Paul Birch proposed transporting trillions of tons of hydrogen from the gas giant planets, like Jupiter, to convert atmospheric CO2 to oceans of water plus mountains of graphite. And he suggested shielding Venus from the sun's heat with enormous shade-panels; collecting solar energy while shrinking the atmosphere.
As it did with ancient peoples of Earth, Venus is still stimulating creativity! Some distant day, human ingenuity and machine intelligence may guide Venus onto a path more like the Earth. But in the nearer term, human recklessness could tip Earth's climate toward the present hellish conditions of Venus. Either way, these twin planets will grow more alike. Now, here in the Anthropocene, let's choose wisely.