Hellish Venus poses many mysteries. New spacecraft aim to solve them.

2021-10-07

If we couldn’t see Venus, we’d hardly dare to imagine such a deceptive world.

Named for the Roman goddess of love and fertility, the planet is one of the nighttime sky’s most beautiful sights. But up close, Venus smells like rotting eggs. Its raindrops would dissolve flesh, and at its surface, temperatures are hot enough that wood and gasoline would spontaneously burst into flame, while the pressure is so high that submarines would crumple.

Though today’s Venus is grotesque, scientists suspect it was once quite different—temperate, perhaps awash in oceans, maybe even inhabited. For billions of years, our solar system may have been home to two blue marbles orbiting the sun, side by side. Yet while life on Earth flourished and thrived, a catastrophic amount of carbon accumulated in Venus’s atmosphere, triggering a runaway greenhouse effect that laid waste to our once-verdant sibling. Now Venus is a deadly marble—a world gone wrong.

Or is it?

“I don’t know that Venus went wrong,” says Lori Glaze, NASA’s planetary science division director. “I prefer to say that Earth went right.”

Maybe, instead of a planetary accident, Venus is an ominous portent: the natural end-state of terrestrial planets in similar orbits. Or maybe Venus never was the watery, life-friendly world that some scientists envision.

Three planetary missions, launching over the next decade, will reveal whether Venus ever had liquid oceans, and if so, how expansive they were and how long they lasted. The robotic trio will precisely map the planet’s surface, look for signs of active volcanism, and peer into the world’s interior. By helping us better understand our sister world, these spacecraft will also help us learn whether the millions of rocky planets in Venus-like orbits around other stars could be habitable.

“I really think that Venus is the key to unlocking planetary habitability,” says Stephen Kane of the University of California, Riverside. “We always obsess about habitability, and we forget to ask the question: What makes a planet uninhabitable? That’s what Venus is.”

But some Venusian mysteries can’t be answered by this new fleet of missions. Key questions such as the amount of ongoing seismic activity will require scrutiny by long-lived landers or rovers, machines that must survive the crushing conditions at the planet’s surface. Scientists are already developing the technologies needed to make such ambitious journeys possible, testing electronics and other hardware in chambers that simulate the alien planet’s unforgiving temperatures and pressures.

“I can’t believe I’m even saying there’s going to be a trio of missions sent to Venus,” says Jennifer Whitten of Tulane University, deputy principal investigator for NASA’s VERITAS Venus mission. “We’re going to get so much information, and I think the next step forward is landing on the surface.”

Siblings on different paths

The confoundingly beautiful planet that gleams in our skies has entranced astronomers since they first fixed it in their telescopic sights in the 1600s. And up until the 1960s, scientists had some pretty wild ideas about what might live on the planet next door.

“Textbooks in those days showed Venus as a tropical jungle,” recalls my dad, Frank Drake, a radio astronomer. “It was reasonable. Venus was closer to the sun and it had clouds, and it was thought to be a lot like Earth.”

But after pointing a radio telescope at the planet in 1961 (the same telescope at the Green Bank Observatory he used to conduct humanity’s first scientific search for intelligent aliens the year before), Dad deduced that the Venusian surface was nowhere near tropical—it was roasting at more than 700 degrees Fahrenheit.

Soon after, he found that the planet’s temperature was always set to “broil,” even on the night-side. That was odd, since Venus itself spins staggeringly slowly. One seemingly interminable day on its wretched surface lasts for 243 days on Earth, which is longer than Venus’s 225-day year. The night-side should have been much cooler after roughly four months without sunlight.

“That was a real surprise,” Dad says. “It told us that the atmosphere of Venus must be very heavy. It was so massive you could turn the sun off and it would not change its temperature for 100 days.”

Follow-up observations confirmed Dad’s calculations as well as an independent prediction made by Carl Sagan, and soon it was evident that all was not exactly well on the planet sometimes referred to as Earth’s twin.

Scientists now know that Earth and Venus are round, similar in size, and probably similar in bulk composition, but that’s where the likenesses end.

While Earth is temperate and watery, Venus is searing and parched. Its surface temperature averages 860 degrees Fahrenheit. Its desiccated landscapes are imprinted with the signatures of flowing lava, not watery rivers—and its surface pressure is 90 times that of Earth’s, similar to the crushing force you’d feel some 3,000 feet underwater.

Overhead, Earth’s clouds billow and vanish, but Venus’s sky is perpetually overcast. Forty-five miles of suffocating clouds block all but a smidgen of sunlight. Strong winds blow in the upper layers, and the entire atmospheric shroud whips around the planet.

Whether Venus has always been an infernal world is among the primary questions scientists are hoping to answer in the next decade. If Venus did have oceans—if it was another blue marble for billions of years—then perhaps the planets like Venus orbiting faraway stars could also be temperate worlds. But if Venus was born this way, a lethal product of its composition and proximity to the sun, then all those “exo-Venuses” may be similarly barren.

“Until you understand why and how and when Venus got to be the way it is, we aren’t really all that well equipped to make sense of what we see in other planetary systems,” says planetary scientist Paul Byrne of Washington University in St. Louis.

“My heart wants it to be this tragic story of a world gone wrong. And I want to think that there were not just oceans there, but there were things swimming around in those oceans. Either way, whatever answer we find is going to be profound.”

The Venus revolution

But since the mid-1960s, Mars has gobbled up the majority of funding for U.S. planetary missions. The last time NASA sent a probe to explore Venus was in 1989, when the Magellan spacecraft set out to make a rough radar map of the planet’s surface. Since then, 14 NASA spacecraft have launched toward Mars.

“When I first joined the Venus community, and invited myself to the meetings and whatnot about 10 years ago, they were a very data-starved community,” says Kane, who had previously focused primarily on exoplanets. “They looked with great envy at their Martian colleagues, who just seemed to get all the missions in the world thrown at them, while our maps of Venus are extremely lacking—they’re very, very poor resolution.”

For decades, the Soviets were the planet’s primary explorers. Starting in the 1960s, they hurled dozens of spacecraft at the shrouded world, some destined for a relatively peaceful orbit while others quickly turned to goop on the planet’s punishing surface. In 1975, one of those doomed landers, Venera 9, beamed a few images of the Venusian landscape back to Earth—the first photos taken from the surface of another planet.

“It was just such a completely alien landscape,” Kane says. “I became obsessed with Venus as a child.” In total, four Soviet spacecraft transmitted images during their short-lived perches on the planet, inspiring artists to imagine what a lander must look like on a broiling landscape beneath sickly orange clouds.

Most recently, Japanese and European orbiters have been surveying the planet, and India is developing a new Venus mission, but for the last couple of years Venus has been orbited by just one lonely spacecraft: Japan’s Akatsuki. For many scientists, it has felt like Venus was forgotten, a world that was always eclipsed by the continual search for water—and signs of life—elsewhere.

Not anymore. In June, NASA associate administrator Thomas Zurbuchen stunned the planetary science community when he announced not one, but two new Venus missions as part of the space agency’s Discovery program. The next week, the European Space Agency announced that it, too, would launch a spacecraft to Venus. NASA’s two missions are known by the acronyms DAVINCI and VERITAS, and ESA calls its mission EnVision.

When asked, “Why Venus, and why now?” Zurbuchen said the NASA selection came down to a handful of key factors: new and intriguing science results about Venus’s climate history, a wealth of planets orbiting other stars that are probably exo-Venuses, and mission designs that could accomplish great science within the Discovery program’s limited budget of a few hundred million dollars—the smallest of NASA’s three interplanetary mission categories.

“I like to say that Venus is the new hot planet,” Zurbuchen says, fully aware of the pun. Although a team of astronomers recently announced tantalizing but controversial evidence of phosphine gas on Venus, a possible sign of life, Zurbuchen says the finding didn’t play a role in NASA’s mission selections. None of the new missions are designed to search for phosphine directly, but they will transform what we know about Venus’s past and future.

“We have an opportunity to study Venus and get more than the sum of the parts of those three individual missions,” says Glaze, who began her career as a volcanologist before being bitten by the Venus bug, and who recused herself from the NASA selection process. Together, the spacecraft will create “a fantastic, complete picture of Venus.”

A whole new world

The trio will set sail for Venus later this decade. DAVINCI will likely launch in 2029 and fly by Venus twice before pulling into orbit. The spacecraft will image the planet’s clouds and surface in multiple wavelengths, but the star of the mission is a probe that, over the course of an hour in 2031, will gently descend through the Venusian clouds, taking the first detailed images of the surface from above before crash-landing.

“It’s a spherical probe, kind of the size of a large beach ball, or some would say a small bean bag chair,” says DAVINCI deputy principal investigator Giada Arney of NASA’s Goddard Space Flight Center. If the probe survives its descent, it could operate for up to 17 minutes on the surface—but that’s not required, Arney says, “that’s just a bonus.”

As it sinks, the titanium probe will sample the planet’s atmosphere. In particular, it’ll sniff around for noble gases—elements such as helium, xenon, krypton, and argon—that linger in the Venusian air like molecular fossils, or direct tracers of the planet’s history.

Those noble gases will reveal the planet’s formation pathway, history of volcanism and giant impacts, and the origin of its water. The probe will also pay close attention to the relative amounts of normal and heavy hydrogen atoms, which will tell scientists how much water made its way into the atmosphere—key to understanding whether Venus ever had oceans.

“Even though we can explain the data we have right now with oceans, we can also explain them with other models that don’t have oceans,” Arney says. “If we do find strong evidence for oceans in the past … that might suggest that habitability might be more tenacious, that it can exist and endure on planets that we would otherwise have considered uninhabitable.”

The probe’s landing site, called Alpha Regio, is one of the planet’s wrinkled, deformed surface regions known as tesserae. Scientists suspect these terrains could be the remains of ancient continents.

One of the big questions about the tesserae—so named by Russian scientists in the 1980s because they resemble parquet flooring—is whether they are made of granitic or basaltic rocks. On Earth, granite requires water to form and makes up our continental crusts, while basaltic rocks are forged by volcanoes. VERITAS, the other NASA mission, will attempt to solve this mystery by studying Venus’s surface composition.

From orbit, VERITAS will also make maps of the planet’s gravity field, which will help scientists study the planet’s interior structure, and it will watch for signs of active Venusian volcanoes—one of the main ways scientists suspect the planet releases heat, given the absence of tectonic plates like those on Earth.

“There’s been these tantalizing hints of volcanic activity, but nobody’s really observed it, or taken a picture of volcanic activity,” Whitten says. “We can see on the surface just this cornucopia of volcanic landforms, really. It’s just awesome.

Piecing together the planet’s geologic history—and current levels of activity—requires much better maps than scientists currently have. VERITAS will gather extremely detailed radar and topographic measurements of the planet’s landscapes—which cover more than three times as much area as Earth’s continents—to generate far superior maps to the grainy data from Magellan. EnVision will also make exquisitely detailed maps of about 25 percent of the planet’s surface—maps that Byrne says will absolutely revolutionize what we know about the alien terrain.

“It’s going to feel like Christmas, to get all this stuff,” Byrne says. “We are going to be introduced to a whole new world.”

Braving the blasted surface

For decades, NASA has followed a set strategy in planetary exploration: First, fly past another world, as the agency did with the Mariner probes that zoomed by Venus in the 1960s. Then send orbiters, followed by landers, and finally, rovers.

Although the Perseverance rover and its kin are currently trundling around the landscapes of Mars, Venus exploration is still largely stuck at the orbiter stage. None of the Soviet landers lasted for long before losing their battle with Venus’s environment—the record is 127 minutes, held by Venera 13.

“Looking down the line, there’s important science questions that we will need to be on the planet for, and that’s really what we’re all about,” says Tibor Kremic, who is spearheading some of the lander technology development at NASA’s Glenn Research Center outside Cleveland.

Answering questions about the planet’s weather, how its atmosphere interacts with the surface, the precise composition of that surface, and its seismic activity requires long-lived surface missions. At NASA and elsewhere, teams are already developing electronics and other hardware that can function in the extreme Venusian environment. Two additional challenges are figuring out how to power a lander, given that solar energy is scarce beneath all those clouds, and designing communications systems that can relay observations back to Earth.

“We’re still a little ways away,” says Alan Mantooth, a distinguished professor of electrical engineering at the University of Arkansas who is developing Venus lander technologies. “But we are getting much closer.”

Mantooth, Kremic, and their colleagues are borrowing from some of the lessons learned while designing electronics for extreme aeronautical applications and natural gas turbines. They’ve developed silicon carbide circuits and transistors that should work perfectly well on Venus. They’ve also made sensors from gallium nitride, another heat-resistant semiconducting material, and they’ve figured out how to package those electronics in casings that will resist crumpling like a tin can under extreme pressures.

Teams test their hardware in chambers that simulate the harshness at Venus’s surface. The biggest of these is called the Glenn Extreme Environment Rig (GEER). There, in the 800-liter pressure cooker, Kremic and his team can pump in a mix of gases that mimics Venus’s reactive, corrosive atmosphere. Already, they’ve graduated from testing small components like circuits, sensors, and shielding to placing entire subsystems in the chamber—for months at a time.

The next big step, Kremic says, would be testing a small prototype Venus lander. A mission concept called LLISSE, or Long-Lived In-Situ Solar System Explorer, would send a roughly 22-pound robot with a suite of sensors to the Venusian surface, where it would attempt to survive for at least 60 days. The team is working toward putting a LLISSE prototype in GEER, which Kremic says will likely happen around the end of 2025.

“I think the ability to survive on the surface of Venus for even a few weeks is going to be an absolute game-changer,” Glaze says.

Seeing Venus’s landscapes from its surface will radically change how we view our next-door neighbor, much as the albums of images from the Martian surface transformed that planet from a ruddy dot into a sweeping world of mountains, craters, and canyons. Even the grainy Venera images, transmitted a half-century ago by melting Soviet landers, had a powerful effect—Kane keeps a painting of the Venera probe on his office wall.

Soon sharper technologies will bring our alien sister into clearer focus, provoking questions we never thought to ask and revealing vistas that are more beautiful and more terrifying than we ever dared to imagine.

nationalgeographic.com, 7 October 2021
; https://www.nationalgeographic.com