One of the main arguments for studying other planets is for what they can tell us about Earth. Did or do they harbor life? What clues can they offer about our planet’s past? If scientists want to investigate an Earth-like planet, one convenient opportunity is Venus.

“Venus is the exoplanet next door,” says Suzanne Smrekar, a NASA planetary scientist and a principal investigator of the upcoming NASA mission to Venus, VERITAS, which will launch around 2031 to study the planet from surface to core.

The similar size and planetary composition of Venus and Earth make them twin planets. Like Earth, Venus is actively evolving. But up close, both planets are nothing alike. While Earth teems with life, our neighbor seems determined to quash it. These divergent stories make Venus all the more fascinating to scientists.

In the next decade, the United States, Europe, China and India will launch a flurry of spacecrafts to the planet. If successful, they’ll join the ranks of the more than 40 robotic missions that humanity has lobbed toward our neighbor. While scientists still have much to discover about Venus, these past missions have been key for overturning centuries of wild misunderstandings that early astronomers made from peering through their ground telescopes. Here are some of the major discoveries so far about the second planet from the sun, Earth’s closest sibling.

1. Contrary to its name, Venus is a hellish place

Surface of Venus
This computer-generated three-dimensional perspective offers a view of Venus’ surface. The simulated hues are based on color images recorded by the Soviet Venera 13 and 14 probes. Stocktrek Images via Getty Images

Venus is named after the Roman goddess of love and fertility, and before the space age, the perception of the planet was driven by science fiction. Authors conjured up lush landscapes populated with dinosaurs, jungles, swamps and oceans. But the first Venus flybys in the 1960s ended such imaginings.

In 1975, the Soviet Union’s Venera 9 lander captured the first images of another planet’s surface when it touched down on Venus. What the lander saw and sensed was a hostile planet: a rocky, barren terrain with a crushing atmospheric pressure 90 times that of Earth and a scorching temperature of 887 degrees Fahrenheit. This world was inhospitable to life.

In 1982, the first color images of Venus’ surface that came from the Venera 13 and 14 probes only confirmed its desolate landscapes. Scientists agree that no liquid water exists on the surface.

While the Venera missions turned fiction authors away from Venus, they threw open the doors to scientific interest. “They were always an incredible turning point,” says Stephen Kane, a planetary astrophysicist at the University of California, Riverside. “I think [the photographs are] one of the key milestones of our civilization.”

2. Venus is still geologically active

Volcano on Venus
This illustration depicts active volcanism and a subduction zone, where the foreground crust plunges into Venus’ interior. NASA / JPL-Caltech / Peter Rubin

The low number of craters on Venus indicates that geologic processes may be recycling aged landscapes into pockets of fresh ground. From the crater density, scientists have calculated that Venus has a crustal surface as young as 150 million years. (While the oldest part of Earth’s continental crust dates back 4.4 billion years, the oldest oceanic crust on Earth was formed 340 million years ago.)

Additionally, Venus is covered with thousands of volcanoes. In the 1990s, the Magellan mission found that at least 85 percent of the surface was covered by lava flows. Moreover, its sulfur-rich atmosphere suggests that the planet’s volcanoes have long been erupting. But these were telltale clues, not conclusive proof. Scientists had never caught the volcanoes in the act—until a study published last year found new evidence among old images.

“One of the biggest recent discoveries for Venus is actually something that was hidden in plain sight in the Magellan data for the last 30-odd years,” says Noam Izenberg, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory.

Revisiting old radar images, the new study matched two images of a fairy ring of volcanic vents that were taken eight months apart. In the later image, fresh lava had spewed to form a new plain. A neighboring vent seemed to have grown in size, as if it had partly blown up its crown. These were signs of a volcano that had gone off.

Venus has more than volcanic outbursts; the planet also sinks in places, like the chest of a recumbent giant exhaling. This process is called subduction, a phenomenon that also occurs on Earth, albeit by a different mechanism. Venus’ crust isn’t clearly fragmented like Earth’s to allow for plate tectonics. So, researchers think that its subduction process is triggered by a bout of volcanic upwelling followed by a longer-lasting sagging. The surviving clues of this breath-like movement are circular depressions in the ground called coronae, which the Magellan mission detected in the early 1990s.

One of VERITAS’s main goals is to confirm and study Venus’ art of subduction. Understanding the finer details could shed light on the early days of Earth itself, before plate tectonics kicked in on our planet, says Smrekar.

3. Nasty clouds populate its crushing atmosphere

Clouds on Venus
An artist’s impression of caustic clouds smothering Venus’ surface ESA

Venus has a dense atmosphere. When the Venera 4 probe descended through that gassy sheath in the mid-1960s, it measured the composition to be primarily carbon dioxide. Scientists calculated that the air was so thick that it would have taken a probe almost an hour to freefall through the entire atmospheric layer before reaching the surface.

Sulfuric acid clouds circle the entire planet at a height of 25 to 37 miles above the surface. They contain tiny acidic aerosols that are about a hundred times thinner than human hair. Together the droplets resemble the air pollution in highly populated cities on Earth. “It’s like a haze that you find when you fly into, say, New Delhi or Beijing,” says Sanjay Limaye, a planetary scientist at the University of Wisconsin-Madison.

This cloud cover strongly reflects sunlight, making Venus appear as the third-brightest object in the sky after the sun and moon. Ancient civilizations saw the gleaming dot of Venus and thought it was a star. They called Venus—somewhat misleadingly—the Morning Star and the Evening Star. Venus’ opaque skies also prevented early astronomers from seeing the surface through their telescopes, which led to erroneous assumptions of the veiled planet until well into the 20th century.

The clouds aren’t placid but constantly on the move, whipping around the planet in two to four days. What drives their super-rotation is still a question that boggles scientists. Some researchers have suggested that the slight temperature difference between the sun-facing and sun-averted hemispheres of Venus propel the warmer clouds to migrate upward and toward the colder face of the planet.

4. Venus twirls in a different direction

These images, taken in 2004 from Earth, show the phases and relative size of Venus as it moves around the sun. Jamie Cooper / SSPL / Getty Images

Almost all planets in our solar system, Earth included, spin counterclockwise on their axes. Venus is the only oddball that pirouettes clockwise.

Scientists discovered Venus’ retrograde rotation after pelting the planet with ground-based, cloud-penetrating radar waves in the 1960s. What they found was the planet completes its rotation in 243 Earth days, 18 more than it takes to loop around the sun.

One hypothesis that could explain Venus’ enigmatic spin is that a planetary-sized object slammed into proto-Venus and reversed the whirl. Other scientists theorize that the combined effects of a soupy atmosphere and sloshing innards of early Venus slowed the original clockwise revolution, perhaps even to a halt. Gradually, the planet may have slipped into a more stable twirl that’s at odds with the rest of its peers in the solar system.

5. Liquid water may have once pooled on Venus

Ancient Venus With Oceans
Observations suggest Venus may have had water oceans in its distant past. NASA

When descending through the atmosphere in 1978, the Pioneer Venus probes noted an abnormally high deuterium-to-hydrogen isotopic ratio. This find indicates that Venus may once have had liquid water, but all of it had evaporated off the surface and escaped from the planet over time.

From Pioneer’s data, scientists calculated that the planet once had enough water to form a globe-spanning ocean with an average depth of 30 feet.

The presence of liquid water once upon a time points to a balmy environment that could have been habitable. Computer models based on Magellan’s topographic scans have proposed that this hydrated landscape could have hung around as recently as 715 million years ago. In other words, Venus’ habitable climate could have persisted for billions of years before the planet dried up for good.

That’s potentially long enough for life to take root. “Life as we know it is extremely tenacious,” Limaye says. “If Venus had liquid water but somehow didn’t have life, that would be a bigger mystery.”

Perhaps Venus’ life-forms, if any, didn’t survive the later scorching climes—or they might have migrated into the clouds, where temperatures are more agreeable and droplets of water, albeit acidic, dot the skies. In recent years, scientists reported to have detected biosignatures on Venus, though these claims were heavily contested.

A private mission, the Venus Life Finder, is underway to study whether Venus’ clouds harbor organics, a tantalizing indicator of life or life-in-the-making. Having secured means of transport from the company Rocket Lab, the study could potentially launch sometime in late 2024 or early 2025.

6. Venus has a runaway greenhouse effect

Surface of Venus
Venus has the hottest planetary surface in our solar system. MARK GARLICK / SCIENCE PHOTO LIBRARY Via Getty Images

Venus’ transformation from a benign planet to a feverish one is due to a runaway greenhouse effect. The hypothesis was proposed by the American astronomer Carl Sagan during his graduate studies. Later, when Mariner 2 became the first spacecraft to skim past Venus in 1962, the torrid temperatures it measured confirmed the runaway greenhouse theory.

Researchers think that the ancient sun was 30 percent dimmer than it is today. As our star warmed, so did Venus. The Venusian surface became hot enough that carbonate rocks broke down and released carbon dioxide into the air, which triggered even more warming.

On Earth, natural geological processes sequester carbon dioxide underground. It dissolves in the ocean, precipitates onto the seafloor and slips beneath Earth’s crust thanks to the conveyor-belt action of plate tectonics. In contrast, Venus has neither ocean nor tectonic activity. Any carbon dioxide in its atmosphere accelerates warming and spurs even more carbon dioxide release from the rocks. Today, all that heat has boiled away the oceans and resulted in hellish surface temperatures high enough to melt lead.

Scientists say Earth may undergo the same sweltering scenario as our hot neighbor. “Will Earth Turn into Venus? It could,” Kane says. After all, the sun will continue to brighten. As Earth’s interior cools, plate tectonics may subside, switching off our world’s ability to balance atmospheric carbon. The runaway greenhouse effect may play out here millions of years from now, making Venus the looking glass into Earth’s future.

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