Return to the Forgotten Planet

With scientists increasingly excited about the possibility that Venus might once have had an ocean capable of supporting life, NASA and the European Space Agency (ESA) have approved not one but three new missions to Earth’s closest neighbouring planet. 

At the same time, Russia is also looking at a potential Venus launch, says Michael Way, an atmospheric scientist at NASA’s Goddard Institute for Space Studies in New York City. “So we might have four missions at Venus in the 2030s.”

It’s an intriguing prospect because, after a spate of missions in the 1970s, ’80s and ’90s, Venus has been recently overlooked. Even our best radar maps of its surface are 35 years old.

“We know the topography better on Pluto than we know Venus,” says Darby Dyar, a planetary geologist at Mount Holyoke College, in Massachusetts.

Dyar is deputy principal investigator of VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, And Spectroscopy), the first of the two new NASA missions, scheduled for launch in 2028. VERITAS is an orbiter whose primary task will be to use radar mapping to massively improve our understanding of the Venusian topography. “This will be two orders of magnitude increase in resolution,” Dyar says.

Mapping is particularly important as researchers dream of visiting the surface with a state-of-the-art lander, engineered to resist the planet’s high-pressure atmosphere and extreme heat. “When you’re thinking about eventually putting a lander on the surface, you want a good topographic map,” she says.

But it will also help scientists piece together the planet’s geologic structure and history, including the question of whether Venus ever had plate tectonics.

Part of the answer, Dyar says, may come from getting better radar images of large circular features called coronae, which appear to be created when large blobs of magma rise from below but don’t quite erupt.

“Instead, they cool underground, but make giant circular features with faults around the edges,” Dyar says.

These faults, she says, may be indications that material from above is being drawn back into the interior of the planet, in essence being recycled from the surface to the depths.

“There are people who look at the edges and think they see subduction,” Dyar says – a geological process that could indicate the presence of tectonic plates. 

Supporting this theory, she says, a lab group in France has conducted experiments in which they fill a large fish tank with a substance akin to silica gel, then heat it from below. That causes blobs of the gel to rise – much like magma. “When they do that,” she says, “they can get subduction going at the edges of the blobs.”

Also of great interest, says Way, are highlands called tesserae, which appear to be among the most ancient features on the planet. They are cut by numerous channels, presumed to have been created by lava flows. 

“But there was a paper recently claiming that some of the channels look like they might have been carved by water,” he says. That doesn’t mean they actually are ancient river valleys, he says, but if they are, it would be a very exciting discovery.

“VERITAS will tell us what’s going on there,” he says. “It’s probably going to revolutionise our understanding of the geology.”

VERITAS’s mapping instruments should also answer another question: how long has Venus had its current thick, hot atmosphere? 

To do this, Way says, the mapping team will look for impact craters too small to show on current low-resolution maps. If they exist, they would reveal the size of the smallest asteroids capable of making it through the Venusian atmosphere without burning up, an important clue to how thick that atmosphere was at various times in the past. That, in turn, is an indicator of how long the planet has existed in its present inferno-like state.

But that’s not all that VERITAS will be able to do. The manner in which tiny variations in the Venusian gravity field tug the spacecraft slightly one way and then another, for example, will allow scientists to measure the planet’s gravity field in detail. From that, they hope to learn about its interior composition, including the size of its core.

VERITAS will also carry an instrument that will allow it to peer through the planet’s dense clouds in infrared wavelengths from which scientists can tease out important clues about the surface composition.

Until recently, Dyar says, it was believed to be impossible to do this because the clouds were too thick. But it turns out that there are a handful of “little windows” between 860 nanometres and 1180nm in which infrared light is capable of penetrating all the way through the clouds. (Red light, for comparison, is 620–750nm.) Using these, Dyar says, it should be possible to distinguish two important types of volcanic rock: basalt (which contains iron) and granite (which is low in iron). “We think we can make an iron map of the surface,” she says.

If that map reveals granite, it would be a very important find, because granite forms from magma that has mixed with moisture in one form or another. “Granite is inherently something that forms in the presence of water,” Dyar says.

A good place to begin, she says, is by taking a close look at the tesserae, which she describes as looking like “crumpled up aluminum foil.” They can be thought of as islands of rock of an unknown type, surrounded by seas of what are probably basaltic plains. 

One theory, she says, is that they are simply “scum” left over from the surface of Venus’ primordial magma ocean. Another is that they are granite. That would make them relics from an ancient, wet Venus. And if Venus was once temperate enough to have a surface ocean, Way’s models of the evolution of its atmosphere indicate that it might have remained temperate, wet and habitable for a long time – even though it is barely more than two-thirds the distance from the Sun as is the Earth.

“Way’s models suggest that Venus could have had water for three billion years,” Dyar says. “Think about what we know about how life evolved on Earth [which was much more quickly than that]. It makes sense that Venus could not only have had oceans but also have had life.”

Meanwhile, scientists using large radio telescopes to study Venus from Earth have found signs that its atmosphere might contain phosphine (PH₃), a gas that on Earth is only formed in significant quantities by microorganisms.

“We’re not claiming we found life on Venus,” says one of the discoverers of this gas, Sara Seager of Massachusetts Institute of Technology (MIT). Rather, says her collaborator William Bains, also of MIT, “We are claiming there is something really unknown, and it might be life.”

The discovery is controversial. But NASA’s second mission, DAVINCI+ (Deep Atmosphere Venus Investigation of Noble gases, Chemistry and Imaging), should put it to the test.

Details of DAVINCI+, expected to launch in 2030, are still uncertain, but its main task will be to release a probe that will descend through the Venusian atmosphere, hopefully all the way to the surface. If phosphine exists, that probe will be equipped to detect it.

But that’s not the only thing it will do. On its way down, the probe will also measure the ratios of hydrogen and deuterium (heavy hydrogen) in what little water vapour remains in the Venusian air – an important clue to how much water Venus has lost to space, and how much it might once have had.

It will also measure other trace gases, such as neon, argon and xenon, including measuring their isotope ratios. Way calls these ratios “fingerprints of the early history of the planet” that can show not only how the early Venusian atmosphere evolved, but how similar its initial composition was to Earth’s. They can also reveal the rate at which volatiles escaped from its interior: clues to whether Venus might once have been habitable and, if so, for how long. “That will tell us whether the planet has been bone dry for 4.5 billion years, or not,” Way says.

The ESA mission, called EnVision, won’t launch until 2031 at the earliest and possibly not until 2033. Like VERITAS, it is an orbiter whose primary tasks will include detailed mapping.

By that time VERITAS will have already completed a global map that will allow scientists to determine the most interesting places for further study with EnVision’s state-of-the-art high-resolution instruments. “VERITAS will have done all this mapping,” Way says “EnVision will clean up and pinpoint.”

“We will have an extremely comprehensive science program at this enigmatic planet well into the next decade,” says Günther Hasinger, the ESA’s Director of Science. 

Meanwhile, the ultimate dream is a lander. “NASA has been pouring money into high-temperature electronics,” Dyar says. “We now think there are landers that could last two or three months.

“That would be the triumvirate,” she says, noting that Venus researchers dream of three things: improved mapping, an atmospheric probe, and a lander. Combined with EnVision, VERITAS and DIVINCI+ “get us two-thirds of the way there,” she says, “and I think the remaining one-third, a lander, will come in a decade or two.”