Both missions will also examine the Tesserae, strange continent-like plateaus on the surface of Venus. Most of the planet is covered in lava flows that must have poured out long after the potentially climate-changing volcanism. The towering tesserae are believed to be the oldest rocks on Venus. “You could be half a billion years old or four billion years old – we don’t know,” Gilmore admits.
In addition, it is not known what the tesserae are made of. If they were indeed continents like the ones on earth, it would have taken a lot of water to produce them. Then they provided evidence that Venus was once a blue planet. “That would blow people away,” O’Rourke is convinced. If the rock were stratified, as suggested by Byrne and his colleagues, this could indicate sedimentation and thus to former waters. Alternatively, there are perhaps pancake-like layers of lava – possible remnants of an ancient volcanic activity that sealed the fate of the atmosphere.
The DAVINCI + probe would take a very close and detailed look at just one tessera, according to O’Rourke. “We don’t even know if all tesserae are made alike, so choosing just one is quite a risk,” he admits. “But DAVINCI + will provide images of geological details that cannot be seen from orbit.” In addition, VERITAS creates a map of each tessera, albeit with fewer details. Meanwhile, EnVision can carefully examine several selected locations from above.
The images collected by VERITAS should show changes by repeatedly mapping a point on the surface. That could clarify whether the planet is still volcanically active today. There is a lot of evidence for this view, which has been held for a long time, but so far no conclusive evidence. “It would be really cool to find an active volcano,” hopes Smrekar. EnVision could help with the search by allowing the spacecraft to record the chemical signature of the gases of an erupting volcano or the heat released in the process.
Confirmation that such an important planetary process is still underway would have far-reaching implications. Any tectonic activity is driven by what is going on deep inside the planet and would open a gateway to the geological heart of Venus. Finally it would be possible to compare its strength and rhythm with that of the earth.
While DAVINCI + would determine how much water Venus has lost, EnVision would determine how much it still has today – for example in the interior of the planet. Trying to get H2To identify O in the gas clouds that emerge from the volcanoes, the scientists want to find out whether the interior of Venus is as dry as its exterior.
More than ten years ago there were mission drafts for VERITAS and DAVINCI +. Her previous versions were finalists in the previous Discovery competition in 2017, but lost to the asteroid exploration missions Psyche and Lucy. When the Discovery selection was announced again in 2021, the first few months of the year were particularly exhausting for both mission teams. They worked around the clock to attract the decision-makers to their concepts. “To properly appreciate the efforts of the past year, one would have to write a novel,” says Smrekar. The report that her team presented in November 2020 already contained “almost the number of pages from ‘War and Peace'”.
VERITAS and DAVINCI + faced two undeniably outstanding competitors. The first was the Io Volcano Observer (IVO), which would have visited Jupiter’s moon Io, the celestial body with the strongest known volcanic activity. It is powered by the gas giant’s pulling force on its satellite. The second mission concept Trident was aimed at the Neptune moon Triton in the outer solar system, the surface of which is constantly renewed by means of a mysterious ice volcanism.
Perhaps the swing in favor of Venus came on September 14, 2020. A research team announcedto have discovered the chemical compound phosphine in the clouds of Venus with two telescopes, just at an altitude at which the prevailing temperatures and pressures could allow the existence of droplets with liquid water.
Phosphine can be produced by volcanism and lightning, but also by microbes. Therefore, some believed the discovery to be an indirect reference to extraterrestrial life. All of a sudden, there was tremendous interest in phosphine and Venus in the public, media, and scientific community.
In the following months the data analysis was called into question, and subsequent evaluations appeared to either refute or support the conclusion, depending on the interpretation. In the end, it wasn’t that important whether there was phosphine or whether it was made by microbes. Rather, the controversy raised awareness that there are regions in the clouds of Venus that are fundamentally neither too hot nor too acidic for specially adapted organisms.
Animated or not, Venus remains fascinating
On earth, single-celled life forms keep reappearing in places that higher plants and animals would instantly kill. Accordingly, microbes may even be at home in places in the solar system that we commonly describe as hostile to life. They could exist in the warmer, moister underground of Mars, for example, and the last word has not been spoken on Venus either. “A planet that looks like hell is not necessarily inhospitable in every respect,” says the astrochemist Clara Sousa-Silva from the Harvard-Smithsonian Center for Astrophysics, Cambridge, who was involved in the potential discovery of phosphine.
The DAVINCI + probe may be able to detect phosphine during its flight through the clouds, but neither it nor VERITAS or EnVision were designed to study this chemical compound. But all three could help identify processes that can produce phosphine, from volcanism to atmospheric chemistry. In any case, the phosphine gave Venus in 2020 a similar PR boost as the fossil-suspected meteorite gave Mars in 1996. Nevertheless, it is “just the icing on the cake,” says Gilmore. “Venus is fascinating regardless of the existence of life.”
Even if neither of the two Discovery missions had been chosen, there would have been reasons to remain optimistic about Venus research as a whole. In addition to Europe with its EnVision mission, other space agencies, including those of Russia and India, have already considered a return to Venus. Because the planet appears, not least in view of the increasing number of discovered worlds that revolve around strange stars, as an attractive destination to learn more about the possible fates of earth-like celestial bodies.
Many earth-sized exoplanets are already known. With today’s telescope technology, however, it is hardly possible to say whether a particular specimen will have such inviting conditions as here or as merciless as on Venus. At the moment, studying Venus up close is perhaps the only way to make reliable assessments of whether there are more Earth-like or more Venus-like planets in the cosmos. Gradually, exoplanet experts are also recognizing the fact that the solar system itself is “an excellent laboratory for exoplanet research,” as Sousa-Silva put it. Smrekar emphasizes: “Only Venus can tell us why our home planet in the solar system is unique and how likely it is that we will find a second earth around another star.”
The inevitable disappointment of those who lost the Discovery program is now giving way to optimism. Proponents of a trip to Io hope to win the next competition or maybe even be selected as part of the more expensive and technically demanding missions of NASA’s New Frontiers program. Those hoping to return to Uranus and Neptune, both of which were last visited by a spacecraft in the late 1980s, are even considering a future flagship mission. This class is at the top of NASA’s exploration programs with a cost and equipment cost of over a billion dollars.
In the coming decade, our neighboring planet will first move into the spotlight. Finally, according to Ghail, “the view has prevailed that we have to achieve on Venus what we have already achieved on Mars”.