The case for Venus
The search for life elsewhere in our universe is exploding. Discoveries of new exoplanets are now a weekly occurrence. Our curiosity about exoplanets is motivated by the tantalizing possibility that we might discover another world where life as we know it could thrive.
Figure 1. Earth is the Goldilocks planet in the habitable zone. But recent research suggests that both Venus and Mars had hydrologic cycles long ago, at least through the first billion years and possibly much longer for Venus. Time, it seems, is an essential dimension in considering the extent of habitability.
Traditional conceptions of the so-called habitable zone for terrestrial bodies assume that within our solar system, only Earth is in the sweet spot for life to form and survive. But that presumption overlooks the temporal variation of habitability: Both Venus and Mars likely had liquid water as far back as 3.6 billion years ago (see figure 1). In fact, exoplanets may even be more Venus-like than Earth-like. Recent research suggests that the number of exo-Venuses is comparable to, or even exceeds, the number of exo-Earths.
Figure 2. As knowledge of and questions about exoplanets explode, our best analogue might be the planet that’s closest to us.
Dan Durda, SwRI
There is growing recognition of the importance of understanding Venus, which has long been known as Earth’s twin. Learning the story of the twin planets—how one ended up hot and dry and the other comfortably warm and wet—would illuminate what makes or breaks a planet’s habitability. As a planetary dynamics laboratory, Venus offers a glimpse of what happens in an extreme greenhouse atmosphere. It also informs our understanding of the evolution of rocky planets. Without figuring out why Venus and Earth evolved differently, we have no hope of predicting all the beautifully bizarre planets that will no doubt be discovered (see figure 2). We may also learn something about the fate of our own planet. That is why NASA needs to green-light a robotic mission to Venus.
Why we need to go
Mariner 2 became the first spacecraft to visit another planet in 1962 when it observed Venus’s carbon dioxide atmosphere and high surface temperature and pressure. During the space race of the 1960s and 1970s, the US and the Soviet Union launched dozens of spacecraft to Venus, and the Soviets accomplished the impressive task of landing and acquiring data on the surface multiple times. The US sent several orbiters and probes to Venus, culminating in the launch of the Magellan mission in 1989, which provided topography, radar imaging, and gravity data. Venus was then abandoned until the European Space Agency sent Venus Express in 2005, followed by Japan’s Akatsuki in 2010.
Figure 3. Striking similarities in the topography of Earth (left) and Venus (right) suggest that the latter might have zones of subduction.
As space agencies have spurned Earth’s twin, scientists have posed more and more compelling science questions that can be addressed only by studying Venus up close. Here are some of the key issues:
Figure 4. Map of flux anomaly (relative emissivity) at the 1.02-micron band, from the Venus Express mission. Although Venus is shrouded in thick clouds, windows in the carbon dioxide spectrum allow mapping of the surface from orbit, with up to six different wavelengths proposed for future missions. High emissivity has been interpreted as evidence for recent volcanism.
Plate tectonics. At one time scientists thought the fundamental process that has shaped Earth’s geologic evolution existed on Venus as well. Although Magellan revealed that Venus lacks plate tectonics today, models suggest it may well develop in the future. Magellan showed evidence of subduction (see figure 3), which is the first step in a planet transitioning away from a single plate. Evidence for recent volcanism at one subduction site suggests it is active today (see figure 4). However, subduction without an interconnected network of faults is not plate tectonics. Some have proposed that Venus’s hot surface temperature seals faults created by subduction, preventing full plate tectonics from developing. Earth may have experienced the same initial phase of subduction, with plate tectonics developing only as the lithosphere cooled enough to support long-lasting plate boundaries. Fully understanding the conditions that are allowing subduction to begin on Venus has implications for both Earth and exoplanets, and it requires improved data sets for radar imaging, topography, and gravity.
Interior dynamics. Venus’s geologic history is key to understanding its interior dynamics. Many textbooks suggest that Venus had a catastrophic pulse of activity, likely volcanism, about a billion years ago that was followed by quiescence. But ongoing activity is equally likely. Venus Express observed regions of fresh, unweathered basalt along with periodic increases in sulfur dioxide concentration attributed to volcanic outgassing. The question of whether Venus is volcanically active is critical for understanding if the interiors of Venus and Earth are fundamentally similar. Via high-resolution topography (see figure 5), scientists could determine whether Venus catastrophically resurfaced by searching for volcanically flooded impact craters and identifying the history of the featureless volcanic plains that cover 40% of the planet.
Figure 5. Hawaii is shown at Magellan resolution and at the resolution easily obtainable in a future Venus orbiter. Topography is key to understanding planetary dynamics. Our knowledge of topography on Venus is one to two orders of magnitude worse than for the Moon and other terrestrial bodies.
TanDEM-X
Water, water everywhere. We need information on how much water existed on Venus in the past and where that water went (see figure 6). Earth and Venus are the only terrestrial planets in the solar system that still have atmospheres, so Venus is a critical laboratory for studying processes that create and destroy an atmosphere. Scientists lack the measurements of heavy and light isotopes of hydrogen within Venus’s clouds that are needed to answer fundamental questions about Venus’s water history. Such measurements can be obtained only from within the planet’s atmosphere.
Planetary evolution. The question of primordial planet composition is fundamental to understanding how a given planet evolved to its current state. Scientists have assumed that Venus and Earth formed from the same building blocks, but it is impossible to distinguish primordial differences from those driven by processes such as major impacts, which blew off the original atmospheres of Earth and Mars. We lack high-accuracy measurements of isotopes and noble gases such as xenon in Venus’s atmosphere, measurements that can be obtained only by in situ mass spectrometers.
Surface mineralogy. The four 1970s Soviet landers are the only sources of images and surface composition measurements because Venus’s thick, CO2-rich atmosphere precludes orbital imagery. But the VIRTIS instrument aboard the Venus Express mission (see figure 4) demonstrated that spectroscopy is possible. There are windows in the CO2 spectrum around 1 micron, which happens to be a pivotal wavelength range for measuring mineralogy and inferring chemical changes caused by surface–atmosphere interactions. On the surface itself, critical questions involve the nature of the tesserae—enigmatic older terrain that may mark a fundamentally different era in Venus’s history, perhaps signaling a period of continent-building and surface water. Detailed measurements of tessera chemistry and mineralogy may hold the key to understanding Venus’s geologic history and weathering environment prior to the volcanic activity that covered the remainder of the surface.
Surface–atmosphere interactions. The complex chemical cycles that operate in the deep atmosphere of Venus may be sustained through chemical weathering of rocks at the surface–atmosphere interface, or they may require a continued supply of volatiles through active processes such as volcanism. Observations of Venus’s deep atmospheric composition are almost nonexistent, forcing scientists to rely almost entirely on models and assumptions. Because of the thick cloud layer, atmospheric composition below about 30 km requires measurements to be made in situ from within the atmosphere.
How to get there
With all the outstanding questions about Earth’s twin, it’s no surprise that the National Academies’ last two planetary decadal surveys have recognized the importance of exploring Venus. And US planetary scientists have done their part: Since the Magellan mission ended in 1994, they have submitted NASA proposals for more than 25 Venus missions, including three in last year’s New Frontiers competition and two of the five finalists in the 2017 Discovery-class selections. Yet NASA has rejected every one.
Figure 6. Venus likely had sufficient water in the past to sustain vast oceans (far left). However, current measurements of the deuterium/hydrogen ratio are insufficient for constraining the volume of water that was present or the timing of water loss. High-precision measurements of those isotopes and others are critical to resolving fundamental questions about how a planet on the inner edge of the habitable zone evolves from potentially habitable to desiccated (far right).
NASA’s Goddard Space Flight Center
The ideal Venus exploration program would have three parts:
- An orbiter, which would measure topography, active deformation, global surface composition, and mineralogy at broad scales.
- An atmospheric sampler, which would conduct in situ analysis of stable and noble gas abundances and isotopes and acquire measurements of trace volatile abundances (H2O, SO2, OCS, CO), including at below 30 km.
- One or more landers, which would measure bulk chemistry, mineralogy, and morphology of rocks in the volcanic plains or tessera regions.
Those three basic missions are all feasible with existing and rapidly maturing technologies, as reviewed by NASA at a 2015 workshop .
By 2025, it will be more than 30 years since Magellan and 40 years since the last in situ mission to Venus. The tragedy is that our understanding of rocky planet evolution is being impeded by NASA’s inexplicable and repeated failure to select scientifically compelling and technically sound missions. The case for Venus exploration needs active recognition by astronomers and planetary scientists who value the important insights into habitability and exoplanet evolution that will result.
Darby Dyar is a senior scientist at the Planetary Science Institute and Kennedy–Schelkunoff Professor of Astronomy at Mount Holyoke College. Suzanne Smrekar is a senior research scientist and deputy principal investigator of the InSight mission at the Jet Propulsion Laboratory. Lori Glaze is chief of the Planetary Geology, Geophysics, and Geochemistry Laboratory at NASA’s Goddard Space Flight Center.
